Abstract
Ultrasound-assisted emulsification is increasingly used to formulate food-grade emulsions and active edible coatings, yet preservation performance is still frequently inferred from droplet size, polydispersity, ζ-potential, and bulk stability. These descriptors do not establish whether an emulsion retains its designed function after spreading, drying, adhesion, and storage on a heterogeneous, metabolically active food surface. This review develops a cavitation-guided framework that connects delivered acoustic energy and reactor-dependent bubble dynamics with emulsifier remodeling, interfacial assembly, emulsion architecture, droplet-to-coating translation, and postharvest function. Direct experimental evidence is distinguished from mechanistic interpretation and testable design hypotheses. Quantitative comparisons of ultrasound, high-pressure homogenization, rotor–stator processing, and hybrid approaches are used to define formulation-specific processing windows rather than a universal optimum. Nanoemulsions, Pickering emulsions, multilayer emulsions, multiple emulsions, and emulgels are assessed according to surface coverage, interfacial robustness, barrier-release coupling, compartmentalization, and residence. Particular emphasis is placed on wetting, drying, droplet immobilization, adhesion, water and gas transport, active-compound flux, and coating failure. Finally, evidence gates are used to distinguish formulation readiness, coating readiness, produce validation, and industry readiness, with throughput, energy use, thermal control, hygienic operation, migration, sensory acceptance, and process reproducibility treated as non-substitutable constraints. This framework shifts ultrasound-assisted coating design from empirical droplet miniaturization toward quantitatively defined, reactor-aware, and commodity-specific sonoprocessing.
Keywords: Acoustic cavitation, Sonoprocessing, Ultrasonic emulsification, Functional emulsions, Interfacial remodeling, Postharvest preservation
1. Introduction
1.1. Postharvest preservation at the produce-environment interface
Fresh fruits and vegetables remain metabolically active after harvest [1], [2], [3], [4]. Their deterioration reflects interacting processes rather than a single dominant pathway: respiration and transpiration deplete reserves and water, oxidative stress accelerates membrane and pigment damage, microorganisms exploit surface injuries and moisture, and cell-wall disassembly progressively weakens tissue structure [5], [6], [7], [8], [9]. The rates and relative importance of these events vary with commodity, maturity, cuticle morphology, mechanical damage, temperature history, and surface wetness. Effective preservation must therefore protect a living, heterogeneous surface without imposing gas-transfer, sensory, or safety penalties. This requirement has become more demanding as food systems seek lower reliance on synthetic preservatives and disposable plastics while maintaining microbial quality, texture, nutritional value, and consumer acceptability [10], [11], [12], [13], [14], [15], [16].
Edible coatings act at the produce–environment boundary, where they can alter water-vapor transfer, O2/CO2 exchange, microbial contact, surface hydration, and the local availability of active compounds [17], [18], [19], [20]. Emulsion-based coatings add dispersed lipid or bioactive domains to a film-forming continuous phase, allowing hydrophilic matrices to carry hydrophobic preservatives while combining barrier and delivery functions [21], [22], [23], [24]. This is especially relevant to essential oils and other plant-derived lipophilic compounds, whose direct application is restricted by poor water dispersibility, volatility, oxidation, and intense aroma [22], [23], [24].
The performance of these systems is nevertheless difficult to infer from conventional emulsion measurements. Droplet size, polydispersity, ζ-potential, and resistance to creaming describe the liquid formulation, whereas preservation depends on events that occur after deposition: wetting, drying, matrix consolidation, droplet immobilization, adhesion, transport, and interaction with living tissue. Produce-level endpoints such as decay, mass loss, microbial load, or firmness are equally insufficient when they are not linked back to acoustic processing and coating structure. The resulting evidence gap lies between two well-developed literatures—ultrasound-assisted emulsification and edible-coating preservation. Existing reviews commonly address ultrasonic emulsification, nanoemulsion preparation, essential-oil delivery, or coating applications separately. What remains unresolved is when an emulsion prepared using ultrasound retains its intended function after it becomes a coating on a real food surface.
1.2. Acoustic cavitation as an interfacial design variable
Ultrasound-assisted emulsification is often interpreted mainly as droplet disruption. High-intensity ultrasound, however, acts through acoustic cavitation: bubbles nucleate, oscillate, and collapse, generating microjets, shock waves, microstreaming, local heating, and radical-mediated effects [25], [26], [27]. These events create and deform oil–water interfaces while changing the rate at which emulsifiers reach and stabilize them. They may also disperse protein aggregates, expose hydrophobic or charged groups, alter polysaccharide conformation, promote macromolecular association, and facilitate particle adsorption at interfaces [25], [28]. The emulsion produced by ultrasound is therefore shaped by both droplet breakup and the acoustic remodeling of its stabilizing materials.
This dual action creates a formulation- and reactor-dependent processing window. Insufficient treatment leaves coarse, weakly protected droplets; excessive treatment may promote recoalescence, aggregation, polymer degradation, oxidation, volatile loss, or thermal damage. Within an appropriate window, cavitation can improve emulsifier dispersion, interfacial coverage, and compatibility between droplets and the film-forming phase [25], [28], [29]. Ultrasound-treated protein-polysaccharide emulsions, for example, show changes in complex dispersion, microstructure, and interfacial stability that cannot be reduced to particle-size effects [28]. In pullulan films containing cinnamon essential-oil nanoemulsions, sonication also influences droplet distribution and the properties of the resulting active film [29].
In this review, cavitation-guided interfacial design refers to the use of a characterized ultrasound field—interpreted through delivered acoustic energy, cavitation behavior, reactor geometry, formulation properties, and thermal history—to control interfacial assembly and its downstream coating function [25], [26], [27], [28], [29]. The term does not imply that cavitation alone determines preservation performance, nor is it used as a synonym for every emulsion prepared by sonication. Acoustic treatment first acts on a defined formulation within a particular reactor; the resulting mechanical, thermal, and sonochemical events may then alter emulsifier conformation, adsorption, droplet breakup, and interfacial organization [25], [27], [28], [29]. Whether these changes remain functionally relevant must be established after incorporation, deposition, drying, and storage on the target food surface. This conditional definition provides a more rigorous basis for interpreting ultrasound-assisted coatings than the assumption that smaller droplets necessarily confer superior preservation.
1.3. Scope, evidence boundaries, and analytical framework
This review follows the sequence by which acoustic processing becomes a preservation function. It begins with the relation between delivered acoustic conditions, cavitation behavior, and interfacial remodeling; proceeds to the selection of nanoemulsions, Pickering emulsions, multilayer emulsions, multiple emulsions, emulgels, and related architectures for defined coating tasks; examines the transition from a liquid dispersion to an adherent, mass-transfer-regulating layer; and concludes with preservation mechanisms, reporting standards, safety, sustainability, and scale-up. The review does not catalogue all edible coatings or all applications of ultrasonic emulsification. Studies are considered according to the causal step they inform.
Two complementary evidence dimensions are used throughout this review. Evidence maturity describes how far a system has progressed from emulsion characterization to coating validation, produce-level efficacy, mechanistic validation, and scale-up or safety assessment. Evidence status, by contrast, describes the strength of a specific scientific statement. A validated finding denotes an outcome measured directly in the cited experiment; a mechanistic interpretation denotes an explanation consistent with the observations but not independently isolated as the sole cause; and a design hypothesis denotes a testable inference that has not yet been validated across the complete ultrasound-emulsion-coating-produce pathway. These dimensions are orthogonal: produce-level efficacy can be experimentally established while the proposed cavitation-mediated mechanism remains inferential. This distinction is retained throughout the discussion to avoid transferring mechanistic certainty from formulation studies to coating or produce responses.
Terminology is also restricted according to the evidence available. Emulsion coating is used as the general term for an edible coating containing dispersed emulsion domains. Nanoemulsion coating is used only when the cited study reports droplets within an explicitly nanometric range. Ultrasound-assisted emulsion coating denotes a system in which ultrasound is used during emulsifier treatment, emulsion formation, or coating preparation, irrespective of whether the cavitation field itself was measured. Cavitation-guided is reserved for the analytical framework and for mechanistic discussions in which delivered acoustic conditions and cavitation-consistent interfacial effects are central. It is therefore not used as a coating category, and ultrasound-assisted coating is avoided because it may imply direct causal control of downstream coating or preservation performance beyond the evidence available. These terms describe different dimensions of the system and are not interchangeable.
Evidence is interpreted by the point reached along this chain. Direct evidence links ultrasound treatment to an emulsion coating applied and evaluated on intact or fresh-cut produce. Intermediate evidence characterizes a film or coating prepared from an ultrasound-treated formulation but stops short of produce-level validation. Supporting evidence addresses cavitation, emulsifier remodeling, emulsion stability, or interfacial assembly without testing the complete coating pathway. This separation is necessary because advanced emulsion technologies are increasingly used to protect and deliver bioactives in foods [30], yet mechanistic advances in formulation do not automatically establish performance after deposition. It also prevents film data, model-emulsion data, or non-ultrasonic coating studies from being treated as equivalent to direct validation.
The analysis is organized around four linked questions. First, which acoustic and formulation conditions generate useful interfacial remodeling without unacceptable molecular or thermal damage? Second, which emulsion architecture best matches the required surface function and release profile? Third, does that architecture survive spreading, drying, adhesion, storage, and interaction with commodity-specific physiology? Fourth, is the evidence sufficient for reproducible and scalable sonoprocessing? The last question is particularly important because laboratory treatments reported by nominal power or sonication time are difficult to compare, whereas reactor-aware studies increasingly emphasize delivered energy, cavitation descriptors, residence time, temperature control, and continuous operation [31], [32], [33]. Recent produce studies involving pullulan, alginate, citral, and sonicated protein-stabilized essential-oil nanoemulsions demonstrate practical preservation potential, but their acoustic reporting, coating characterization, and mechanistic depth remain uneven [34], [35], [36], [37].
Fig. 1 provides the organizing evidence chain used throughout this review. It deliberately remains at the level of translational stages, from reactor-defined acoustic treatment to cavitation-mediated interface formation, coating translation, produce validation, and process translation; the mechanistic detail of the acoustic-to-interface step is developed separately in Fig. 2. Three transitions require direct evidence rather than extrapolation: formulation-to-coating translation, coating-to-produce efficacy, and laboratory-to-throughput scale-up. These boundaries are used in the following sections to distinguish direct evidence from intermediate validation and mechanistic interpretation.
Fig. 1.
Evidence pathway linking ultrasound processing to postharvest coating performance. Reactor-defined acoustic treatment generates a cavitation field that remodels the emulsion interface; the resulting architecture must remain functional during coating formation, produce storage, and process translation. Arrows indicate sequential evidence links rather than universal one-to-one causality.
Fig. 2.
Acoustic processing window for cavitation-guided interfacial remodeling. The upper pathway links acoustic input to cavitation events and interfacial remodeling, whereas the lower panel illustrates the transition from underprocessing to a formulation-specific functional window and, at excessive exposure, molecular or oxidative damage. The regime boundaries are schematic and are not intended as universal quantitative thresholds.
2. Cavitation-guided interfacial remodeling
Within the cavitation-guided framework developed here, ultrasound-assisted emulsion coatings are examined according to how acoustic processing modifies interfacial structure. In liquid formulation media, high-intensity ultrasound generates cavitation fields in which bubbles nucleate, oscillate, and collapse, concentrating mechanical, thermal, and chemical effects over short spatial and temporal scales [25], [26], [27]. These events can fragment droplets, accelerate mass transfer, disrupt biopolymer aggregates, expose buried functional groups, and create new oil–water interfaces that must be rapidly stabilized by proteins, polysaccharides, particles, or molecular complexes [25], [38], [39]. Recent emulsion-focused sonication studies further indicate that cavitation efficiency depends strongly on both formulation properties and reactor configuration, rather than on nominal input power alone [40]. For edible coating systems, however, cavitation is useful only when it produces an interface that remains stable during spreading, drying, adhesion, and storage. Fig. 2 therefore expands the acoustic-to-interface segment of Fig. 1, linking defined acoustic inputs to cavitation events, biopolymer remodeling, interfacial assembly, and the transition from useful processing to overprocessing.
