Next-generation probiotic delivery: Engineering complex coacervation systems for targeted gut release and bioactive synergy

Abstract

Probiotics offer diverse health benefits, yet their efficacy is severely constrained by low viability during food processing, storage, and gastrointestinal transit. Complex coacervation, driven by electrostatic interactions between oppositely charged biopolymers, has emerged as a versatile encapsulation platform, but its recent evolution through integrated engineering strategies and co-delivery architectures has not been systematically reviewed. This review critically examines the design of complex coacervation systems for probiotic survival, analyzing synergistic technologies (drying, tableting, emulsification, crosslinking, and physical field) and co-delivery systems with bioactive compounds from an engineering perspective, with emphasis on protective mechanisms, formulation-process relationships, and translational bottlenecks. Complex coacervation provides a customizable, pH-responsive barrier that shields probiotics from thermal, oxidative, acidic, and bile stresses. Importantly, the physicochemical properties of coacervate networks, including polymer composition, crosslinking density, permeability, and swelling behavior, also determine gastrointestinal release profiles by allowing protection in the stomach while facilitating probiotic liberation under intestinal conditions. When integrated with complementary technologies, this barrier achieves hierarchical protection—drying for powder stability, tableting for macroscopic shielding, emulsification for multi-compartment architectures, crosslinking for mechanical reinforcement, and low-frequency static magnetic field for clean-label physical modulation. Co-delivery further preserves probiotic viability while enhancing bioactive bioavailability, unlocking synergistic gut-health benefits. Nevertheless, system performance remains highly formulation- and process-dependent, governed by polymer selection, charge balance, crosslinking density, drying regimes, and packaging. Translating these engineered systems into next-generation commercial products requires deeper mechanistic insights, standardized characterization, precise release kinetics, robust in vivo validation, and scalable manufacturing solutions.

Graphical abstract

Highlights

• •Complex coacervation is a versatile pH-responsive platform for probiotic encapsulation.
• •Hierarchical protection is achieved via synergistic multi-technology integration.
• •Co-encapsulation with bioactives unlocks synergistic gut-health benefits.
• •Performance depends on polymer selection, charge balance, and processing.
• •Translating these systems requires standardized characterization and scale-up.

1. Introduction

Probiotics represent an important microbial resource with a steadily growing global market share, owing to their diverse physiological functions such as alleviating colitis (Chen et al., 2025a; Ji et al., 2025), enhancing immunity (Chen et al., 2024; Lee et al., 2025), and modulating the gut microbiota (Deng et al., 2025; Mafe et al., 2025). To exert these health-promoting effects, probiotics must not only survive processing and storage but also remain viable and functionally active after gastrointestinal transit and reach the desired intestinal sites (Yuan et al., 2024). However, probiotic cells are extremely sensitive to processing, storage, and gastrointestinal environments, which significantly compromises their viability (Barajas-Álvarez et al., 2023). Processing-induced thermal, dehydration, and oxidative stresses, together with subsequent exposure to oxygen, moisture, gastric acid, and bile salts, can progressively compromise cell viability from manufacture to gastrointestinal transit (Ge et al., 2024; Li et al., 2025; Shu et al., 2025a, b; Yang et al., 2023). These sequential stresses indicate that improving viability at a single processing stage is insufficient. Next-generation delivery systems must provide protection throughout the product lifecycle while retaining the capacity for appropriate intestinal release. However, probiotics are not a homogeneous group, and their responses to processing, storage, and gastrointestinal stresses vary considerably among species and strains. Differences in cell wall composition, surface properties, oxygen sensitivity, stress tolerance, and metabolic characteristics can influence their interactions with encapsulation materials and determine their requirements for protection (Shu et al., 2026a). For instance, oxygen-sensitive probiotics such as Bifidobacterium species generally require stronger protection against oxidative stress, whereas Lactobacillus species may exhibit different responses depending on strain-specific physiological properties (Shu et al., 2026b). Consequently, probiotic delivery systems should not be designed as universal platforms, but rather tailored by considering the characteristics of the target probiotic strain and its intended application.

Encapsulation has emerged as an effective strategy to address these challenges (Bu et al., 2025; Zhao et al., 2024). Multiple encapsulation systems have been developed, such as microcapsules, emulsions, edible films, hydrogels, and liposomes, each with specific advantages and limitations suitable for diverse food applications (Liu et al., 2025; Wang et al., 2024b, 2025c). Among them, complex coacervation is a traditional yet highly efficient encapsulation technique that has gained wide recognition. This liquid–liquid phase separation process, driven by electrostatic interactions between oppositely charged polymers, offers advantages such as high encapsulation efficiency, simple processing, and relatively low cost (Fu et al., 2025; Hu et al., 2024, Hu et al., 2024; Sathyavageeswaran et al., 2024). More importantly, the tunable composition, interfacial structure, and environmental responsiveness of coacervate matrices provide opportunities to regulate gastrointestinal protection, release behavior, and the delivery efficiency of encapsulated probiotics and bioactive compounds. Complex coacervation has been successfully applied to encapsulate various bioactive compounds, such as polyphenols and essential oils (Zeng et al., 2023; Zhang et al., 2024). It also shows promising potential for probiotics. For example, a Persian gum–soy protein isolate coacervate substantially reduced the loss of Lactobacillus acidophilus NCFM during simulated gastric exposure while maintaining high encapsulation and subsequent release (Mirzapour-Kouhdasht et al., 2024). In another material configuration, a gelatin–amidated low-methoxyl pectin coacervate hydrogel improved the gastrointestinal survival of Bifidobacterium longum and Lactobacillus acidophilus (González et al., 2024). Rather than simply demonstrating that coacervation is protective, these studies collectively show that probiotic performance is closely coupled to matrix composition and structural properties. Dense protein–polysaccharide barriers can improve resistance to gastrointestinal stress, whereas highly hydrated or porous networks may favor swelling and release but provide less robust long-term protection. Thus, increasing encapsulation efficiency or barrier strength alone does not necessarily translate into an optimal delivery system. The central engineering challenge is to balance mutually competing requirements—processing and storage stability, gastric protection, and timely intestinal release—through rational control of coacervate composition, structure, and responsiveness.

In recent years, notable progress has been achieved in probiotic encapsulation based on complex coacervation, yet systematic summaries from an engineering integration perspective remain scarce. More importantly, existing studies often evaluate individual formulations or processing strategies using different endpoints, making it difficult to establish generalizable relationships among material composition, processing conditions, coacervate structure, probiotic survival, and gastrointestinal release. This review presents a systematic engineering framework for designing complex coacervation systems to improve probiotic survival, gastrointestinal-responsive release, and targeted delivery performance, with particular attention to synergistic combined technologies—drying, tableting, emulsification, crosslinking, and low-frequency static magnetic fields—and co-delivery approaches (Fig. 1). Notably, engineering strategies should be adapted according to probiotic-specific characteristics and application requirements, rather than considered as universally transferable solutions for all probiotic strains. Accordingly, rather than cataloguing results from individual studies, this review compares their formulation–process–structure–function relationships, identifies the conditions under which different engineering strategies provide benefits or introduce trade-offs, and critically examines translational bottlenecks. On this basis, we propose strategic roadmaps to guide the development of industrially viable probiotic products with application-specific protection and intestinal delivery functions.

Fig. 1.

Overview diagram of this review.

2. Engineering powder stability: drying-induced vitrification and barrier compaction

To engineer powder stability, complex coacervation is paired with a downstream drying step (typically spray-drying or freeze-drying), transforming soft, charge-balanced coacervate droplets into transportable, shelf-stable carriers. Importantly, drying is not merely a dehydration step. It restructures the coacervate matrix through vitrification and compaction, thereby altering water activity, glass transition behavior, interfacial permeability, and ultimately probiotic survival and release (Wang and Zhong, 2024; Yin et al., 2022). Recent studies on drying-assisted complex coacervation for probiotic encapsulation are summarized in Table 1. Although drying-assisted complex coacervation generally improves probiotic stability, the magnitude of protection varies substantially with probiotic strain, matrix composition, coacervate structure, drying conditions, and subsequent storage. Thus, drying performance should not be attributed to the drying method alone but interpreted through formulation–process interactions. A central design trade-off is that matrix densification can improve resistance to moisture, oxygen, and gastrointestinal stresses, whereas excessive compaction may impair rehydration and intestinal release. Accordingly, optimization should consider not only immediate survival after drying but also water activity, glass transition behavior, permeability, release characteristics, and long-term storage performance.

Table 1.

A summary of recent probiotic encapsulation by complex coacervation combined drying technology.

Probiotic strain	Wall material	Drying method	Integration mode/Key parameters	Key findings	Reference
Lactobacillus rhamnosus GG	Pea protein isolate/sodium caseinate + sugar beet pectin	Spray-drying vs freeze-drying	Complex coacervation → spray-drying or freeze-drying; protein: sugar beet pectin = 5:1 or 2:1 (w/w); pH 3.0	Spray-dried coacervates, especially with pea protein isolate–sugar beet pectin, maintained superior probiotic viability after digestion (initial survival of 8.5-9.0 log CFU/g; survival loss of ∼1 log CFU/g during SGF of 2 h and ∼2 log CFU/g during SIF of 2.5 h); drying method and formulation critically influenced stability	Qi et al. (2021)
Lactobacillus plantarum LN66	Gelatine + gum arabic	Freeze-drying	Complex coacervation → freeze-drying; gelatin: gum arabic = 1:1 (w/w); pH 4.0	The microcapsules presented the particle size of 196.57 ± 1.46 μm and the encapsulation efficiency of 75.26 ± 1.95% (w/w); encapsulation markedly improved survival during gastrointestinal digestion (survival of encapsulated cells was 71.33 ± 0.99% (w/w) and 70.39 ± 0.86% (w/w) during SGF of 4 h and SIF of 4 h, separately, while that of free cells was only 45.45 ± 0.5% (w/w) and 8.59 ± 0.67% (w/w)); storage stability was higher in glass bottles compared to aluminum foil (increased 1.1-fold and 1.4-fold at 4 °C and 25 °C, respectively)	Zhang et al. (2022)
Lactobacillus plantarum 550	Soy protein isolate + peach gum	Spray-drying	Complex coacervation → spray-drying; soy protein isolate: peach gum = 3:1 (w/w); total solids = 5% (w/v); pH 3.75	Coacervates enhanced gastrointestinal resistance (7.88 log CFU/g after SGF and SIF of 6 h with survival loss of ∼1 log CFU/g, while free cells lost all their survival in only 0.5 h of GSF) and storage stability (survival loss of ∼0.1 log CFU/g at 2 weeks to ∼0.35 log CFU/g at 8 weeks and 4 °C); peach gum improved protection by reducing moisture (from 5.01 ± 0.01 to 4.14 ± 0.02 g/100g) and scavenging free radicals	Yao et al. (2023)
Limosilactobacillus reuteri	Kudzu starch + Hemp protein	Spray-drying	Complex coacervation → spray-drying; Kudzu starch: Hemp protein = 1:2 (w/w); pH 5.0	Encapsulation improved techno-functional properties (water and fat holding capacities) and provided enhanced probiotic survival during digestion (∼89%), storage of 30 days at 25 °C (83.2%), and heat treatment of 50 °C for 10 min (94.2%) compared with those of individual Kudzu starch- and Hemp protein-encapsulated cells	Hamdi et al. (2025)
Lactobacillus acidophilus	Whey protein isolate + octenyl succinic anhydride starch	Spray-drying	Complex coacervation → spray-drying; whey protein isolate: octenyl succinic anhydride starch = 2:1 (w/w); pH 4.0	Optimized formulations produced compact coacervates (survival of 95.94 ± 1.64% after spray-drying; water content of 4.68 ± 0.28%; water activity of 0.208 ± 0.005) that improved protection during digestion (9.16 ± 0.15 log CFU/g with survival loss less than 0.3 log CFU/g, while the survival of free cells decreased from 10.12 ± 0.18 log CFU/mL to 2.47 ± 0.19 log CFU/mL)	Liu et al. (2023)
Lactobacillus rhamnosus GG	Gelatin type B + gum arabic	Spray-drying vs freeze-drying	Complex coacervation → spray-drying or freeze-drying; gelatin: gum arabic = 1:1 (w/w); pH 3.75	The water content of microcapsules was less than 2.80%, and the encapsulation efficiency was higher than 93.21%; spray-drying enhanced bile salt resistance (110.89%) and low water activity (0.11) stability, while freeze-drying provided better stability under high water activity (0.75) and long-term storage at 25 °C and 4 °C for 8 weeks (survival loss of ∼2 log CFU/g with spray-drying and ∼1 log CFU/g with freeze-drying)	Bi et al. (2022)
Lactobacillus plantarum	Buckwheat protein + shellac	Freeze-drying	Complex coacervation → freeze-drying; buckwheat protein: shellac = 1–5 (w/w); pH 7.0	Hydrophobic interactions reinforced stability during freeze-drying (0.69-1.44 log N0/N; 1.67 log N0/N for the control of buckwheat protein) and digestion (survival loss of ∼0.2 log CFU/mL, while survival loss was 6.24 log CFU/mL for free cells), and improved colon adhesion (30.92% higher than free cells)	Guo et al. (2023)

Note: SGF is simulated gastric fluid. SIF is simulated intestinal fluid.

Formulation governs what drying can preserve because protein–polysaccharide composition determines the charge balance, viscoelasticity, and compactness of the wet coacervate before dehydration. Across reported systems, several recurring protective mechanisms can be identified. Electrostatic compaction in pea protein–sugar beet pectin, soy protein–peach gum, and whey protein–octenyl succinic anhydride starch systems promote denser matrices that restrict moisture and acid transport during drying and digestion (Liu et al., 2023; Qi et al., 2021; Yao et al., 2023). Hydrophobic reinforcement, as demonstrated by buckwheat protein–shellac coacervates, can further limit water and acid ingress, whereas rheological and water-holding regulation in kudzu starch–hemp protein systems can improve film formation and resistance to atomization and thermal stresses (Guo et al., 2023; Hamdi et al., 2025). Collectively, these findings indicate that drying performance depends less on identifying a universally optimal polymer pair than on matching specific matrix properties—such as compactness, hydrophobicity, and rheological resilience—to the dominant stresses imposed by the selected drying process. Importantly, greater matrix compactness is not invariably beneficial. A dense shell can improve resistance to moisture, oxygen, and gastric stress, but excessive compaction may impede rehydration, swelling, and probiotic liberation in the intestine. Likewise, hydrophobic reinforcement can improve moisture resistance but may reduce matrix wettability and alter release kinetics. Different protein sources also provide distinct formulation advantages rather than a simple hierarchy of performance: plant protein-based systems offer sustainability and tunable interfacial properties but may show greater structural heterogeneity, whereas animal protein-based systems can provide relatively predictable shell formation but may be less compatible with plant-based product positioning. Therefore, formulation optimization should target an application-specific balance among barrier strength, rehydration, gastrointestinal release, reproducibility, and compatibility with the intended food matrix rather than maximizing any single structural parameter.