2.1. Acoustic dose: Frequency, amplitude, energy density, and pulse control
Acoustic dose cannot be reduced to the nominal power displayed on a generator. Frequency, amplitude, delivered acoustic power, power density, energy density, sonication time, pulse ratio, probe diameter, immersion depth, treatment volume, liquid height, viscosity, gas content, temperature, and reactor geometry collectively define the cavitation regime [31], [32], [40], [41], [42]. This distinction is essential because nominally identical ultrasound treatments may deliver different acoustic power to the liquid phase and generate different cavitation distributions. Early nanoemulsion studies showed that acoustic power, surfactant concentration, flow-cell geometry, and residence time strongly affect the final droplet size obtained by ultrasound, whereas later reactor-scale work demonstrated that frequency, liquid height, power density, and transducer position can change sonochemical efficiency as measured by calorimetric power and potassium iodide (KI) oxidation [38], [39], [42]. Within the overall pathway in Fig. 1, these variables define the reactor-level acoustic input; Fig. 2 resolves how frequency, amplitude, energy density, pulse mode, temperature, and reactor configuration shape cavitation behavior and the processing window.
Frequency controls bubble dynamics and therefore the balance between mechanical disruption and sonochemical activity. Low-frequency ultrasound typically produces more violent transient cavitation, stronger droplet disruption, and intense local shear, whereas higher-frequency operation can generate smaller and more distributed cavitation events with different radical and heating profiles [26], [42]. Amplitude and power density determine whether droplet breakup is merely initiated or driven into the submicron/nanoscale range. However, increasing energy input does not linearly improve emulsion quality. Beyond a formulation-specific threshold, additional acoustic energy may broaden size distributions, promote recoalescence through excessive collision frequency, increase temperature, or chemically damage the dispersed oil phase [38], [39].
Energy density is therefore a more interpretable descriptor than time or power alone. For coating precursors, reporting energy per processed volume or mass is necessary because the same treatment time can have different effects in low-viscosity emulsions, hydrocolloid-rich matrices, or protein-polysaccharide complexes. Pulse control is equally important. Intermittent sonication can maintain cavitation-assisted droplet disruption while reducing cumulative heating, volatile loss, and macromolecular damage. Continuous sonication may be useful for rapid emulsification but requires active cooling and residence-time control, particularly when lipidic active phases or essential oils are present. Flow-through and microreactor systems offer better residence-time definition and scale-up potential than simple probe sonication, but they introduce new dependencies on channel geometry, surface roughness, flow rate, and acoustic coupling [31], [43]. Thus, acoustic dose should be treated as a formulation-reactor descriptor rather than a generic processing condition.
Representative studies illustrate both the attainable processing range and its strong formulation dependence. Kentish et al. used batch and focused flow-through ultrasound at 20 ∼ 24 kHz and obtained flaxseed-oil emulsions with mean droplet diameters as low as 135 ± 5 nm, comparable with emulsions prepared using a microfluidizer at 100 MPa [39]. Leong et al. subsequently demonstrated that an optimized surfactant/co-surfactant/oil system could reach mean diameters of approximately 40 nm using ultrasound or high-shear homogenization; the minimum size depended jointly on droplet deformability and equipment geometry, while an overpressure of up to 400 kPa altered the relationship between total energy density and emulsification efficiency [38]. More recently, Xu et al. reported continuous soybean-oil nanoemulsification in a 20 kHz ultrasonic microreactor. The optimized system produced droplets of 96.4 nm with a polydispersity index (PDI) of 0.195; numbering-up from one to four reactor units increased the total flow rate from 0.66 to 2.64 mL min−1 while maintaining droplet sizes near 120 nm, and a larger 160 W reactor reached 20 mL min−1 with a droplet size of 120.5 nm and PDI of 0.219 [43]. These values should be interpreted as study-specific benchmarks rather than universal operating optima because formulation composition, viscosity, acoustic coupling, residence time, temperature, and reactor geometry all shift the effective processing window.
2.2. Cavitation events: Bubble collapse, microjets, shock waves, and microstreaming
Cavitation provides the physical basis for ultrasound-assisted emulsification. During acoustic rarefaction and compression cycles, bubbles grow, oscillate, and collapse, producing microstreaming, shock waves, microjets, localized shear, transient hotspots, and radical-generating events [26], [27]. In oil-water systems, these events deform and fragment droplets, increase interfacial area, disperse emulsifier aggregates, and accelerate the transport of stabilizers to newly formed interfaces [38], [44]. The final droplet size is therefore governed by the competition between cavitation-induced breakup and the rate at which the new interface becomes protected against recoalescence. These localized phenomena correspond to the cavitation-event domain in Fig. 2 and provide the physical link between reactor-level acoustic input and interfacial restructuring.
Recent mechanistic studies have refined the conventional view of ultrasonic emulsification. Cavitation-induced emulsification is not driven by a single universal mechanism; it depends on bubble location, liquid–liquid interfacial deformation, jet direction, viscosity ratio, interfacial tension, and confinement [44], [45], [46], [47]. Bubble collapse near a liquid–liquid interface can produce directed liquid jets and local interface penetration, providing a physical route for droplet fragmentation beyond bulk turbulence [45], [46]. In microreactors, surface roughness and ultrasound frequency can alter nucleation sites and bubble behavior, thereby changing emulsification efficiency and the reproducibility of droplet production [47]. These findings support a key point for coating research: ultrasound modifies emulsions through a spatially structured cavitation field, not through uniform agitation.
Evidence for the local physics of emulsification is stronger than evidence connecting these events directly to coating performance. Bubble collapse close to liquid-liquid interfaces has been shown to generate directed liquid jets, local interface penetration, and deformation [44], [45], [46], while ultrasonic microreactor studies demonstrate that surface roughness and acoustic frequency alter bubble nucleation and emulsification behavior [47]. These observations directly support spatially localized cavitation mechanisms. The subsequent proposition—that such spatial heterogeneity generates corresponding heterogeneity in interfacial coverage or dried coating structure—remains a mechanistic interpretation that requires direct verification in coating-forming formulations. Cavitation imaging or dosimetry therefore establishes how acoustic energy acts in the reactor but does not by itself demonstrate adhesion, transport control, or preservation after deposition.
This spatial structure is a major source of both opportunity and variability. Near a probe tip, the liquid experiences intense cavitation, high shear, and rapid heating; farther away, acoustic exposure may be weaker and less reproducible. In viscous biopolymer matrices, acoustic attenuation and damped bubble collapse can reduce emulsification efficiency. In protein-rich systems, localized high energy can simultaneously disrupt aggregates and promote reaggregation after unfolding. Consequently, mechanistic claims should not rely on droplet size alone. At minimum, emulsion studies should report acoustic dose, reactor geometry, temperature profile, and formulation context; stronger mechanistic claims require cavitation-related indicators such as calorimetry, hydrophone mapping, sonochemiluminescence, radical probes, KI dosimetry, or high-speed visualization [31], [32], [41], [42], [44], [45], [46], [47]. Without these descriptors, ultrasound remains a black-box homogenization step rather than a controlled interfacial engineering field.
2.3. Biopolymer remodeling: protein unfolding, polysaccharide scission, and complex co-assembly
Natural emulsifiers are actively remodeled during sonication. Proteins can undergo aggregate disruption, partial unfolding, hydrophobic exposure, sulfhydryl exposure, secondary-structure rearrangement, and changes in solubility or adsorption kinetics [48], [49]. These modifications may improve emulsifying performance by increasing molecular flexibility and exposing interfacial anchoring sites. Ultrasound-treated insoluble potato protein isolates, for example, showed improved structural, rheological, emulsifying, and gelling properties under appropriate high-intensity conditions [49]. These findings indicate that ultrasound can convert poorly dispersible plant proteins into more functional emulsifiers, but only within a controlled energy window. These molecular responses constitute the biopolymer-remodeling domain of Fig. 2 and explain why the effect of ultrasound cannot be represented by terminal droplet size alone.
Polysaccharides and protein-polysaccharide complexes provide a different mode of interfacial control. Ultrasound can induce chain scission, conformational loosening, viscosity reduction, and improved dispersion in polysaccharide-rich systems [50]. These effects are beneficial when they improve flowability, sprayability, or complex dispersion, but detrimental when they weaken the film-forming network required after drying. In protein-polysaccharide emulsions, ultrasound can alter complex size, interfacial adsorption, steric stabilization, and continuous-phase rheology. Soy protein isolate–pectin complexes and soy protein isolate-pectin emulsions illustrate how acoustic treatment can tune particle dispersion, droplet size, microstructure, protein adsorption, and emulsion stability [28], [51]. Pea protein isolate-xylan conjugate-stabilized nanoemulsions further show that ultrasound homogenization can improve the structure and stability of plant-protein/polysaccharide interfacial systems [52].
Complex co-assembly extends this mechanism beyond simple protein unfolding or polysaccharide scission. Protein-polysaccharide systems can form multilayer-like interfacial structures through electrostatic interaction, steric stabilization, or competitive adsorption, depending on pH, ionic strength, complexation state, and acoustic energy [28], [51], [52]. Protein-polyphenol systems and Pickering systems introduce additional routes in which moderate ultrasound can expose binding domains or promote particle-mediated droplet armoring. Table 1 [25], [28], [49], [50], [51], [53], [54], [55], [56], [57], [58], [59], [60] summarizes these cavitation-guided remodeling modes, while Table S1 [24], [34], [35], [36], [37], [49], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64] expands the material decision space by linking food-grade emulsifiers and bioactive phases to ultrasound response, interfacial role, coating scenario, failure mode, and readiness. Together, these tables establish that material selection and acoustic dosing must be optimized as a coupled design problem.
Table 1.
Cavitation-guided remodeling modes of natural emulsifiers and their coating relevance.
| Remodeling mode | Typical interface unit | Cavitation trigger | Interfacial outcome | Coating function | Failure threshold | Refs. |
|---|---|---|---|---|---|---|
| Conformational exposure | Soy protein; potato protein | Shear; microstreaming | Hydrophobic/R-SH exposure; faster adsorption | Compact oil-water interface | Aggregation; oxidation | [49], [51], [53] |
| Complex dispersion | Soy protein-pectin | Moderate HIU power | Smaller complexes; higher solubility | Stable coating-forming emulsion | Reaggregation at excess power | [28], [51] |
| Interfacial co-assembly | Protein–pectin complexes | pH-dependent cavitation response | Higher adsorbed protein; lower tension | Multilayer-like barrier | Competitive adsorption | [28], [54] |
| Chain tuning | Polysaccharide-rich matrices | Mechanical scission | Lower viscosity; better flowability | Sprayable coating solution | Weak film network | [25], [50] |
| Antioxidant skin | Protein-polyphenol complexes | Exposure of binding sites | Higher flavonoid loading; radical buffering | Oxidation-resistant droplet interface | Binding loss at high power | [55] |
| Particle armoring | Chickpea/WPI/quinoa particles | Nanoparticle fragmentation | Dense interfacial particle layer | Pickering stability; low surfactant | Particle reaggregation | [56], [57], [58] |
| Upcycled composite interface | Seed protein-chitosan | Ultrasound-assisted phosphorylation | Higher solubility; stronger charge | Biopolymer Pickering coating | Added modification complexity | [59] |
| Film-forming protein matrix | Pea protein isolate | Protein unfolding | Denser film network; altered optics | Edible film/coating precursor | Brittleness; opacity | [60] |
Note: HIU, high-intensity ultrasound; WPI, whey protein isolate. The table is organized by remodeling mode rather than by material name, so that the review can extract transferable design rules from heterogeneous studies.
2.4. Processing window: interface reinforcement versus molecular damage
Ultrasound-assisted emulsification operates within a formulation- and reactor-specific processing window. At insufficient acoustic input, droplet disruption remains incomplete, interfacial area is limited, and stabilizer adsorption may be too slow to prevent creaming or coalescence. Moderate treatment can reduce droplet size, improve emulsifier dispersion, expose interfacial anchoring groups, and strengthen interfacial films [28], [38], [39], [51]. Beyond this window, however, further energy input can become counterproductive through protein over-unfolding and aggregation, polysaccharide depolymerization, volatile loss, or lipid oxidation under intensified thermal and radical exposure [48], [49], [50]. This non-monotonic response has been demonstrated experimentally in soybean protein isolate-lecithin emulsions, in which 150 W for 24 min provided the best emulsifying performance and stability among the tested conditions, whereas treatment at 450 W increased particle size and reduced emulsion stability in association with protein aggregation [65]. The optimum therefore cannot be defined by droplet size alone.
The position and width of this window depend on formulation chemistry, rheology, dispersed-phase volatility, acoustic delivery, reactor geometry, residence time, and temperature. Globular proteins, insoluble protein aggregates, protein-polysaccharide complexes, and particle-stabilized interfaces consequently require different acoustic conditions. A treatment that improves dispersion in a low-viscosity nanoemulsion may weaken a polysaccharide-rich film-forming matrix or accelerate degradation of a volatile active phase. Processing optimization should therefore combine emulsion-level readouts with indicators of molecular integrity and downstream coating function.
Direct comparisons with alternative homogenization routes reinforce this point. Combined ultrasound-high-pressure homogenization (HPH) reduced the energy required to obtain small, stable droplets, with ultrasound preceding HPH lowering energy demand by up to approximately sixfold in the formulation examined [66]. In an energy-normalized comparison over 47∼175 MJ m−3, particle size depended not only on total energy but also on its partition between ultrasound and HPH; minimum size required at least half of the total energy to be supplied by HPH [67]. Peanut-protein emulsions treated by ultrasound at 300 W for 20 min retained larger initial particles than those treated by HPH at 300 or 500 bar, yet exhibited greater interfacial protein accumulation, viscosity, and resistance to physical destabilization [68]. Rotor-stator pre-homogenization followed by sonication likewise reproduced selected structural attributes of pilot-scale HPH emulsions, but the required conditions depended on emulsifier type [69]. These study-matched contrasts are visualized in Fig. 3A and assembled quantitatively in Table 2 [33], [38], [39], [43], [66], [67], [68], [69], [70]. Together, they show why process selection cannot be reduced to a ranking by terminal droplet size: ultrasound, HPH, hybrid US-HPH processing, and rotor-stator-assisted sonication differ in energy partitioning, interfacial structure, and physical stability, with the apparent advantage of each route remaining formulation- and reactor-dependent. Table 2 therefore defines the quantitative boundary for the processing-window concept in Fig. 2, whereas Fig. 3A provides the corresponding study-level visual comparison.
Fig. 3.
Literature-grounded processing benchmarks and architecture-specific evidence coverage for ultrasound-assisted emulsion coating systems. (A) Study-matched processing comparisons based on reported droplet-size, interfacial, and energy benchmarks for ultrasound, high-pressure homogenization (HPH)/microfluidization, rotor-stator-assisted sonication, and combined ultrasound-HPH processing [38], [39], [66], [67], [68], [69]. Values are source-specific observations and are not pooled across formulations. (B) Representative continuous-processing benchmarks relating throughput to reported droplet size and polydispersity index (PDI) in ultrasonic microreactor and temperature-controlled tandem systems [43], [70]. Numerical values represent published study-specific operating points rather than recommended or transferable operating ranges. (C) Architecture-specific evidence coverage across emulsion characterization, coating functionality, produce validation, and scale-up/manufacturing assessment, synthesized from the study-level evidence summarized in Table S2. Evidence categories indicate the breadth and continuity of validation rather than effect magnitude or a formal meta-analytic score.
Table 2.
Quantitative benchmarks and direct process comparisons for ultrasound-assisted food emulsion formation.
| System | Ultrasound/process condition | Comparator or scale-up route | Quantitative outcome | Throughput/scale-up evidence | Process interpretation | Evidence boundary | Refs. |
|---|---|---|---|---|---|---|---|
| Flaxseed-oil O/W emulsion | 20 ∼ 24 kHz; batch and focused flow-through US | Microfluidization, 100 MPa | Mean droplet diameter down to 135 ± 5 nm | Batch and flow-through configurations | US can approach microfluidization-scale droplets under matched formulation conditions | Emulsion formation; no coating validation | [39] |
| Model food-grade O/W nanoemulsion | Ultrasound emulsification; overpressure tested to 400 kPa | High-shear homogenization | Minimum mean diameter ∼ 40 nm under optimized formulation/device conditions | Batch comparison | Minimum size depends jointly on droplet deformability, formulation and equipment geometry | Formulation-specific benchmark; not a universal optimum | [38] |
| Sunflower-oil nanoemulsion | HPH, 20 ∼ 100 MPa, combined with 20 or 60 s US | HPH → US; US → HPH; individual HPH or US | Combined treatment reduced mean size and improved stability at lower energy input; up to sixfold energy reduction | Sequential hybrid processing | US → HPH gave the highest energy efficiency | Emulsion/process comparison only | [66] |
| Sunflower-oil nanoemulsions stabilized by Tween 80 or WPI | Combined US-HPH; 47 ∼ 175 MJ m−3 | Energy-normalized HPH/US allocation | Tween 80: 140 ∼ 190 nm at 175 MJ m−3; WPI: ∼ 220 ∼ 250 nm at 120 MJ m−3 | Energy-partition comparison | ≥50% of total energy supplied by HPH was required for minimum size in tested systems | Values depend on oil/emulsifier content | [67] |
| Peanut-protein O/W emulsion | HIU: 300 W, 20 min; 4% protein; 30% oil | HPH at 300 or 500 bar | d4,3: 2.56 ± 0.22 μm (HIU), 1.83 ± 0.25 μm (300 bar), 1.39 ± 0.23 μm (500 bar); |ζ|: 29.92 ± 0.88, 26.70 ± 2.43 and 31.47 ± 1.60 mV | Batch comparison | Smaller droplets did not alone determine physical performance; HIU produced distinct interfacial/rheological behavior | Protein-stabilized model emulsion; no coating | [68] |
| Lecithin- and WPI-stabilized emulsions | Rotor–stator, 5000 ∼ 10000 rpm, 5 min; US, 2 ∼ 10 min | Pilot HPH | Lecithin: 10000 rpm + 10 min US reproduced pilot-scale structure; WPI required 5000 rpm to limit aggregation | Laboratory scale-down of pilot process | Scale-down conditions depend on emulsifier chemistry; excessive shear can denature/aggregate protein | Scale equivalence of emulsion structure, not produce efficacy | [69] |
| Translucent O/W nanoemulsion | High-amplitude US; optimum ∼ 90 μm peak-to-peak | Barbell-horn scale-up | Product quality maintained after direct 10-fold scale-up; additional fivefold scale-up considered feasible | Direct 10× scale-up | Acoustic amplitude, rather than vessel volume alone, must be preserved during scale-up | Engineering demonstration; no edible-coating line | [33] |
| Soybean-oil nanoemulsion | Ultrasonic microreactor, 20 kHz; optimum 50 W, 114 s | Numbering-up; enlarged reactor | Optimum: 96.4 nm, PDI 0.195; 0.66 → 2.64 mL min−1 gave ∼ 120 nm droplets; 160 W enlarged reactor: 120.5 nm, PDI 0.219 | 0.66 → 20 mL min−1 | Numbering-up and reactor enlargement can raise throughput while retaining droplet scale | Continuous emulsion manufacture; no coating application | [43] |
| Soybean-oil nanoemulsion | Tandem continuous unit, 21.1 kHz; 11 ∼ 24 W; 45 ∼ 65 °C; 0.5 ∼ 6 mL min−1 | Temperature-controlled two-stage US | At 20/22 W, 55 °C and 1/2 mL min−1: 42/55 nm; PDI 0.138/0.152 | Continuous 0.5 ∼ 6 mL min−1; stable for 220 d at 4 or 25 °C | Coupled power, temperature and residence time permit continuous ∼ 50 nm production | Reactor/emulsion evidence; no postharvest coating line | [70] |
Note: Values are representative study-specific benchmarks, not recommended operating ranges. Direct comparison across studies requires normalization for formulation, reactor geometry, treatment volume, delivered acoustic power, temperature and residence time. US, ultrasound; HIU, high-intensity ultrasound; HPH, high-pressure homogenization; WPI, whey protein isolate; PDI, polydispersity index; ζ, zeta potential; O/W, oil-in-water.
A defensible processing window therefore requires evidence at four levels. First, acoustic conditions should report frequency, amplitude, delivered power, power or energy density, pulse mode, treatment volume, temperature, and reactor geometry. Second, claims involving cavitation should be supported, where appropriate, by calorimetry, hydrophone or passive acoustic measurements, chemical dosimetry, sonochemiluminescence, or high-speed imaging [31], [32], [41], [42], [44], [45], [46], [47]. Third, emulsion performance should be evaluated beyond terminal droplet size using polydispersity, ζ-potential, interfacial properties, rheology, physical stability, and oxidative stability. Fourth, coating relevance requires evidence for wetting, drying, thickness, adhesion, release, and barrier performance. Table 3 [26], [29], [31], [32], [34], [37], [39], [43], [64], [66], [67], [70] summarizes the corresponding reporting requirements and distinguishes minimum descriptors from evidence needed for mechanistic or translational claims. Collectively, these criteria shift process optimization from empirical droplet miniaturization toward identification of an acoustic window that preserves both interfacial functionality and coating-forming capacity.
Table 3.