Direct comparisons further demonstrate that spray- and freeze-drying confer different, rather than universally superior, forms of protection. Spray-dried coacervates frequently show better digestive resistance and stability under low water activity conditions, whereas freeze-dried coacervates generally provide better cellular preservation and may offer advantages for long-term storage (Bi et al., 2022). For example, Lactobacillus rhamnosus GG coacervates achieved >93% encapsulation efficiency and >96% survival under simulated gastrointestinal conditions with either drying route; yet spray-dried powders showed better bile salt resistance and greater stability under low water activity conditions, whereas freeze-dried powders provided better cellular preservation and showed advantages for long-term storage when moisture was appropriately controlled (Bi et al., 2022). Similar trade-offs have also been reported: the protein–pectin systems in Qi et al. (2021) favored spray-drying for post-digestive viability, while Lactobacillus plantarum LN66 coacervates demonstrated robust freeze-dried survival and benefited markedly from moisture-barrier packaging choices (Zhang et al., 2022). These comparisons indicate that neither drying route is intrinsically superior. Their relative advantages depend on whether the primary design objective is rapid powder production and low-water-activity stability or minimization of cellular injury during dehydration. Mechanistically, the different advantages of spray-drying and freeze-drying are determined by their distinct dehydration processes and the resulting matrix structures. Spray-drying rapidly removes water and promotes glass-state formation, generating a relatively dense matrix that limits the diffusion of bile salts, oxygen, and moisture, thereby improving gastrointestinal protection and stability under low water activity conditions. However, the accompanying thermal, shear, and osmotic stresses during atomization and dehydration may cause sublethal cellular damage, making the dried cells more dependent on strict moisture control during storage (Yin et al., 2022). In contrast, freeze-drying minimizes thermal stress through low-temperature and vacuum-assisted dehydration, thereby better preserving cellular integrity and metabolic activity. However, the porous structure generated after ice sublimation increases susceptibility to moisture uptake and oxygen diffusion, requiring appropriate protectants and moisture-barrier packaging for long-term storage. These structural differences also affect gastrointestinal release. Dense spray-dried matrices may delay probiotic exposure to gastric acid and bile salts, whereas porous freeze-dried matrices may hydrate and reconstitute more rapidly, potentially facilitating subsequent release. Consequently, drying conditions should be optimized against two competing objectives—upper-gastrointestinal protection and timely intestinal release—rather than viability retention alone (Bi et al., 2022; Qi et al., 2021).

Alternative drying technologies may address some limitations of conventional routes, although direct evidence in complex-coacervate probiotic systems remains limited. Vacuum drying provides a low-temperature and oxygen-limited environment that may minimize oxidative damage and preserve the functionality of sensitive probiotic cells and bioactive compounds (Kiepś and Dembczyński, 2022). Compared with conventional thermal drying, vacuum-assisted approaches can reduce heat exposure. However, relatively long drying times and challenges in achieving efficient moisture removal remain important limitations for industrial application. Fluidized bed granulation drying represents another promising strategy for probiotic powder production, particularly when improved flowability, particle uniformity, and additional coating functionality are desired (Kiepś and Dembczyński, 2022). By suspending particles in a controlled airflow, this technology enables efficient heat and mass transfer and allows simultaneous coating or layering of protective materials. For complex coacervation-based probiotic systems, fluidized bed drying may provide potential opportunities to construct additional protective barriers. However, optimization of airflow conditions, particle aggregation behavior, and mechanical stress is necessary to prevent disruption of fragile coacervate structures. Microwave-assisted vacuum drying, although not yet widely applied in probiotic encapsulation, represents an emerging drying approach with potential advantages in reducing drying time and thermal exposure through combined volumetric heating and reduced-pressure conditions (Dehghannya and Habibi-Ghods, 2025). This approach could be advantageous for future applications involving heat-sensitive probiotic carriers by shortening processing time and improving drying efficiency. Nevertheless, non-uniform microwave energy distribution, localized overheating, and difficulties in achieving consistent treatment at large scales remain major challenges requiring further investigation. Accordingly, these technologies should currently be regarded as promising process options rather than established alternatives to spray- or freeze-drying for coacervate-based probiotic delivery. Their practical value requires direct comparative studies using matched formulations and standardized endpoints, including probiotic survival, water activity, glass transition behavior, structural integrity, and long-term storage stability.

Drying performance should also not be interpreted independently of post-drying storage. For Lactobacillus plantarum LN66 microcapsules, storing powders in glass bottles improved viability 1.1-fold (4 °C) and 1.4-fold (25 °C) over aluminum foil, highlighting moisture ingress and headspace effects as hidden variables in “drying” performance (Zhang et al., 2022). This echoes the water-activity dependence seen by Bi et al. (2022) and indicates that apparent differences among drying strategies may partly reflect unstandardized moisture ingress, headspace oxygen, and packaging conditions rather than dehydration history alone. Residual moisture, water activity, and packaging barrier properties should therefore be routinely reported and controlled when benchmarking drying methods.

Different matrix combinations contribute distinct protective mechanisms that explain variations in probiotic survival across drying methods. Electrostatic compaction, as seen in whey protein isolate–octenyl succinic anhydride starch, soy protein isolate–peach gum, and pea protein isolate–sugar beet pectin systems, increases coacervate density and smoothness, thereby restricting mass-transfer pathways and reducing the penetration of moisture and gastrointestinal stressors (Liu et al., 2023; Qi et al., 2021; Yao et al., 2023). Hydrophobic reinforcement from buckwheat protein–shellac can reduce water and acid permeability and modify cell–matrix interactions, thereby improving probiotic protection and intestinal adhesion (Guo et al., 2023). Meanwhile, rheological tuning and enhanced holding capacity in kudzu starch–hemp protein coacervates strengthen film formation and droplet protection during atomization, buffering spray-drying stress and thermal challenge (Hamdi et al., 2025). Together, these mechanisms illustrate how material chemistry dictates coacervate behavior and why optimal ratios are highly formulation-specific.

Cross-study comparison is further complicated by differences in probiotic strains, formulation compositions, drying processes, and evaluation methods. Many studies vary pH set-points, salt levels, inlet/outlet spray-dryer temperatures, and feed solids without reporting full moisture/water-activity trajectories; yet these variables strongly confound viability. Future work should standardize (or fully disclose) drying thermograms, residual moisture, and water activity to enable fair cross-study comparisons. More fundamentally, short-term CFU measurements remain the dominant endpoint and provide limited information on why a formulation succeeds or fails. Few papers link powder glass transition, rehydration kinetics, and barrier durability to time-resolved survival in bile and pancreatic fluids. Incorporating permeability assays and microstructure–transport modeling would move the field from empirical recipes to predictive design. Scale-up considerations are underexplored. Atomization shear, nozzle type, and feed rheology in pilot-scale spray-dryers can shift survival outcomes relative to bench units. Similarly, primary drying shelf temperature and secondary drying in freeze-dryers need rational tuning against coacervate mechanics.