Minimum reporting requirements and representative quantitative benchmarks for ultrasound-assisted emulsion-coating studies.
| Evidence domain | Descriptor | Recommended reporting | Representative study-specific benchmark / reported value | Evidence role | Tier | Risk if omitted |
|---|---|---|---|---|---|---|
| Acoustic source | Device; frequency; amplitude | Probe/bath/flow reactor; kHz; amplitude (%) or tip displacement | 20 ∼ 24 kHz [39]; 20 kHz microreactor [43]; 21.1 kHz tandem reactor [70] | Defines acoustic/cavitation regime | Minimum | Treatments become non-comparable |
| Acoustic power | Applied and delivered power | Generator setting plus calorimetric/otherwise measured delivered power, W | Applied power: 40 ∼ 160 W in USMR scale-up [43]; 11 ∼ 24 W in tandem reactor [70]; delivered calorimetric power often NR in coating studies | Separates electrical input from energy delivered to liquid | Minimum | Nominal-power bias |
| Energy dose | Power density; energy density | W mL−1 or W kg−1; kJ mL−1, kJ g−1 or MJ m−3 | 47 ∼ 175 MJ m−3 in controlled US-HPH comparisons [67] | Normalizes treatment across scale/time | Minimum | False dose-response relationships |
| Exposure mode | Time; pulse regime; residence time | Total time; on/off cycle; duty %; residence-time distribution | 20 or 60 s US in hybrid US-HPH [66]; 114 s optimum in USMR [43]; continuous 0.5 ∼ 6 mL min−1 in tandem reactor [70] | Controls cumulative exposure and heating | Minimum | Unresolved dose/history effects |
| Thermal history | Initial/final temperature; cooling | T0/Tf; ΔT(t); cooling mode; isothermal control | 45 ∼ 65 °C examined and 55 °C selected in tandem processing [70] | Separates cavitation-mediated from thermal effects | Minimum | Thermal confounding |
| Reactor geometry | Probe/vessel/channel geometry | Probe diameter; immersion depth; vessel dimensions; liquid height; working volume; channel dimensions | Batch/focused flow-through [39]; continuous microreactor [43]; tandem flow unit [70] | Determines spatial acoustic field | Minimum | Poor reproducibility |
| Cavitation field | Cavitation intensity/distribution | Hydrophone/PCD; calorimetry; SCL; KI/coumarin dosimetry; high-speed imaging | Reactor-specific methods established [26], [31], [32]; quantitative cavitation maps remain NR in most edible-coating studies | Tests whether the invoked cavitation field was measured | Advanced | Black-box mechanism |
| Formulation state | Oil/emulsifier fraction; viscosity; pH; ionic strength | Composition, rheology and physicochemical state before US | Oil 10 ∼ 20% and emulsifier 1 ∼ 3% across energy-response design [67] | Defines formulation-dependent cavitation threshold | Minimum | Confounded process comparison |
| Emulsion output | Size; PDI; ζ-potential; stability | nm/μm; PDI; mV; creaming/TSI; oxidation where relevant | 96.4 nm/PDI 0.195 [43]; 42 ∼ 55 nm/PDI 0.138 ∼ 0.152 [70]; 162.1 nm/PDI 0.193 for a coating-forming NE [34] | Confirms interfacial state after processing | Minimum | Size-only or stability-only claim |
| Coating formation | Wetting; thickness; morphology; adhesion | Contact angle; μm; CLSM/SEM; adhesion or coverage metric | Strawberry coating ∼ 20 ∼ 25 μm [37]; tomato coating ∼ 5 μm [64]; coating/film morphology [29] | Confirms droplet-to-coating translation | Advanced | Stable emulsion assumed to equal functional coating |
| Transport/release | WVTR/WVP; O2/CO2 transfer; active release | Standardized barrier units; release curve/kinetic coefficient | Film WVP 0.050 g m−1 kPa−1 h−1[29]; OEO release 6.2% at 1 h → 74.5% at 96 h; Dapp = 9.17 × 10−7 cm2 s−1[64] | Links coating structure to preservation exposure | Advanced | Mechanism inferred from endpoint quality alone |
| Produce validation | Storage condition; comparator; preservation endpoint | Commodity; T/RH/time; untreated/matrix/active/process controls; decay, CFU, water loss, firmness, respiration as appropriate | 6-d strawberry test: bacteria 2.544 and molds 1.958 log CFU g−1[34]; 14-d EEO-NE strawberry test: decay 10.21% vs 93.12%, weight loss 8.12% vs 48.87%, firmness 0.85 vs 0.46 N [37] | Establishes commodity-level function | Minimum for preservation claims | Efficacy cannot be attributed to coating/process |
| Scale-up | Throughput; residence-time variability; yield | mL min−1 or kg h−1; RTD; batch/run CV; product yield | 0.66 → 20 mL min−1 while retaining ∼ 120 nm droplets [43]; direct 10-fold acoustic scale-up [33] | Tests process reproducibility at increased throughput | Advanced | Laboratory optimization presented as manufacturing readiness |
| Hygiene/safety | Fouling; erosion; migration; sensory response | Cleaning interval/CIP; metal release; migration/exposure; sensory panel | Quantitative coating-line CIP and migration benchmarks: NR; transducer erosion is experimentally documented in food sonoprocessing | Defines use-related constraints | Advanced | Unresolved regulatory/consumer risk |
Note: Reported numerical values are representative study-specific observations, not recommended process windows or acceptance limits. NR, not reported; PCD, passive cavitation detection; SCL, sonochemiluminescence; KI, potassium iodide; USMR, ultrasonic microreactor; HPH, high-pressure homogenization; PDI, polydispersity index; ζ, zeta potential; WVP, water-vapor permeability; WVTR, water-vapor transmission rate; RTD, residence-time distribution; CV, coefficient of variation; CIP, cleaning in place.
3. Emulsion architectures matched to coating functions
Emulsion architecture determines how acoustic processing is retained as a functional interface rather than merely as a reduction in droplet size. For edible coatings, architecture selection begins with the preservation task: rapid surface delivery, prolonged retention, barrier–release coordination, co-delivery, or extended residence. These functions require different distributions of interfacial material, droplet mobility, internal compartmentalization, and compatibility with the film-forming phase. Architecture also changes the susceptibility of a formulation to sonication. A field suitable for primary nanoemulsion formation may damage particle shells, disrupt electrostatic layers, rupture internal droplets, or weaken a gel network. Architecture and acoustic treatment are therefore coupled design variables.
Building on the cavitation-guided remodeling modes in Table 1 and the material options in Table S1, Fig. 3 integrates three complementary evidence dimensions that recur across the review. Fig. 3A compares selected study-matched processing routes without pooling heterogeneous formulations into a technology ranking. Fig. 3B extends the processing perspective to continuous systems, relating throughput to reported droplet size and PDI using representative published operating points. Fig. 3C then shifts from process performance to architecture-specific evidence coverage, comparing how far nanoemulsion, Pickering, multilayer, multiple-emulsion, and emulgel systems have progressed from emulsion characterization to coating functionality, produce validation, and manufacturing-relevant assessment. Table 4 [25], [29], [34], [35], [36], [56], [57], [58], [71], [72], [73] retains the architecture-specific selection logic, whereas the study-level basis for Fig. 3C is summarized in Table S2 [29], [34], [35], [36], [37], [61], [62], [63], [64], [71], [72]. Accordingly, Fig. 3A is discussed with the processing-window evidence in Section 2.4, Fig. 3C provides the immediate evidence context for the architecture discussion below, and Fig. 3B is revisited in Section 6.2 when continuous processing and scale-up are considered.
Table 4.
Architecture-selection map for ultrasound-assisted emulsion coatings.
| Architecture | Acoustic-processing feature | Coating task | Target surface rationale | Highest demonstrated evidence tier | Best-supported outcome | Architecture-specific limit | Refs. |
|---|---|---|---|---|---|---|---|
| EO nanoemulsion | Droplet miniaturization; narrow PDI | Fast antimicrobial contact | Soft berries; fungal-prone cuticle | E3 | Lower decay; firmness retention | Aroma burst; volatility | [29], [34] |
| Pullulan-EO nanoemulsion | Ultrasound-assisted EO dispersion | Transparent active coating | Strawberry; high respiration | E3 | Smaller droplets; lower mass/firmness loss | Ambient-storage bias | [34] |
| Nanoemulsion film precursor | Droplet retention in film matrix | Active film/coating base | Packaging-to-coating translation | E2 | Better dispersion; film activity | Film ≠ real fruit surface | [29] |
| Alginate-EO nanoemulsion | Nanometric EO droplets in hydrocolloid | Antibiofilm coating | Tomato; biofilm risk | E3 | Lower pathogens; lower weight loss | pH/matrix dependence | [35] |
| Citral nanoemulsion coating | Lipophilic aldehyde delivery | Fresh-cut pathogen control | Pineapple; wounded surface | E3 | Reduced pathogens; color retention | High-dose texture loss | [36] |
| Pickering emulsion film | Particle-armored oil droplets | Delayed coalescence; oil retention | Long-storage coatings | E2 | Better film stability; slower oil migration | Opacity; rough mouthfeel | [71], [72] |
| Protein-particle Pickering emulsion | Ultrasound-assisted particle dispersion and interfacial assembly | Low-surfactant interface | Clean-label coatings | E1 | Higher freeze-thaw/thermal stability | Particle aggregation | [56], [57], [58] |
| Emulgel/HIPE-like system | Cavitation-assisted dense droplet packing | Residence; sustained release | Waxy or uneven surfaces | E1 | Higher viscosity; longer retention | Tackiness; spreadability | [73] |
| Multiple emulsion | Two-step sonication possible | Compartmentalized delivery | Sensitive actives | E1 | Encapsulation potential | Leakage; scale-up fragility | [25] |
Note: EO, essential oil; PDI, polydispersity index; HIPE, high-internal-phase emulsion. Evidence levels: E1, emulsion stability only; E2, film/coating property validation; E3, produce-level validation; E4, mechanism plus produce validation; E5, scale-up/safety validation. Most current food-coating studies remain at E2-E3; E4-E5 evidence is still scarce.
3.1. Nanoemulsions: coverage, transparency, and rapid bioactive contact
Nanoemulsions are the most established architecture for thin coatings that require broad surface coverage, limited visible scattering, and prompt delivery of hydrophobic actives. Their small droplets and large interfacial area facilitate dispersion of essential oils and plant-derived compounds within aqueous film-forming matrices. Ultrasound can assist this function by creating new interfacial area, accelerating emulsifier adsorption, and reducing local enrichment of the oil phase [24], [25], [34], [35], [36], [37], [61], [62], [63], [64]. The benefit is most evident on visually sensitive commodities and exposed fresh-cut surfaces, where rapid antimicrobial contact is more important than prolonged sequestration of the active phase.
Produce studies support this functional role across several matrices. Cinnamaldehyde nanoemulsions incorporated into alginate coatings suppressed microbial deterioration while maintaining mushroom firmness, color, and antioxidant properties [74]. Thyme essential-oil nanoemulsions in chitosan/pullulan coatings improved strawberry preservation [75], whereas chitosan-stabilized lemon essential-oil nanoemulsions limited black mold and quality loss in table grapes [76]. Xanthan gum coatings containing pomelo peel extract nanoemulsions extended the same principle to plant-extract delivery and by-product valorization [77]. Together with the ultrasound-assisted coating systems summarized in Table S2 [22], [24], [29], [34], [35], [36], [37], [61], [62], [63], [64], [71], [72], these studies show that nanoemulsions currently possess the broadest produce-level evidence among the architectures considered here, although not every coating study resolves the independent contribution of acoustic treatment.
Their high interfacial area also creates liabilities. Rapid partitioning into the coating matrix or headspace can produce an early antimicrobial pulse followed by premature depletion. Oxidation and aroma exposure may increase when volatile oils are highly dispersed, and further droplet miniaturization may add little once surface coverage is no longer limiting. The relevant design target is therefore a stable droplet distribution that provides adequate coverage while retaining sufficient active phase over the intended storage period. Nanoemulsions are strongest when optical quality, rapid contact, and straightforward processing dominate; they are less compelling when prolonged retention or delayed release is the primary requirement.
3.2. Pickering emulsions: interfacial armor and active retention
Pickering emulsions replace or supplement molecular emulsifiers with food-grade particles that become anchored at the oil-water interface. The resulting interfacial armor can suppress coalescence, constrain droplet deformation, and reduce migration of the dispersed phase. These features are attractive for coatings intended to retain volatile actives, limit oil separation, or reduce dependence on low-molecular-weight surfactants. Their advantage does not arise from particle stabilization alone, however. The adsorbed particles must remain compatible with the continuous matrix and must not generate unacceptable opacity, roughness, or solids loading after application.
Sonication serves several functions in Pickering systems: dispersing particle aggregates, generating oil-water area, and increasing collision between particles and newly formed interfaces. These processes must occur without destroying particle morphology or shifting surface properties beyond the range required for irreversible adsorption. Excessive treatment can produce fine fragments with inadequate anchoring energy, promote continuous-phase aggregation, or alter rheology to the point that coating application becomes difficult. The useful acoustic window is consequently defined by interfacial coverage and particle integrity rather than by the lowest attainable droplet diameter.