Adjunct strategies and material innovations show promise but need systematic evaluation. Antioxidant polysaccharides such as peach gum and hydrophobic polymers such as shellac can enhance probiotic stability by reducing oxidative damage, improving matrix cohesion, or limiting moisture and oxygen diffusion; however, their protective effects may vary depending on the drying-induced microstructure of coacervate matrices. For example, dense glassy structures generated by spray-drying may enhance the barrier function of hydrophobic or antioxidant components, whereas porous structures formed during freeze-drying may require additional moisture-control strategies to fully preserve their protective effects. Therefore, future studies should move beyond evaluating individual materials and establish material–process–storage relationships through factorial designs integrating matrix chemistry, drying conditions, and packaging strategies to distinguish additive, synergistic, or redundant effects. Although plant-derived systems may additionally meet clean-label and sustainability demands, their practical advantages should be evaluated together with batch reproducibility, scalability, and compatibility with the intended food matrix rather than inferred from material origin alone.

From an engineering perspective, no single drying route is universally optimal. Spray-dried compact coacervates may be advantageous when low water activity, powder handling, and resistance to gastrointestinal stresses are prioritized, whereas freeze-drying may be preferable for heat-sensitive probiotics provided that subsequent moisture and oxygen exposure are rigorously controlled. Ultimately, drying should be viewed as a structure-forming operation rather than a simple preservation step. Its effectiveness depends on matching formulation chemistry, charge balance, and rheology with dehydration conditions and post-drying moisture management. Establishing these formulation–process–structure–function relationships, together with standardized reporting and pilot-scale validation, will be essential for moving drying-assisted coacervation from empirical optimization toward rational engineering of probiotic powders.

3. Macro-scale dosage engineering: tableting as a secondary physical shield

Beyond powder engineering, macro-scale dosage design offers an additional protective layer. By integrating low-temperature drying with subsequent tableting, fragile coacervate microcapsules can be converted into dose-accurate, easy-to-handle solid forms that gain a second, macroscopic barrier against moisture, oxygen, and mechanical abrasion. Importantly, the engineering value of tableting lies not simply in converting coacervate powders into a solid dosage form, but in introducing an additional structural level at which mechanical protection, moisture transport, and gastrointestinal disintegration can be regulated. In a representative study, three nanochitin morphologies—chitin nanocrystals, nanofibers, and nanospheres—were coacervated with whey protein isolate, oven-dried at 42 °C, and compressed into tablets; tableting further improved structural integrity and protection compared with the powder alone (Luo et al., 2024). Mechanistically, nanochitin can densify and reinforce the coacervate shell via electrostatic and hydrogen-bond networks, while the tablet matrix limits particle-particle friction and provides geometric shielding during storage and gastrointestinal transit. These results suggest that the protective effect arises from hierarchical reinforcement: the coacervate shell protects individual probiotic-containing particles, whereas the tablet provides a secondary macro-scale barrier. However, evidence for this added benefit remains limited because relatively few studies have directly compared the same coacervate formulation before and after compression under otherwise matched conditions. Importantly, tableting provides not only an additional physical barrier for probiotic preservation but also an opportunity to regulate gastrointestinal release behavior. By controlling tablet disintegration, porosity, and coating characteristics, the macro-scale dosage form can complement the pH-responsive properties of the coacervate shell, enabling protection during gastric transit while facilitating probiotic release at intestinal sites (Wang et al., 2025d). This creates an important design trade-off: increasing tablet density and mechanical strength may improve physical protection but can simultaneously slow water penetration, microcapsule rehydration, and probiotic release. Therefore, tablet hardness should not be maximized independently of disintegration and release performance.

A critical appraisal of this hybrid route highlights both promise and caveats. Mild oven drying reduces thermal shock relative to spray-drying but often leaves higher residual moisture than freeze-drying, which may compromise long-term stability unless water activity is tightly controlled. The reported viability gains after tableting are encouraging (Luo et al., 2024), yet many tablet-related variables that strongly affect cells—compression force and speed, die wall lubrication, ejection force, and post-compression relaxation—are rarely disclosed (Wang et al., 2024a). Without standardized reporting of residual moisture/water activity, tablet porosity, hardness/friability, and disintegration time, cross-study inference remains limited (Chatzidopavlaki et al., 2024). Moreover, direct head-to-head comparisons of oven-dried vs spray- or freeze-dried coacervates taken through the same compaction protocol are still scarce, making it difficult to separate the benefits of the tablet from those of the drying history and matrix chemistry. Consequently, improved viability in a final tablet cannot automatically be attributed to compression itself; drying history, residual moisture, coacervate mechanics, and tablet architecture are important confounding factors that should be experimentally separated.

From a formulation perspective, tableting introduces several coupled design trade-offs rather than independent optimization variables. (i) Particle integrity under compression. Coacervate microcapsules can fracture or delaminate under axial loads; cushioning excipients (e.g., plastically deforming fillers) and low-shear dry granulation can mitigate damage, whereas brittle fillers or aggressive wet granulation may be detrimental (Wang et al., 2025). Thus, increasing compaction pressure may strengthen the tablet as a whole while simultaneously damaging the micro-scale carriers it is intended to protect. (ii) Moisture management. Hygroscopic binders and super-disintegrants speed rehydration but can raise water activity during storage; selecting low-hygroscopic diluents and using desiccant packaging or high-barrier blisters helps preserve a glassy state (Yee et al., 2024). This illustrates a second trade-off between storage stability and rapid rehydration. (iii) Lubrication vs rehydration. Hydrophobic lubricants improve manufacturability but can impair tablet wetting and slow capsule rehydration; minimal, well-distributed lubrication balances these trade-offs (Zheng et al., 2024). (iv) Release programming. The coacervate shell provides intrinsic pH-responsive protection, while the tablet layer offers an additional engineering tool to regulate disintegration timing and gastrointestinal transit. Enteric coatings or tailored tablet architectures can delay exposure of coacervates to gastric conditions and promote release at intestinal sites (Wan et al., 2025). However, excessive retardation at both the tablet and coacervate levels could delay probiotic liberation beyond the desired intestinal region, emphasizing the need to coordinate rather than independently maximize the barrier properties of the two structural levels. (v) Functional excipients. Prebiotic fillers can serve as binders while supporting probiotic growth post-release, but their fermentability and moisture uptake must be managed (Zhang et al., 2025). Taken together, these considerations show that successful tableting requires simultaneous optimization of microcapsule mechanics, tablet manufacturability, moisture stability, and disintegration rather than simply maximizing tablet strength or probiotic survival after compression.

Current evidence is therefore insufficient to establish whether tableting provides a general advantage over well-designed coacervate powders. Future studies should prioritize matched factorial comparisons that independently vary nano-reinforcers, coacervate composition, drying mode, and compaction parameters. Mechanistic evaluation should also extend beyond colony-forming units to include microcapsule fracture before and after compression, acid/bile permeability, tablet porosity and mechanical properties, water and oxygen transport, and time-resolved disintegration and probiotic release. Pilot-scale roller compaction and high-speed tableting are particularly important because compression dwell time, ejection stress, and powder flow behavior can differ substantially from laboratory-scale conditions. Finally, in vivo gastrointestinal localization and colonization studies are required to determine whether the secondary tablet barrier produces functional benefits beyond those already provided by the coacervate shell.