Direct coating evidence is less extensive than for nanoemulsions but is becoming more persuasive. Chitosan-particle-stabilized Pickering emulsions containing thyme essential oil combined shear-thinning behavior, gel-like structure, sustained volatile release, storage stability, and strawberry preservation [78]. Nanocellulose microgel particles have also stabilized lemon essential oil for antimicrobial packaging applications [79]. Supporting film studies show that zein-pectin nanoparticle-stabilized corn germ oil-oregano oil emulsions can improve ionically crosslinked konjac glucomannan films [80], while cellulose nanocrystal-stabilized Thymus vulgaris essential-oil emulsions can enhance water-vapor and ultraviolet barriers and preserve fish fillets when incorporated into starch/polyvinyl alcohol films [81].
The distinction between direct coating evidence and film-based support remains important. Film studies demonstrate interfacial retention and matrix reinforcement, but do not resolve deposition, appearance, or performance on metabolically active produce. Pickering emulsions are therefore most defensible for applications in which retained delivery and interfacial robustness outweigh the need for near-complete transparency. Their practical value will depend on controlling particle loading, rheology, dried-layer appearance, and the fate of the particle shell under commodity-specific storage conditions.
3.3. Multilayer emulsions: interfacial stratification and barrier–release coordination
Multilayer emulsions introduce sequentially assembled protein, polysaccharide, or complex layers around a primary droplet. Interfacial stratification can increase steric and electrostatic protection, reduce exposure of oxidation-sensitive oils, and alter partitioning between the droplet, coating matrix, food surface, and headspace. This architecture is most relevant when a primary interface provides insufficient stability or when active retention and release must be adjusted without substantially increasing dispersed-phase loading.
Its compatibility with ultrasound depends strongly on the sequence of processing. Sonication before secondary-layer deposition can improve primary emulsification and disperse the components required for subsequent assembly. The protein–polysaccharide systems discussed in Section 2 show that acoustic treatment can modify complex size, adsorption, and steric stabilization [28], [51], [52]. Once a multilayer shell has formed, however, further cavitation may disturb electrostatic association, change charge distribution, or remove weakly bound material. A whey protein isolate-chitosan hydrochloride system, in which a second layer improved the storage stability of thyme-oil nanoemulsions, illustrates the protective potential of sequential assembly [82].
The increased number of formulation variables narrows the operational margin. Layer order, pH, ionic strength, charge reversal, biopolymer ratio, and interfacial saturation all affect stability, while drying and contact with acidic or mineral-rich food surfaces can change the electrostatic environment. Consequently, evidence based only on droplet size and ζ-potential is insufficient to establish multilayer integrity. Interfacial composition, resistance to environmental stress, and release behavior are needed to distinguish a persistent shell from transient charge association.
Multilayer emulsions offer a rational route to barrier-release coordination, particularly for labile or highly volatile actives. Their current limitation is not conceptual plausibility but incomplete coating-level validation. Evidence remains scarce on whether the assembled layers survive incorporation into a film-forming matrix, deposition, water removal, and storage on real produce. Until those transitions are resolved, multilayer systems are better regarded as advanced interfacial carriers with coating potential than as a mature coating platform.
3.4. Multiple emulsions and emulgels: compartmentalization, retention, and sustained delivery
Multiple emulsions and emulgels address functions that simple oil-in-water droplets cannot readily provide. Water-in-oil-in-water emulsions can separate hydrophilic and hydrophobic compounds within the same carrier, enabling co-delivery or staged availability. A double emulsion containing brassinolide and cinnamon essential oil delayed broccoli senescence through combined delivery of a plant regulator and a volatile antimicrobial phase [83]. This example demonstrates the functional value of compartmentalization, although broader evidence is required to determine whether comparable structures remain stable during ultrasound processing, coating incorporation, and storage.
The main acoustic constraint is preservation of the internal droplets. Energy sufficient to refine the external emulsion may rupture internal compartments, increase solute leakage, or convert a multiple emulsion into a simpler structure. Sequential processing is therefore preferable where the primary and secondary emulsification steps require different energy inputs. Stability must be judged from encapsulation retention and internal-droplet integrity, rather than from the external droplet diameter alone.
Emulgels solve a different problem by immobilizing droplets within a structured continuous phase. Their yield stress and viscoelasticity can reduce drainage and prolong residence on rough, inclined, porous, or poorly wettable surfaces. Droplet confinement may also slow volatile transport and provide a more persistent reservoir than a low-viscosity coating solution. Ultrasound can improve initial dispersion, but excessive exposure may lower molecular weight, disrupt junction zones, or reduce the rheological structure on which residence depends. The relevant balance is therefore between efficient droplet incorporation and retention of the gel network.
These architectures incur higher processing and application costs. Multiple emulsions are sensitive to osmotic imbalance, internal leakage, and sequential-emulsification conditions. Emulgels may be difficult to atomize and can produce thick, tacky, opaque, or slowly drying layers. Direct produce-level evidence remains sparse, particularly for emulgels prepared using ultrasound. Their use is most justified when compartmentalized delivery or extended surface residence provides a clear functional gain that cannot be achieved with a nanoemulsion or a particle-stabilized system.
Across the architectures, functional sophistication and evidence maturity do not increase in parallel. Nanoemulsions currently have the strongest produce-level support, whereas Pickering, multilayer, multiple-emulsion, and emulgel systems progressively rely more heavily on formulation- or film-level evidence. Table 4 retains the task-based architecture selection criteria, whereas Fig. 3C summarizes architecture-specific evidence coverage across emulsion characterization, coating functionality, produce validation, and scale-up/manufacturing assessment. Based on the study-level evidence compiled in Table S2, this synthesis reflects the breadth and continuity of validation rather than effect magnitude or a formal meta-analytic score. Architecture selection can therefore identify a plausible coating function, but cannot establish that the function survives deposition, drying, and storage on real produce; that transition is examined in Section 4.
4. Droplet-to-coating translation on food surfaces
A stable emulsion is only a coating precursor. Its preservation value emerges only after droplets are deposited, spread, immobilized, dried, retained, and coupled to mass transfer and active migration on a heterogeneous food surface. This distinction is particularly important for ultrasound-assisted emulsion coatings because acoustic cavitation can generate small droplets, narrow size distributions, and favorable ζ-potential values, yet these colloidal descriptors do not prove that the formulation will form a continuous, adherent, transparent, breathable, and acceptable in sensory terms. Postharvest commodities such as fruits, vegetables, and edible mushrooms undergo water loss, discoloration, microbial susceptibility, and texture deterioration after harvest, and coatings are useful only when they modify these surface-driven processes under real storage conditions [84]. Section 3 established how emulsion architectures should be matched to coating functions rather than selected by droplet size alone. The present section extends that logic from emulsion architecture to coating translation.
Droplet-to-coating translation involves four coupled interfaces: the oil-water interface around droplets, the droplet-biopolymer matrix interface during drying, the coating-food surface interface during adhesion, and the coating-environment interface during storage. Fig. 4 summarizes this conversion from spreading, drying, droplet packing, film formation, adhesion, gas/water transport, active migration, and surface failure to produce-level responses. The critical point is that an emulsion can remain physically stable in a vial but fail as a coating if it drains from the cuticle, dries into a cracked layer, bleeds oil, whitens the fruit surface, releases essential oils too rapidly, blocks gas exchange excessively, or causes sensory rejection. These coating-level endpoints provide the mechanistic foundation for the evidence matrix developed later in Section 6.1.
Fig. 4.
Droplet-to-coating translation and preservation responses at the produce surface. Spreading, drying, droplet immobilization, and matrix consolidation determine water, gas, and active-compound transport across the coating-produce interface. These fluxes influence microbial growth, redox balance, respiration, water loss, texture, and sensory quality. Arrows indicate expected responses under a suitably permeable coating. ROS, reactive oxygen species.
4.1. Film formation: droplet packing, drying, coalescence arrest, and matrix evolution
Film formation begins when the coating liquid is deposited on the food surface and water evaporation concentrates biopolymers, emulsifiers, droplets, salts, sugars, and active compounds into a confined interfacial layer. During this transition, droplets become increasingly crowded. If the continuous matrix reaches sufficient viscosity and mechanical integrity before droplet collision leads to coalescence, the emulsion is converted into a stable droplet-filled coating. If matrix consolidation is too slow, droplets may cream, fuse, migrate to the air-facing surface, or generate oil-rich domains. Thus, coating formation is governed by a kinetic competition between droplet mobility and matrix arrest.
This process cannot be inferred from emulsion stability alone. Sodium-alginate coating studies showed that plasticizers, vegetable oils, and surfactants change coating solution properties, surface wetting, and final coating behavior, indicating that droplet incorporation must be evaluated together with matrix evolution rather than as a separate colloidal event [85]. Work on edible films and coatings prepared from Anacardium occidentale polysaccharide further established that surface tension, spreading coefficient, adhesion work, cohesion work, and permeability are interdependent coating descriptors [86]. A uniform alginate coating on cantaloupe and strawberries also demonstrated that actual fruit surfaces reveal water-barrier behavior and coating uniformity that bulk emulsion stability tests cannot capture [87].
For emulsion-based coatings, droplets exert two opposing effects during drying. Moderate droplet loading can interrupt polymer packing, increase tortuosity, introduce hydrophobic domains, and improve water resistance or active retention. Excessive droplet loading can weaken matrix cohesion, create discontinuities, reduce tensile integrity, and promote oil exudation. Essential-oil-loaded emulsion-based edible films prepared from soy protein isolate–gum acacia conjugates illustrate how emulsion structure, droplet distribution, matrix morphology, water vapor permeability, mechanical behavior, and release kinetics are coupled [88]. Cassava starch-carnauba wax coatings for fresh-cut apples similarly show that hydrophobic wax domains improve barrier performance only when the lipid phase remains compatible with the starch network and the dried coating remains mechanically coherent [89].
Ultrasound can improve this conversion when cavitation creates droplets small enough to distribute uniformly and interfaces strong enough to resist coalescence during drying. However, overprocessing may generate the opposite result: excessive droplet surface area can demand more stabilizer than the matrix can supply, while acoustic damage to proteins or polysaccharides may weaken the network required for coating formation. Coating-relevant sonication should therefore be judged by dried-layer morphology, thickness uniformity, oil retention, mechanical continuity, and active-release behavior, not only by droplet size or PDI.
4.2. Surface compatibility: wetting, spreading, adhesion, and cuticle interaction
Food surfaces are not passive substrates. Fruit and vegetable cuticles contain wax crystals, pores, lenticels, microcracks, trichomes, epidermal curvature, and cultivar-dependent surface energy. Fresh-cut surfaces introduce another class of interfaces, where exposed cell-wall polysaccharides and tissue exudates increase hydrophilicity but also accelerate microbial attachment and enzymatic deterioration. These features determine whether a coating droplet spreads into a continuous layer or retracts into islands. A formulation with excellent emulsion stability can fail immediately if its surface tension is too high, its viscosity prevents leveling, or its dispersed oil phase disrupts adhesion to the cuticle.
Wettability studies on starch-gellan coatings showed that essential oils and surfactants substantially alter contact angle and spreading on fruit surfaces, confirming that antimicrobial enrichment and surface compatibility must be optimized together [90]. Oregano essential oil nanoemulsion coatings for tomatoes further demonstrated the relevance of coating-surface descriptors, including contact angle, work of adhesion, work of cohesion, and spreading coefficient, alongside preservation outcomes [64]. These studies support a key design rule: droplet architecture should be evaluated at the food surface, not only in the storage vessel.
Application mode adds another translation layer. Dipping favors full immersion and high solution-surface contact, but may create thick coatings, runoff, and cross-contamination risks [91]. Spraying is more compatible with industrial processing, but depends on atomization, droplet impact, viscosity, surface drying, and repeated passes. Optimization of plantain epicarp spray-coating conditions showed that application parameters directly influence active edible coating performance, highlighting that coating effectiveness depends on deposition physics as much as on formulation composition [92].
Surface compatibility also explains why the same emulsion may perform differently across commodities. A coating optimized for a hydrophilic cut fruit surface may not spread on a waxy mango or tomato cuticle. A nanoemulsion that forms a transparent layer on strawberry may produce visible whitening on grapes if drying creates refractive-index mismatch or droplet aggregation. A Pickering emulsion that improves long-term stability may increase roughness or opacity on glossy fruit. Thus, architecture selection should be coupled to surface class: wounded hydrophilic tissues favor fast-spreading nanoemulsions; waxy intact cuticles require wetting and adhesion control; rough, porous, or vertical surfaces may benefit from higher-viscosity emulgels or particle-stabilized systems.
4.3. Barrier-release coupling: water vapor, O2/CO2 transfer, and active migration
The most important functional transition occurs when the dried emulsion coating becomes both a barrier and a delivery layer. These two roles are inseparable. A coating that slows water vapor transfer may also restrict O2 influx and CO2 efflux; a coating that retains essential oils may delay antimicrobial contact; a coating that releases actives rapidly may suppress microbes early but lose function before the end of storage. Therefore, barrier properties and release behavior should be evaluated as coupled outputs of droplet-matrix architecture rather than as independent endpoints.
Carnauba wax emulsion coatings incorporated with ginger essential oil and hydroxypropyl methylcellulose delayed papaya decay and quality loss, showing how hydrophobic droplets, waxy domains, and an edible polymer matrix can jointly regulate surface protection and postharvest responses [93]. Carboxymethyl chitosan coatings infused with linalool-loaded molten-globular β-lactoglobulin nanoparticles extended fresh-cut apple preservation, supporting the concept that active-loaded colloidal domains can function as localized reservoirs rather than freely dissolved antimicrobials [94]. Carvacrol emulsions improved microbiological and physicochemical properties of fresh-cut apples, further indicating that emulsion delivery must be interpreted through both antimicrobial efficacy and tissue-quality outcomes [95]. Nanochitosan emulsion coatings containing pomegranate peel extract extended apricot shelf life, illustrating the advantage of coupling nanoscale dispersion with antioxidant-rich plant extracts [96].
Barrier-release coupling must also be evaluated in film models because dried coating matrices often behave differently from liquid emulsions. Banana-starch films containing curcumin-loaded nanoemulsions provide a useful example of how nanoemulsified actives can be integrated into a polysaccharide network to generate active edible films with modified structure and functionality [97]. Natural-plant-compound co-assembly offers an additional conceptual basis for this section: non-covalent nanocomplex formation can improve biocompatibility, biodegradability, pharmacological efficacy, and bioavailability of plant small molecules [98]. For emulsion coatings, this concept should not be overextended as direct coating evidence, but it supports the broader principle that natural bioactives may require interfacial organization, not merely physical addition, to achieve controlled delivery.
For living commodities, the optimal coating is not the one with the lowest permeability or the slowest release, but the one that balances water retention, O2/CO2 exchange, and active migration for a specific commodity. Excessively strong barriers may increase anaerobic risk, whereas overly permeable coatings may fail to reduce transpiration or volatile loss. Active migration should therefore be described as flux through a structured coating rather than total active loading. Relevant variables include droplet size, interfacial shell strength, oil–polymer partitioning, matrix hydration, food-surface affinity, storage humidity, and temperature. Nanoemulsions generally favor early release and rapid microbial contact; Pickering and multilayer emulsions favor delayed release and better retention; multiple emulsions and emulgels may support compartmentalized or sustained delivery. This hierarchy is consistent with the architecture-selection logic in Table 4 and the transport pathways exhibited in Fig. 4.
4.4. Failure modes: oil bleeding, cracking, whitening, off-flavor, and sensory rejection
Failure modes provide a more stringent test of coating translation than emulsion stability. Oil bleeding indicates that the dispersed phase has not been immobilized within the drying matrix or that the interfacial layer cannot withstand droplet crowding and storage stress. Cracking reflects drying shrinkage, insufficient polymer flexibility, excessive solid loading, or poor adhesion to the food surface. Whitening and haze usually indicate droplet coarsening, phase separation, surface roughening, air voids, or refractive-index mismatch. Off-flavor and sensory rejection reveal that antimicrobial efficacy has exceeded the consumer-acceptable release window.
These endpoints are especially important for essential-oil emulsion coatings. Essential oils are attractive because of their antimicrobial and antioxidant activity, but their volatility, hydrophobicity, oxidation sensitivity, and strong aroma frequently limit direct application [99]. Chitosan–essential oil film and coating systems illustrate this duality: essential oils can strengthen antimicrobial function, but high loading can compromise mechanical properties, barrier uniformity, appearance, and sensory acceptability [100]. Nanoencapsulation-based edible coatings can reduce some of these problems by improving dispersion and controlled delivery, but they do not eliminate the need to validate flavor, appearance, dose, and release behavior on real food surfaces [101].
A rigorous failure analysis should distinguish five levels of failure. Formulation failure includes creaming, coalescence, phase separation, and oxidative instability before application. Surface failure includes poor wetting, runoff, weak adhesion, and non-uniform coverage. Structural failure includes cracking, whitening, oil bleeding, and loss of mechanical continuity after drying. Transport failure includes excessive water loss, insufficient gas exchange, uncontrolled active release, and anaerobic risk. Validation failure occurs when the coating appears effective under simplified tests but lacks robust microbial, sensory, physiological, or storage-distribution evidence. Rapid analytical tools for foodborne pathogens, including portable biosensing platforms reported in Journal of Analysis and Testing, illustrate how microbial validation can move beyond slow or endpoint-only assays, although such tools should be treated as validation methods rather than coating-design evidence [102].
The most useful failure analysis therefore asks not only whether decay is delayed, but how the coating achieved that effect and at what cost. A coating that suppresses fungal growth but produces visible whitening may be unsuitable for premium berries. A coating that reduces weight loss but induces anaerobic flavor may be unacceptable for climacteric fruit. A coating that releases essential oil rapidly may pass short-term microbial tests but fail during distribution. A coating that is physically uniform immediately after application may crack after hydration–dehydration cycling. These failure modes should be treated as design endpoints rather than secondary observations.
Droplet-to-coating translation ultimately defines whether acoustic cavitation creates a real preservation interface or only a stable colloidal formulation. The strongest evidence links acoustic dose and interfacial architecture to coating deposition, dried-layer structure, surface adhesion, barrier-release behavior, failure thresholds, and produce-level responses. These coating-level endpoints provide the mechanistic bridge to Section 5, where antimicrobial activity, oxidative control, respiration regulation, transpiration reduction, and texture preservation are interpreted as outcomes of droplet-matrix-surface-physiology coupling rather than as isolated preservation effects.
5. Preservation mechanisms beyond passive barriers
Once an emulsion coating has consolidated on the produce surface, preservation is governed by the exposure profile and physiological effects created by that interface. Antimicrobial delivery, redox control, gas and water transport, and texture retention depend jointly on active-compound availability, coating permeability, surface hydration, and commodity metabolism. These processes interact: microbial suppression limits infection-associated oxidative and cell-wall damage; water retention sustains turgor; O2/CO2 transfer alters respiration, ethylene production, and redox balance; and cell-wall metabolism integrates these responses. Fig. 4 links the droplet-matrix-surface organization established during coating formation to the resulting preservation functions. Mechanistic interpretation should therefore focus on how the deposited layer controls the location, duration, and intensity of activity.
5.1. Antimicrobial delivery: surface contact, volatile flux, and sustained exposure
Antimicrobial performance is determined by the concentration and persistence of active compounds at colonization sites, rather than by total loading. Essential oils, phenolics, aldehydes, terpenoids, and organic acids can disrupt membranes, proton gradients, permeability, enzyme function, and local pH or redox conditions. On coated produce, however, their effective dose reflects partitioning among droplets, interfacial layers, the hydrated matrix, the produce surface, and the headspace. Rapid volatilization shortens protection, whereas excessive sequestration delays microbial contact.
Nanoemulsified actives favor early surface protection by dispersing hydrophobic compounds throughout aqueous coating matrices and reducing local oil enrichment. Eugenol nanoemulsion-loaded shellac reduced decay and maintained firmness in Kaji lemon, showing that a wax-containing matrix can combine volatile retention with produce protection [103]. Nano-emulsification of Monarda didyma essential oil improved antifungal efficacy and blueberry preservation relative to non-emulsified delivery [104], while nanostructured Pimpinella anisum essential oil showed antifungal and anti-aflatoxigenic activity with less nutritional deterioration [105]. These findings support nanoscale dispersion as a means of improving active distribution, but not necessarily exposure duration.
Reservoir-type systems address retention. Clove essential oil-loaded chitosan nanocapsules extended blueberry shelf life by confining the volatile phase within a biodegradable cationic carrier [106]. More broadly, pullulan-, alginate-, chitosan-, shellac-, and related emulsion coatings [34], [35], [36], [37], [71], [72], [73], [74], [75], [103], [107] span a continuum from rapid-contact delivery to sustained availability. Release profile is therefore more informative than nominal active concentration when protection must persist beyond the initial antimicrobial pulse.
Vapor-phase transport can extend activity to stem scars, calyx regions, wounds, lenticels, and cuticular microcracks that are incompletely covered, but the same pathway accelerates depletion and may cause aroma rejection. The useful flux must remain microbiologically effective without exceeding sensory tolerance. Microbial counts, lesion development, and biofilm measurements remain the primary endpoints for validating antimicrobial performance.
5.2. Oxidative control: interfacial protection and tissue redox status
Postharvest oxidative deterioration arises when reactive oxygen species production exceeds tissue antioxidant capacity. Wounding, respiration, senescence, chilling, infection, and membrane disruption intensify oxidative pressure, promoting lipid peroxidation, electrolyte leakage, browning, pigment and aroma loss, and cell-wall disassembly. Emulsion coatings can moderate these responses by reducing external stress, retaining antioxidants near the surface, and controlling their transfer into tissues. These routes act together and cannot be inferred from antioxidant loading alone.
Chitosan coatings containing ascorbic acid and curcumin reduced weight loss, decay, malondialdehyde and hydrogen peroxide accumulation, and softening-related enzyme activities in strawberries [5]. The coordinated response is consistent with active delivery combined with altered water and gas exchange. Lemon essential oil-gelatin coatings preserved kiwifruit flavor through antioxidant and antimicrobial activity, lower respiration, and reduced formation of undesirable volatiles [108]. In both systems, lower oxidative markers may reflect direct scavenging, reduced respiratory stress, membrane protection, and lower microbial pressure.
Fermented Rosa sterilis and edible Artemisia princeps illustrate the diversity of food-compatible antioxidant resources and the importance of sensory quality [109], [110]. Their suitability for coatings depends on aqueous dispersibility, interfacial affinity, stability during processing, and release under storage conditions; high extract activity alone is insufficient.
Ultrasound adds a formulation-specific constraint because the active phase and carrier experience the same cavitation field used for emulsification. Controlled treatment can improve dispersion and interfacial localization, whereas excessive exposure may oxidize volatile oils or damage stabilizing biopolymers. Chemical activity must therefore be retained after sonication and throughout storage. Tissue redox markers should be interpreted together with coating oxidative stability, since an oxidized dispersed phase cannot function as a durable antioxidant reservoir.
5.3. Physiological regulation: gas exchange, water loss, ethylene, and anaerobic limits
Emulsion coatings alter the local atmosphere and hydration state of living produce. Water-vapor resistance can reduce transpiration and preserve turgor, while O2 influx and CO2 efflux influence respiration, ethylene biosynthesis, substrate consumption, and volatile metabolism. The required permeability is commodity-specific: a barrier suitable for a slowly respiring intact fruit may be excessive for wounded or highly climacteric tissue.