Overall, coacervate-to-tablet processing offers a rational route for integrating micro-scale encapsulation with macro-scale dosage engineering, but the additional structural level introduces both protection and resistance to release. Its value therefore depends on coordinating coacervate mechanics and drying history with compression behavior, moisture management, tablet disintegration, and gastrointestinal release. Rather than treating tableting as an inherently beneficial secondary shield, future work should establish when the macro-scale barrier provides a measurable advantage over the coacervate powder alone.

4. Hierarchical interfacial engineering: emulsion-templated multi-compartment architectures

To achieve hierarchical interfacial engineering, emulsion templating is married with complex coacervation to create multi-compartment carriers that partition probiotics into protected inner domains while building a charge-balanced outer shell. Two architectures have been reported: (i) pre-emulsified W/O systems, in which a protein-stabilized oil layer (often a high-melting-point or phospholipid-rich fat) surrounds an aqueous probiotic suspension and subsequently recruits a polyelectrolyte shell via coacervation; and (ii) W₁/O/W₂ double emulsions, where the inner aqueous phase carries the cells, the oil phase modulates heat and mass transfer, and the external aqueous phase supplies oppositely charged polymers for coacervate deposition. Unlike simple coacervates, these architectures introduce an additional oil–water interface and spatially separate probiotic cells from the external environment, allowing protection and release to be regulated at multiple structural levels. Reported improvements in survival during drying and gastrointestinal transit therefore arise not from coacervation alone, but from coupled effects of compartmentalization, interfacial composition, oil-phase properties, and coacervate-shell formation. Beyond improving probiotic survival, emulsion templating introduces spatial control over probiotic localization and shell organization, providing a means to balance upper-gastrointestinal protection with subsequent intestinal release rather than simply maximizing barrier strength.

The added value of emulsion templating is illustrated by several structurally distinct systems. A preformed W/O interface can act as a thermal and diffusional buffer during dehydration: shortening oils with elevated melting points reduced the effective heat exposure area of cells during spray-drying and improved post-drying/storage survival; as shown in Fig. 2A, whey protein isolate first formed the W/O emulsion, and gum arabic–driven coacervation captured the droplets and cells in one step (Yin et al., 2024). In double emulsions, as shown in Fig. 2B, gelatin–carboxymethyl cellulose shells built at pH ≈ 3.5 around W₁/O/W₂ droplets achieved high encapsulation and controlled intestinal release; placing retinyl palmitate in the oil layer and keeping the probiotic suspension in W₁ leveraged both hydrophobic and electrostatic barriers (Xu et al., 2025). The same gelatin–carboxymethyl cellulose design demonstrated in vivo efficacy: microcapsules mitigated dextran sodium sulfate–induced colitis, increased fecal short-chain fatty acids, and shifted microbiota (notably, Muribaculaceae), linking hierarchical structure to function beyond colony counts (Zhu et al., 2024). Adding krill oil to the middle oil layer further compacted the coacervate shell through phospholipid–gelatin interactions, reduced water loss and freeze-drying injury, and delivered dose-dependent gains in stress protection (Chen et al., 2025). Taken together, these studies indicate that the oil phase should not be regarded merely as an inert physical barrier. Depending on its melting behavior and molecular composition, it can regulate heat and mass transfer, participate in interfacial interactions, and alter subsequent coacervate-shell organization. However, because these studies differ in probiotic strain, oil composition, emulsion architecture, drying route, and evaluation endpoints, their reported survival gains cannot be directly attributed to a single structural variable.

Fig. 2.

Preparation diagram of complex coacervation combined emulsion technology. A, water-in-oil emulsion (Yin et al., 2024). B, double emulsion (Xu et al., 2025). PGRP is polyglyceryl-6 polyricinoleate. GEL is Gelatin. CMC is carboxymethyl cellulose.

Across these studies, three recurring interfacial mechanisms provide a more useful basis for comparison than absolute survival values. (i) Interfacial rheology and binding: protein-stabilized W/O interfaces (whey protein isolate; gelatin) present reactive groups that promote polyelectrolyte complexation and yield smoother, denser shells. (ii) Oil-phase functionality: high–melting–point fats and phospholipid-rich oils (shortening; krill oil; retinyl palmitate carriers) can modify heat and moisture transport around probiotic cells and, where specific molecular interactions occur, contribute to shell compaction. (iii) Hierarchical compartmentalization: W₁ isolates probiotics from the external digestive milieu during gastric transit, while the surrounding oil phase and coacervate shell provide sequential diffusion barriers. Under intestinal conditions, pH-triggered changes and matrix relaxation loosen the gelatin/carboxymethyl cellulose complex, thereby synchronizing probiotic release with the target intestinal environment (Xu et al., 2025; Zhu et al., 2024). These mechanisms are complementary but not necessarily additive: increasing oil-phase hydrophobicity or shell compactness may strengthen gastric protection while simultaneously slowing hydration and intestinal release. Thus, the relevant design objective is not maximal interfacial resistance, but an architecture that provides sufficient sequential barriers during processing and gastric transit while retaining responsiveness under intestinal conditions.

Despite promising outcomes, emulsion–coacervate hybrids face instability constraints uncommon in simple coacervates. Double emulsions are vulnerable to coalescence, swelling, and osmotic pumping between W₁ and W₂; without osmolality matching and low–hydrophilicity emulsifiers (e.g., polyglycerol polyricinoleate), W₁ leakage can precede shell formation, eroding yield and uniformity (Kim et al., 2025; Luo and Wei, 2023; Negro et al., 2025). Formation of the coacervate at low pH can displace interfacial proteins/surfactants, alter droplet Laplace pressure, and trigger bridging flocculation (Agalya et al., 2023; Edwards et al., 2023); robust interfacial rheology (interfacial storage modulus) should be confirmed before shell deposition (Ma et al., 2025; Tao et al., 2024; Zhao et al., 2025). Oil phases bring oxidation risk (krill oil; retinyl carriers), potentially generating peroxides that undermine probiotic membranes unless antioxidant systems and oxygen-barrier packaging are specified (He et al., 2025; Ngamekaue et al., 2024; Shi et al., 2024). These limitations illustrate an important trade-off introduced by hierarchical architectures: adding interfaces can provide additional protection and functional control, but each additional interface also creates another potential site for physical destabilization, molecular displacement, or oxidative deterioration. Therefore, increasing structural complexity does not necessarily translate into greater probiotic stability. Methodologically, many reports do not disclose droplet-size distributions (D_[3,2], D_[4,3]) before and after coacervation, interfacial viscoelasticity, osmotic gradients, or water-activity trajectories during drying—gaps that hinder causal inference and scale-up. Finally, relative to spray-dried emulsions, freeze-dried double emulsions can retain higher porosity unless secondary drying and packaging are optimized; thus, gains seen in freeze-drying studies (e.g., with krill oil) should be interpreted alongside moisture and oxygen control (Chen et al., 2025b). More fundamentally, most studies optimize emulsion formulation, coacervation conditions, and drying parameters simultaneously, making it difficult to distinguish whether improved survival originates primarily from compartmentalization, oil chemistry, shell densification, or the drying process itself. The confounding limits direct cross-study comparison and prevents robust structure–function relationships from being established.

To move from recipe-based success to predictive engineering, studies should (i) co-report interfacial rheology, droplet-size distributions pre/post coacervation and post-drying, osmolality of W₁ and W₂, and water activity through processing; (ii) test permeability of the final microcapsules to acid and bile and quantify oxygen/water vapor transmission when oil phases are present; (iii) use factorial designs that independently vary oil chemistry (melting point, phospholipid content), emulsifier type, coacervate pair and pH window, and drying/packaging; and (iv) link structure to function via time-resolved digestion assays and, where relevant, in vivo colonization and efficacy. Such designs would help separate the individual and interactive contributions of emulsion templating and coacervation, allowing the field to determine when these two technologies are genuinely synergistic rather than simply additive.

Overall, emulsion-assisted complex coacervation provides a structurally rational route to protect probiotics: the oil layer shapes thermal and moisture fluxes and can chemically reinforce the shell; the coacervate adds pH-responsive gating; and the hierarchical architecture provides spatial and temporal control over gastrointestinal release, aligning probiotic liberation with intestinal conditions. However, the available evidence does not support a universally optimal emulsion architecture or oil phase. Instead, performance depends on matching interfacial rheology, oil chemistry, osmotic balance, coacervate assembly, and downstream drying conditions to the intended probiotic strain and delivery objective. The above case studies collectively argue that success hinges on interfacial design as much as polymer chemistry. Addressing the emulsion instability and oxidation challenges noted here will be essential for industrial translation, as further elaborated in Section 8.

5. Mechanical reinforcement and barrier tuning: crosslinking strategies for shell densification

To mechanically reinforce the coacervate shell and tune its barrier properties, crosslinking introduces covalent or ionic bonds that strengthen the polyelectrolyte network and enhance the physical stability of probiotic carriers (Wang et al., 2025b; Yang et al., 2026). Shell densification via crosslinking reduces permeability to oxygen, acids, and bile salts, extends storage stability, and improves probiotic survival across processing, digestion, and diverse food matrices (Gharibzahedi and Altintas, 2024; Laurujisawat et al., 2025; Zang et al., 2025). However, crosslinking also introduces critical trade-offs: excessive rigidity may impair release, certain crosslinkers raise safety or sensory concerns, and the interplay between coacervate assembly and crosslink formation can yield unexpected weaknesses if not carefully tuned. Therefore, crosslinking should not be considered solely as a shell-strengthening strategy. Rather, controlled network formation provides an additional approach to regulate permeability, degradation behavior, and gastrointestinal release profiles. By adjusting crosslink density and chemistry, crosslinked coacervates can be designed to maintain integrity during gastric transit while enabling subsequent probiotic release under intestinal conditions.

Several types of crosslinkers have been investigated for probiotic coacervate systems, including ionic crosslinkers, enzymatic crosslinkers, and naturally derived molecular crosslinkers. The selection of crosslinkers determines not only the degree of network reinforcement but also the balance between mechanical stability, release behavior, and regulatory acceptance. Representative examples of these crosslinking strategies in combination with complex coacervation are summarized in Table 2, highlighting how different crosslinkers modulate coacervate structure and probiotic protection performance. Rather than providing interchangeable routes to network reinforcement, these crosslinker classes operate through different molecular mechanisms and consequently introduce different performance constraints. Among these strategies, ionic crosslinking using sodium tripolyphosphate (TPP) is one of the most commonly investigated approaches, effectively bridging cationic chitosan chains and producing tighter networks (Bhatia et al., 2024; J. Hu et al., 2024; Tian et al., 2024). Similarly, calcium ions can promote ionic interactions with negatively charged polysaccharides such as alginate and pectin, increasing network density and improving mechanical strength. However, excessive ionic crosslinking may generate overly compact structures that restrict swelling, delay probiotic release, and reduce accessibility of encapsulated cells. In gum arabic–chitosan and gum arabic–trehalose–chitosan coacervates, tripolyphosphate crosslinking significantly improved encapsulation efficiency after freeze-drying and prolonged probiotic survival across a range of storage temperatures, with trehalose further buffering dehydration stress (Barajas-Álvarez et al., 2024). This result also illustrates an important interpretive limitation: improved survival in a crosslinked formulation cannot necessarily be attributed to crosslinking alone when additional protectants such as trehalose simultaneously stabilize proteins and cellular membranes. Factorial designs that independently vary crosslinker concentration and adjunct protectants are therefore needed to distinguish network reinforcement from complementary cellular protection. Casein–chitosan coacervates subjected to crosslinking with tripolyphosphate, calcium salts, or vanillin demonstrated that the choice of crosslinker matters: vanillin not only provided excellent stability (up to 260 days) but also conferred immune-modulatory properties in vivo, while mineral crosslinkers offered moderate protection (Peñalva et al., 2023). Although vanillin provides additional antioxidant functionality and can contribute to network stabilization, its interactions with proteins and potential effects on sensory properties require careful optimization. Importantly, this comparison suggests that crosslinker performance should not be evaluated solely from storage survival. Naturally derived molecular crosslinkers may provide functions beyond mechanical reinforcement, whereas ionic crosslinkers primarily alter network organization and permeability. Consequently, comparisons based only on CFU retention may overlook differences in release behavior, sensory compatibility, and biological functionality. Enzymatic crosslinking with transglutaminase represents another avenue. In fruit juice matrices, it stabilized Lactobacillus acidophilus for more than two months, particularly in acidic orange juice, proving its applicability for functional beverages (da Silva et al., 2021). Compared with ionic crosslinkers, enzymatic approaches offer milder reaction conditions and improved compatibility with food applications. However, enzyme cost, reaction control, and batch-to-batch variability remain important limitations for industrial implementation. More importantly, enzymatic crosslinking illustrates why stronger network formation cannot automatically be equated with better probiotic protection. When soy protein isolate–based coacervates were combined with transglutaminase or pectin, structural density strongly influenced outcomes: while pectin-based coacervates provided robust protection during spray-drying and gastrointestinal digestion, the combination of coacervation and enzymatic crosslinking unexpectedly weakened protection due to reduced crosslinking efficiency and a less compact shell (Hu et al., 2023). This counterintuitive result provides particularly important evidence for understanding the interaction between coacervation and crosslinking. Coacervation and enzymatic crosslinking do not necessarily act additively. At the molecular level, complex coacervation relies primarily on electrostatic association and subsequent molecular rearrangement between oppositely charged biopolymers, whereas transglutaminase-mediated crosslinking requires sufficient accessibility and mobility of reactive protein residues. Indeed, the spatial organization and compactness of biopolymer networks can restrict transglutaminase diffusion and accessibility, thereby decreasing the rate of enzymatic crosslinking (Grossmann et al., 2017). Coacervate formation may therefore restrict enzyme accessibility to potential crosslinking sites. Conversely, transglutaminase-induced protein crosslinking can alter protein conformation and protein–polysaccharide interactions, including complexation affinity, stoichiometry, and particle formation (Madsen et al., 2022), suggesting that premature or excessive crosslinking may interfere with optimal coacervate assembly. Such bidirectional interference may consequently reduce effective crosslinking or generate heterogeneous and less compact networks, as observed by Hu et al. (2023). Thus, combining two individually beneficial stabilization strategies does not guarantee synergy; their effectiveness depends on whether the molecular requirements for coacervation and crosslink formation are compatible in both space and time. From a formulation perspective, these negative trade-offs may be mitigated by controlling the sequence and timing of coacervation and crosslinking and systematically optimizing pH, polymer ratio, enzyme dosage, reaction time, and crosslinking degree rather than simply maximizing network density. Factorial experimental designs can further distinguish additive, synergistic, and antagonistic interactions among these variables. In parallel, molecular dynamics and coarse-grained simulations could help predict polymer–polymer interactions, conformational changes, and accessibility of reactive sites, thereby identifying conditions under which coacervation and crosslinking become complementary rather than competitive. This predictive approach is further discussed in Section 8.5.