A bio-inspired shellac gel/chitosan coating formed a nanoporous layer on fresh-cut lotus root and reduced respiration, ethylene release, water loss, and enzymatic browning while preserving color, texture, and nutritional quality [111]. This study directly connected dried-layer microstructure with gas transfer and physiology. Cellulose nanocrystal-stabilized Pickering emulsion chitosan coatings also reduced ethylene production and physiological disorders in cold-stored Bartlett pears [112], showing that particle-stabilized interfaces can regulate metabolism as well as carry antimicrobials.
Some components act at both the coating and tissue levels. Calcium ions promote polymer crosslinking while supporting cell-wall stabilization and postharvest quality [113]. Beeswax-in-water Pickering emulsions stabilized by cellulose nanofibrils and carboxymethyl chitosan distribute hydrophobic domains through an edible matrix to increase water-transfer resistance [114]. In both cases, the response depends on the final interfacial and matrix structure, not on the isolated ingredient.
The useful operating region lies between negligible transport control and excessive restriction. Moderate changes in the surface atmosphere can lower respiration and transpiration; severe O2 depletion or CO2 accumulation induces fermentative metabolism, ethanol and acetaldehyde formation, tissue injury, and off-flavor. The kiwifruit study with lemon essential oil-gelatin coatings illustrates why respiration and aroma preservation must be assessed together [108]. Weight-loss reduction alone does not establish physiological suitability. Gas composition, respiration, ethylene, volatile metabolites, and sensory quality are needed to determine whether metabolism has been moderated without crossing the anaerobic threshold.
5.4. Texture preservation: water status, pectin remodeling, and cell-wall enzymes
Firmness integrates turgor, membrane integrity, microbial injury, respiration, ethylene signaling, oxidative status, and cell-wall metabolism. Emulsion coatings influence these determinants through water retention, infection control, modified gas exchange, and the delivery of antioxidants, antimicrobials, calcium, or other actives. Texture retention is thus a downstream outcome of interfacial transport and tissue physiology.
Pectin remodeling is central to softening in many fleshy fruits [115]. Pectin methylesterase (PME) alters homogalacturonan methyl esterification and its susceptibility to calcium association or depolymerization. Polygalacturonase (PG) and pectate lyase (PL) cleave pectic backbones, while β-galactosidase removes galactosyl side chains. The conversion of wall-associated pectin into soluble fractions weakens the middle lamella and intercellular adhesion; hemicellulose and cellulose remodeling may contribute during advanced ripening.
Conventional coating studies provide supporting evidence for these pathways but generally do not isolate the contribution of ultrasound. Chitosan delayed strawberry softening by limiting water-soluble pectin accumulation and suppressing PME and PG activities and related gene expression [116]. Konjac glucomannan/low-acyl gellan gum coatings containing thymol microcapsules preserved blueberry cell-wall ultrastructure and reduced PME, PG, cellulase, and β-galactosidase activities [117]. Carboxymethyl cellulose coatings delayed banana ripening by lowering respiration and ethylene production and restricting cell-wall polysaccharide disassembly [118], while layer-by-layer chitosan/carboxymethyl cellulose coatings similarly delayed mango softening [119]. These studies identify plausible downstream markers for structured active coatings but do not establish cavitation as the cause of the tissue response.
For ultrasound-assisted coating systems, acoustic treatment may modify interfacial structure and thereby influence active retention, release, and transport, whereas texture preservation reflects subsequent changes in water status, microbial injury, oxidative stress, and cell-wall metabolism. These processes may act concurrently, but their contributions cannot be inferred from firmness alone. Reduced dehydration does not establish suppression of pectin disassembly, and lower enzyme activity does not demonstrate direct delivery of an inhibitor. Attribution of texture preservation to cavitation requires characterized acoustic conditions, appropriate sonicated and nonsonicated controls with matched formulation and thermal history, and experimental evidence linking changes in interfacial structure and coating transport to corresponding physiological responses. Without this causal linkage, improvements in firmness, pectin fractions, or enzyme activity support coating-associated preservation but do not establish a cavitation-mediated mechanism.
6. Standardization and scalable sonoprocessing
Translation of ultrasound-assisted emulsion coatings requires continuity between acoustic processing, interfacial structure, coating function, and produce-level efficacy. Field characterization, energy delivery, thermal control, reactor design, and reproducibility remain central scale-up requirements [120]. Table 5 [25], [27], [29], [31], [32], [33], [34], [36], [37], [43], [64], [67], [70], [120], [121] identifies where translation can fail across acoustic control, interfacial architecture, coating performance, and produce validation, whereas Fig. 5 converts these evidence requirements into sequential readiness gates. The distinction is deliberate: Table 5 diagnoses failure modes, while Fig. 5 determines whether the available evidence is sufficient to advance to the next translational stage.
Table 5.
Translational barriers, failure endpoints, quantitative evidence, and design rules for scalable ultrasound-assisted emulsion coatings.
| Translational barrier | Failure mechanism / endpoint | Required metric | Published benchmark / current evidence gap | Design rule | Priority | Refs. |
|---|---|---|---|---|---|---|
| Acoustic heterogeneity | Local cavitation zones → variable droplet/interfacial state | Pressure/cavitation field map; spatial power distribution; droplet-size CV | Flow-reactor field characterization is established; coating-manufacturing studies rarely couple field maps with coating/produce outcomes | Map or normalize acoustic exposure before scale-up | Critical | [31], [32], [120] |
| Power-only reporting | Nominal electrical setting ≠ delivered acoustic dose | Calorimetric power; power density; energy density | Controlled US-HPH work spans 47 ∼ 175 MJ m−3[67]; delivered energy remains NR in many coating studies | Report delivered power and energy per processed volume/mass | Critical | [31], [67] |
| Thermal confounding | Cavitation heating → altered viscosity, oxidation or volatile loss | Full T(t); cooling load; isothermal control | Continuous tandem processing evaluated 45 ∼ 65 °C and selected 55 °C for a specific formulation [70]; coating studies seldom include matched thermal controls | Separate acoustic and thermal contributions | Critical | [27], [70] |
| Overprocessing | Radical/shear/hotspot damage → aggregation, scission, oxidation | Size drift; active retention; PV/TBARS; protein carbonyls | No transferable threshold; optimum is formulation- and reactor-specific | Define insufficient-functional-excessive exposure experimentally | Critical | [25], [27] |
| Viscosity/acoustic attenuation | High matrix viscosity → damped cavitation and incomplete breakup | Viscosity/flow curve; acoustic power; size versus rheology | No cross-study coating threshold; quantitative coupling remains insufficiently reported | Optimize acoustic dose after addition of the film-forming matrix, not only in water-like precursors | High | [25], [31] |
| Continuous operation, fouling and CIP | Protein/biopolymer deposition; erosion; unstable run | Run duration; pressure drop; fouling mass; cleaning interval; erosion/metal release | Continuous emulsion production reaches 20 mL min−1[43] and 0.5 ∼ 6 mL min−1[70]; coating-line fouling/CIP datasets are NR; transducer erosion is documented | Use hygienic, inspectable flow paths; quantify cleaning and erosion | Critical | [31], [32], [43], [70] |
| Emulsion-to-coating mismatch | Stable dispersion → poor wetting, cracking, oil bleeding or nonuniform film | Contact angle; coverage; thickness; microscopy; adhesion; leakage | Pullulan-CEO film WVP = 0.050 g m−1 kPa−1 h−1[29]; EEO-NE strawberry layer ∼ 20 ∼ 25 μm [37]; alginate-OEO tomato layer ∼ 5 μm [64] | Validate the deposited layer as a separate evidence gate | Critical | [29], [37], [64] |
| Uncontrolled active release | Burst release/volatilization → short antimicrobial persistence or sensory excess | Headspace concentration; retention; release rate; kinetic coefficient | OEO release: 6.2% at 1 h → 74.5% at 96 h; Dapp = 9.17 × 10−7 cm2 s−1[64] | Tune interfacial retention and coating transport to the required exposure period | High | [29], [64] |
| Weak comparator design | Produce efficacy cannot separate matrix, active and US effects | Untreated; matrix-only; active-only; non-US/process comparator where feasible | Direct produce studies quantify efficacy, but matched acoustic comparators remain uncommon | Use controls that isolate formulation, coating and sonication contributions | Critical | [34], [36], [37] |
| Sensory rejection | Volatile aroma, opacity or tackiness → unacceptable product | Sensory panel; odor/flavor score; transparency; surface tack | 1% citral coating reduced texture and sensory acceptance, whereas 0.5% remained acceptable [36]; EEO-NE improved sensory retention relative to free EEO during storage [37] | Define functional dose below sensory failure threshold | High | [36], [37] |
| Safety/regulatory uncertainty | Probe erosion; migration; allergenicity; persistent nano-state | Metal particulates; migration/exposure; allergen assessment; particle-state characterization | Transducer-derived particulates are experimentally demonstrated; coating-specific migration/exposure datasets remain sparse | Couple food-contact safety assessment to the final coating and process | Critical | [121] |
| Sustainability/economic gap | Energy, cooling, cleaning or material burden offsets preservation benefit | kWh kg−1; water use; cooling load; yield; CAPEX/OPEX; LCA/TEA | Energy density is quantified in process studies, but cost per kg usable coating and full coating-line TEA are generally NR | Normalize resource use to usable coating and avoided food loss | High | [31], [33], [67] |
| Scale-up fragility | Larger reactor fails to reproduce target interface/function | Throughput; RTD; between-run CV; droplet/coating equivalence | Direct 10× acoustic scale-up demonstrated [33]; USMR 0.66 → 20 mL min−1[43]; tandem system 0.5 ∼ 6 mL min−1[70]; integrated industrial coating-line validation remains NR | Scale the required interfacial state and coating function, not reactor volume alone | Critical | [33], [43], [70] |
Note: Published numerical values denote study-specific engineering or coating benchmarks. NR denotes a parameter not reported sufficiently for quantitative cross-study comparison. “Critical” denotes evidence required before a translational or industry-ready claim; “High” denotes evidence strongly recommended for coating-system validation. CV, coefficient of variation; RTD, residence-time distribution; CIP, cleaning in place; PV, peroxide value; TBARS, thiobarbituric acid reactive substances; LCA, life-cycle assessment; TEA, techno-economic analysis; CAPEX, capital expenditure; OPEX, operating expenditure.
Fig. 5.
Non-substitutable evidence gates from formulation to industrial sonoprocessing. The four gates are formulation-ready, coating-ready, produce-validated, and industry-ready. Advancement requires the evidence generated at each gate to remain valid at the next scale. PDI, polydispersity index; CIP, cleaning in place.
6.1. Evidence matrix: minimum descriptors and claim-specific validation
The acoustic layer should establish what treatment was actually delivered rather than report nominal generator settings alone. Frequency, geometry, processed volume, temperature, formulation state, and calorimetrically determined power or energy density provide the minimum context summarized in Table 3 [31], [32], [41], [42]. When cavitation itself is invoked as the causal agent, at least one validated field or cavitation descriptor is additionally required.
The interfacial layer identifies what acoustic treatment produced. Conventional colloidal descriptors such as droplet size, polydispersity index, ζ-potential, and storage stability remain necessary but do not identify the structure responsible for stabilization or delivery. Claims concerning interfacial organization therefore require complementary measurements of interfacial composition, mechanics, morphology, active retention, or release, selected according to the proposed mechanism. Endpoint droplet size should be treated as evidence of structure formation rather than as a proxy for coating efficacy [122].
Coating and produce validation should likewise be claim-specific. The formulation must demonstrate spreading, drying, adhesion, structural integrity, and the transport or release function required by the intended application. Produce experiments should then use commodity-relevant storage and controls that separate matrix, active-compound, and ultrasound effects. Endpoints should test the proposed mechanism rather than accumulate as an undifferentiated quality panel. Table 5 and Table S3 [22], [25], [26], [27], [28], [29], [31], [32], [33], [34], [35], [36], [37], [48], [54], [64], [65], [71], [116], [119] summarize the corresponding evidence requirements.