Table 2.

Case studies of complex coacervation combined with crosslinking for probiotic encapsulation.

Probiotic strain	Coacervate matrix	Crosslinker	Integration mode/Key parameters	Key advantages	Limitations	Reference
Lacticaseibacillus rhamnosus	Gum arabic–chitosan/gum arabic–trehalose–chitosan	Sodium tripolyphosphate	Complex coacervation → ionic crosslinking; gum arabic: chitosan = 5:1 (w/w); final pH 4.60-5.64 depending on sodium tripolyphosphate level	Improved encapsulation efficiency after crosslinking (from 26% to 110%); trehalose enhanced cryoprotection; prolonged survival during storage (7.69 log CFU/g with survival loss of 1.34 log CFU/g, while free cells inactivated at 20 °C for 12 weeks); improved viability during digestion (survival loss of 2.37 log CFU/g, while for free cells, it was 4.08 log CFU/g) with altered metabolite production	Ionic crosslinking depends strongly on the chitosan ratio and pH; potential regulatory concerns for phosphate additives	Barajas-Álvarez et al. (2024)
Lactobacillus plantarum (CECT 220 and WCFS1 strains)	Sodium caseinate –chitosan	Sodium tripolyphosphate, calcium salts, or vanillin	Complex coacervation → crosslinking reinforcement; sodium caseinate: chitosan = 15.6:1 (w/w); pH 5.5–6.0 (chitosan phase)	High bacterial loading (bacteria loading of about 11 log CFU/g with a mean size of about 7.5 μm) and long-term stability at 25 °C/60% RH (without significant modification of the payload in 260 days); vanillin-crosslinked microparticles maintained viability for months and induced Th1/Th17 immune responses in vivo	Stability and efficacy vary with crosslinker; crosslinkers pose trade-offs between functionality and consumer acceptance	Peñalva et al. (2023)
Lactobacillus acidophilus LA-02	Gelatin-gum arabic	Transglutaminase	Complex coacervation → enzymatic crosslinking; gelatin: gum arabic = 1:1 (w/w); pH 4.0; transglutaminase 2.5 and 5.0 U/g of protein, crosslinking at 25 °C for 15 h	Prolonged probiotic survival in acidic fruit juices; stable in orange juice at 4 °C for 63 days (from 8.64 to 8.12 log CFU/mL, while for free cells, survival decreased from 9.88 to 5.41 log CFU/mL); suitable for functional beverages	Enzymatic reactions sensitive to process conditions; variability in different juice matrices; requires controlled enzyme dosing	da Silva et al. (2021)
Lactobacillus plantarum 550	Soy protein isolate alone or with pectin	Transglutaminase	Complex coacervation with/without enzymatic crosslinking comparison; soy protein isolate: pectin = 8:1 (w/w); transglutaminase 12.5 U/g; crosslinking at 40 °C for 1 h	Pectin–soy protein isolate coacervates provided strong protection during spray-drying (from 9.65 to 9.35 log CFU/g, while for free cells, survival decreased from 10.24 to 0 log CFU/mL), digestion (only 0.43 log CFU/g lost, while free cells were inactivated), and storage at 4 °C for 4 weeks (9.25 log CFU/g with survival loss of ∼0.05 log CFU/g).	Combination of coacervation and enzymatic crosslinking reduced protection due to incomplete crosslinking and a weaker structure	Hu et al. (2023)

Across the available studies, the protective effects of crosslinking can be interpreted through three interrelated mechanisms rather than as isolated formulation-specific outcomes: (i) Network densification, which reduces porosity and improves resistance to acid and bile penetration; (ii) Moisture and oxygen control, limiting diffusion into the capsule and delaying oxidative or desiccation damage during storage; (iii) Matrix-specific interactions, such as trehalose's role in stabilizing proteins and membranes during dehydration, or vanillin's ability to form reversible covalent bonds and impart antioxidative effects. These mechanisms explain why crosslinking can simultaneously affect processing resistance, storage stability, and gastrointestinal behavior, but they also reveal a common protection–release trade-off. Moderately reinforced networks can provide gastric protection while allowing gradual swelling, degradation, or diffusion-driven release in intestinal environments. In contrast, excessive crosslinking may generate overly compact structures that restrict probiotic liberation and compromise functional delivery. At still higher or poorly controlled crosslinking levels, increased brittleness may also undermine the intended benefit by promoting fracture during drying, handling, or storage. Thus, crosslink density should be treated as a tunable transport and mechanical parameter rather than simply an indicator of successful network formation.

Crosslinking has the advantage of extending coacervation into demanding matrices such as juices, which present acidity, osmotic stress, and high water activity. More importantly, crosslinking provides an additional strategy for engineering gastrointestinal release by tuning shell permeability, degradation behavior, and mechanical stability. It also opens doors to targeted delivery: for instance, vanillin-crosslinked casein–chitosan microparticles reached the distal ileum and colon and stimulated Th1 and Th17 responses, linking structural reinforcement to immunological outcomes (Peñalva et al., 2023). This finding highlights that crosslinking can bridge structural protection and functional delivery by maintaining carrier integrity during upper gastrointestinal transit while enabling biological activity at specific intestinal sites. However, this type of in vivo evidence remains uncommon, and most available studies do not establish a quantitative relationship among crosslinking degree, gastrointestinal degradation, probiotic release, and subsequent biological function. Therefore, targeted delivery should currently be interpreted as a promising capability of selected crosslinked systems rather than a general property of crosslinked coacervates. Crosslinker selection also introduces distinct practical trade-offs. Ionic crosslinkers such as TPP and calcium salts provide effective network reinforcement, but their concentration-dependent effects on swelling and release require optimization. Enzymatic crosslinkers such as transglutaminase offer food-compatible and relatively mild alternatives but face challenges related to enzyme cost, reaction control, substrate accessibility, and process consistency. Naturally derived molecular crosslinkers such as vanillin may provide additional antioxidant or biological functionality, but sensory effects, formulation-specific interactions, and scalability require further evaluation. Accordingly, no crosslinker class can be considered universally superior. Selection should be based on the required balance among network strength, release responsiveness, food compatibility, processing robustness, sensory quality, and regulatory considerations.