6.2. Scale-up and manufacturing constraints: throughput, energy, heat removal, fouling, and hygienic design
Scale-up is principally a problem of preserving acoustic exposure and residence-time control as throughput increases. Simply increasing vessel volume or nominal power can intensify local overprocessing while leaving underexposed regions. Continuous, recirculating, and flow-through reactors provide better control, but performance remains dependent on reactor geometry, acoustic-field distribution, flow, temperature, and coupling [123], [124]. Fig. S1 summarizes the principal reactor configurations relevant to coating manufacture.
Published scale-up studies provide useful engineering benchmarks but do not yet establish application-specific commercial validation of ultrasound-assisted postharvest coating manufacture. High-power ultrasonic nanoemulsification has been scaled by approximately one order of magnitude while maintaining an acoustic amplitude near 90 μm peak-to-peak and comparable product quality [33]. Continuous microreactor studies further show that throughput can be increased by numbering-up or reactor enlargement while largely retaining droplet characteristics, with representative systems progressing from sub-mL min−1 operation to 20 mL min−1 [43]. More recently, a temperature-controlled 21.1 kHz tandem unit operated over 0.5∼6 mL min−1, 11∼24 W, and 45∼65 °C, illustrating the importance of coordinated residence-time and thermal control during continuous emulsification [70]. These continuous-processing benchmarks, introduced in Fig. 3B and revisited here in the context of scale-up, indicate that throughput should be evaluated together with retention of the target droplet/interfacial state rather than by nominal ultrasonic power or reactor volume alone. The plotted values are study-specific operating points rather than transferable processing windows. Collectively, these studies support acoustic-field control, numbering-up, residence-time management, thermal regulation, and continuous operation as practical scale-up strategies, while integrated validation of emulsion production, coating application, drying, sanitation, and preservation efficacy at commercial throughput remains unavailable. Scale-up-relevant multi-transducer and continuous-flow configurations have also been described in the engineering literature [120], [123], [124], but their availability should not be interpreted as application-specific validation of a complete postharvest coating process.
These scale-up studies also show that reactor and formulation variables cannot be optimized independently. Residence time, channel geometry, acoustic coupling, flow regime, pre-homogenization, recirculation, and frequency configuration affect droplet formation and process efficiency [125], [126]. In continuous vitamin E and soybean-oil nanoemulsion systems, ultrasonic power, flow rate, temperature, emulsifier chemistry, phase ratio, and antioxidant protection jointly determined physical and chemical stability [70], [127]. These studies do not establish direct industrial readiness for edible coatings, but they demonstrate that cavitation, flow, heat removal, formulation chemistry, and equipment hygiene must be co-designed.
Thermal control is part of this design rather than an auxiliary correction. Heat accumulation alters viscosity, interfacial tension, adsorption kinetics, droplet breakup, volatile retention, and oxidation [128]. Temperature feedback, cooling capacity, pulse or residence-time control, and isothermal comparison are therefore needed to separate acoustic effects from thermal effects. Biopolymer-rich fluids add fouling and cleaning constraints, while sonotrode or transducer erosion may introduce metallic particulates [129]. Equipment qualification should consequently include material compatibility, erosion monitoring, hygienic design, cleaning validation, and the effect of fouling on acoustic delivery.
Process efficiency should be evaluated at the manufacturing boundary rather than by terminal droplet size alone [130], [131]. Translational studies should report throughput, delivered energy per unit product, residence-time distribution, thermal load, yield, active retention, and process variability [130], [131]. For coatings, these variables should be linked to application yield, drying demand, coating consumption, and preservation benefit.
Detailed economic evidence remains substantially weaker than process-level evidence [130], [131]. Capital cost, maintenance frequency, sonotrode replacement, cleaning-water demand, downtime, and cost per kilogram of usable coating are rarely reported together and should not be inferred from laboratory power measurements. Future techno-economic comparisons should instead normalize energy, cooling, cleaning, maintenance, emulsifier consumption, product losses, and shelf-life benefit to the same functional output. Hygienic design is equally important: biopolymer deposition can alter process performance, while transducer erosion may introduce metallic particulates [129], [132]. Equipment qualification should therefore include fouling monitoring, clean-in-place compatibility, material integrity, erosion surveillance, and verification that cleaning restores both sanitary status and process performance [132]. Regulatory readiness is considered separately in Section 6.3 because equipment scalability does not substitute for ingredient-specific food-contact compliance, migration/exposure assessment, or nanospecific evaluation where applicable.
6.3. Safety and sustainability: demonstrated hazards, exposure-dependent concerns, and regulatory triggers
Safety considerations should be separated according to the strength and nature of the evidence. Demonstrated process hazards include, for example, metallic particulate release associated with cavitation erosion of ultrasonic components, which justifies equipment-material selection and erosion monitoring [129]. Formulation-dependent concerns include oxidative damage, volatile loss, allergenicity or irritation, excessive exposure, and sensory rejection [133], [134], [135]. Their occurrence depends on composition, acoustic dose, temperature, storage, and consumption pattern and therefore requires measurement in the final formulation rather than being presumed from sonication alone [134], [135]. A third category comprises regulatory triggers, for which the relevant question is not whether the word “nano” is used but whether the final food contains a material whose composition, persistence, particle state, or exposure profile requires nanospecific assessment under the applicable regulatory framework [131], [133].
This distinction is particularly important for nanoemulsions. Food-grade lipid nanoemulsion droplets are generally dynamic and digestible colloidal structures and should not automatically be equated with persistent engineered nanoparticles [131], [134]. Nanospecific characterization becomes more relevant when insoluble particles, mineral phases, engineered nanomaterials, nanocellulose, or other persistent particulate stabilizers remain associated with the consumed coating [133]. Likewise, enhanced dispersion or bioavailability is not itself evidence of toxicity; gastrointestinal fate depends on emulsion structure, food matrix, digestion conditions, and the physicochemical properties of the encapsulated compound [131], [134]. Safety evaluation should instead resolve composition, particle fate, digestion or dissolution where relevant, migrated dose, exposure frequency, and the regulatory status of individual ingredients and processing aids [133], [134], [136], [137].
Migration and exposure should be measured in the relevant coating-food system using validated analytical methods capable of distinguishing extractable inventories from species that migrate under defined contact conditions [136], [137], [138]. Volatile actives require separate assessment of headspace loss and retained or migrated fractions, whereas persistent particulate stabilizers require fate characterization where relevant.
Sustainability should likewise be assessed at a defined system boundary [139], [140]. Biodegradability of the coating matrix does not establish an overall environmental advantage if ingredient extraction, sonoprocessing, cooling, water use, cleaning, drying, rejected product, or short equipment lifetime impose substantial burdens. Life-cycle and techno-economic analyses should therefore relate material and energy inputs to the functional outcome achieved, preferably including avoided food loss and shelf-life extension [139], [140]. Increasing structural sophistication is justified only when the additional processing burden yields a measurable gain in active-dose reduction, coating efficiency, preservation duration, or product acceptance.
6.4. Translational readiness: non-substitutable decision gates
Translational readiness is determined by the weakest unverified link in the evidence chain. Fig. 5 distinguishes four non-substitutable gates: formulation-ready, coating-ready, produce-validated, and industry-ready. Progression requires a reproducible acoustic treatment and interfacial structure, demonstration that this structure survives deposition as a functional coating, validation of commodity-relevant preservation with appropriate controls, and finally reproduction of the target coating function at relevant throughput [31], [32], [126], [127].
A formulation-ready system has a reproducible composition, a defined reactor and delivered acoustic dose, controlled temperature, a characterized interfacial architecture, stability over the intended holding period, active retention, and a plausible safety basis. Complex protein-polysaccharide, emulsion-gel, and hydrogel platforms may satisfy this gate when their structure and processing history are resolved [141], [142], [143], [144], [145], but they remain coating precursors.
Industry readiness further requires process reliability, thermal and residence-time control, hygienic operation, cleaning recovery, equipment integrity, food-contact compliance, sensory acceptability, and economically meaningful throughput. Evidence at one gate cannot compensate for failure at another: precise cavitation characterization does not establish coating function, and produce efficacy alone does not establish a scalable acoustic process.
7. Conclusions and outlook
Ultrasound-assisted emulsion coatings should be evaluated as multiscale interfacial systems rather than as emulsions whose value is defined by the minimum attainable droplet size. The principal contribution of this review is a cavitation-guided framework that links delivered acoustic energy and reactor-dependent bubble dynamics to emulsifier remodeling and interfacial assembly while explicitly separating experimentally validated observations from mechanistic interpretation. This framework identifies a formulation-specific processing window in which acoustic treatment can improve interfacial organization without unacceptable thermal, oxidative, or macromolecular damage.
The second contribution is the identification of the droplet-to-coating transition as the critical translational step. A stable emulsion becomes preservation-relevant only if its architecture survives spreading, drying, and immobilization and subsequently provides an appropriate combination of adhesion, water and gas transport, and active-compound flux on the target commodity. Current produce-level evidence is strongest for essential-oil nanoemulsions. Pickering, multilayer, multiple-emulsion, and emulgel systems offer distinct advantages in retention, compartmentalization, or residence, but their coating- and produce-level evidence remains less mature and should not be assigned equivalent readiness on the basis of formulation performance alone.
Industrial translation ultimately depends on reproducing the required interfacial state at relevant throughput rather than scaling nominal ultrasonic power. Continuous and flow-through reactors, delivered-energy normalization, residence-time control, thermal management, hygienic design, and erosion control provide an increasingly credible engineering basis. However, food-coating studies still rarely quantify process energy and cost, cleaning validation, migration, sensory acceptance, and scale-dependent variability within the same system. The next decisive studies should therefore combine acoustic dosimetry and appropriate nonsonicated or conventionally homogenized comparators with coating microstructure, transport measurements, commodity physiology, and quantitative manufacturing metrics. Such evidence would move ultrasound-assisted coating development from empirical formulation screening toward reactor-aware, commodity-specific, and industrially testable postharvest preservation.
Declaration of generative AI and AI-assisted technologies in the manuscript preparation process
During the preparation and revision of this work, the authors used ChatGPT-5.6 (OpenAI) to assist with language refinement, document organization, and selected graphical components and visual elements. The authors independently verified all scientific statements, evidence classifications, numerical summaries, and references, and take full responsibility for the content of the publication.
CRediT authorship contribution statement
Zhao Yang: Writing – original draft. Ruyi Wang: Writing – original draft. Chun-Yan Shen: Resources. Jialiang Pan: Resources. Wenzhen Liao: Writing – review & editing. Xingfen Yang: Supervision. Qi He: Writing – review & editing, Supervision, Project administration, Conceptualization.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 31901750) and the Natural Science Research Project from the Guangdong Food and Drug Vocational College (2025ZR08).
Footnotes
This article is part of a special issue entitled: ‘Ultrasound-Driven Functional Emulsion Formulations’ published in Ultrasonics Sonochemistry.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.ultsonch.2026.108095.
Contributor Information
Xingfen Yang, Email: Yangalice79@smu.edu.cn.
Qi He, Email: heqi18@smu.edu.cn.
Appendix A. Supplementary data
The following are the Supplementary data to this article:
Supplementary material. Ultrasonic reactor configurations and scale-up constraints, material-selection guidance, study-level quantitative evidence, and analytical validation matrices for ultrasound-assisted emulsion coatings.
Data availability
No primary experimental data were generated.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Supplementary material. Ultrasonic reactor configurations and scale-up constraints, material-selection guidance, study-level quantitative evidence, and analytical validation matrices for ultrasound-assisted emulsion coatings.
Data Availability Statement
No primary experimental data were generated.