The current literature demonstrates the protective power of crosslinking, but systematic comparisons across different crosslinkers, coacervate chemistries, and food applications remain sparse. Many studies emphasize encapsulation efficiency and short-term CFU counts, while neglecting release kinetics, structural dynamics, and metabolite-level impacts during digestion. Few report detailed physicochemical parameters such as crosslinking degree, porosity, or permeability, limiting mechanistic understanding. This methodological heterogeneity also makes apparently superior crosslinkers difficult to compare across studies because differences in probiotic strain, polymer pair, crosslinker concentration, drying history, storage conditions, and digestion protocols may contribute as much to the outcome as crosslinker chemistry itself. Moreover, the synergy or antagonism between coacervation and crosslinking deserves closer scrutiny: as Hu et al. (2023) showed, combining two stabilization strategies can paradoxically weaken protection if they interfere at the molecular level. Future research should adopt factorial designs, include real food systems with industrially relevant processing, and prioritize biocompatible crosslinkers to meet both performance and regulatory demands. Standardized reporting of crosslinking degree, network porosity, permeability, mechanical properties, swelling/degradation kinetics, and time-resolved probiotic release would be particularly valuable for establishing transferable structure–function relationships.

Combining complex coacervation with crosslinking expands the functional space of probiotic encapsulation by reinforcing shell integrity and enhancing long-term stability. Although trehalose-assisted tripolyphosphate, vanillin, and transglutaminase have all shown promising results for improving probiotic protection, their effectiveness depends on crosslinker chemistry, probiotic characteristics, coacervate composition, and processing conditions. More importantly, the available evidence indicates that crosslinking should be viewed as a means of tuning network mechanics and mass transport rather than simply increasing shell strength. The next generation of research must refine crosslinker selection, optimize the balance between protection and release, and establish rational relationships between crosslinking degree, gastrointestinal degradation, and targeted release behavior, extending evaluation beyond laboratory media into real foods and in vivo models to fully harness the promise of crosslinked coacervates in probiotic delivery.

6. Physical field-assisted assembly: low-frequency static magnetic field as a clean-label modulator

Among various physical field-assisted processing strategies, low-frequency static magnetic field (LF-SMF) has received the most attention for modulating complex coacervation-based probiotic encapsulation. Although other physical fields, including ultrasound, electric fields, pulsed electric fields, and electromagnetic radiation, have shown potential in food processing and biomaterial modification, their application in combination with complex coacervation for probiotic delivery remains largely unexplored. Importantly, even for LF-SMF, direct evidence in probiotic complex-coacervation systems remains limited. Therefore, its current significance lies primarily in its potential as a process-level modulator rather than as an established encapsulation technology.

As a clean-label physical modulation tool, LF-SMF offers a non-chemical lever for enhancing coacervate assembly. LF-SMF refers to a weak, non-ionizing magnetic field, typically within 1--300 mT, characterized by quasi-static behavior (Oravec et al., 2021). Unlike high-frequency electromagnetic fields, LF-SMF does not generate significant heating but can influence ion transport, hydration shells, and molecular orientation in biological and polymeric systems (Krylov and Osipova, 2023; Radil et al., 2024). Studies in protein and biopolymer systems suggest that LF-SMF may alter electrostatic interactions, protein conformation and rearrangement, and the packing of biopolymer aggregates (Jiang et al., 2025; Yang et al., 2024; Yuan et al., 2025). These structural modifications may contribute to enhanced coacervate integrity, thereby improving encapsulation efficiency and probiotic survival during processing, storage, and gastrointestinal digestion. Beyond improving structural protection, LF-SMF-induced changes in polymer packing and network organization may also influence mass transfer properties, including water penetration, matrix swelling, and probiotic release behavior during gastrointestinal digestion. However, these proposed relationships should be interpreted cautiously because direct causal links among magnetic-field exposure, molecular rearrangement, coacervate densification, and probiotic protection have not yet been systematically established across different protein–polysaccharide systems.

In a recent study, as shown in Fig. 3, highland barley polysaccharides were combined with casein under acid-induced coacervation, with the process further modified by LF-SMF exposure (Wang et al., 2025a). The polysaccharides promoted casein denaturation and aggregation, while the magnetic field amplified these changes, leading to greater coacervate yield, larger particle size, higher viscoelasticity, and a more compact microstructure (Wang et al., 2025a). As a result, the modified coacervates showed superior encapsulation efficiency and provided stronger protection to probiotics under adverse conditions. This study provides proof-of-concept that a physical field can alter coacervate assembly without introducing an additional chemical crosslinker. Nevertheless, because polymer composition, structural properties, and probiotic performance changed within the same formulation system, the relative contributions of altered protein conformation, electrostatic association, particle aggregation, and network densification to the observed protection remain difficult to separate. Although the precise mechanisms remain under investigation, LF-SMF has been proposed to influence electrostatic interactions, hydration shells, and molecular orientation. Changes in ζ-potential and hydration dynamics could facilitate charge neutralization during acidification and thereby promote aggregation. Similarly, magnetic-field-induced changes in molecular orientation have been proposed as a possible contributor to tighter packing and stronger viscoelastic properties (Chen et al., 2024, Chen et al., 2024; He et al., 2025, He et al., 2025; Jiang et al., 2024). These mechanisms are mechanistically plausible but should not yet be treated as universally established for probiotic coacervates, because magnetic responsiveness is likely to depend strongly on protein conformation, polysaccharide chemistry, ionic environment, and field conditions. Thus, the central mechanistic question is not simply whether LF-SMF strengthens a coacervate, but under which material and processing conditions field-induced molecular changes are sufficiently large to produce a functionally relevant barrier effect.

Fig. 3.

Schematic diagram of complex coacervation combined with low-frequency static magnetic field technology (Wang et al., 2025a). CS is casein. HBP is a highland barley polysaccharide.

The main advantage of LF-SMF is that it strengthens coacervates without introducing additional chemicals or modifying the formulation environment, making it appealing for clean-label applications. In principle, physical-field modulation could also reduce dependence on chemical or enzymatic crosslinkers when comparable structural reinforcement can be achieved. However, whether LF-SMF can reduce the polymer or crosslinker concentration required for probiotic protection has not yet been established systematically and should therefore be considered a potential rather than demonstrated advantage. Furthermore, as a process-level intervention, LF-SMF is tunable through field strength, exposure duration, and treatment stage, providing an additional processing variable for regulating coacervate assembly. These potential advantages must be weighed against three major limitations. First, reproducibility is limited by incomplete reporting of field parameters such as strength, frequency content, exposure time, and coil configuration. Without standardized field characterization, differences among studies cannot readily be attributed to magnetic-field intensity itself. Second, responsiveness is highly dependent on the chemistry of the protein–polysaccharide pair. While casein–barley polysaccharide systems respond strongly, other proteins (e.g., globular whey proteins) may show weaker effects. This material dependence limits direct extrapolation from a single coacervate system and reinforces the need to distinguish general magnetic-field effects from formulation-specific responses. Third, scale-up is not trivial. In small-scale systems, magnetic field intensity and exposure conditions can be precisely controlled because of the short distance between the coil and treatment zone, whereas maintaining a homogeneous field in large processing volumes becomes increasingly difficult because field distribution depends on reactor geometry, coil arrangement, treatment distance, and material properties. Consequently, a laboratory-scale increase in coacervate yield or structural compactness does not necessarily predict equivalent performance in industrial reactors. Industrial implementation will therefore require appropriate coil design and field monitoring to ensure consistent treatment throughout the processing volume. Energy consumption is another practical consideration: although LF-SMF is a non-thermal technology, continuous large-scale operation may require additional electrical input. Its industrial value should therefore be assessed on the basis of the functional gain achieved per unit of processing complexity and energy input, rather than on its non-thermal character alone. Techno-economic evaluation, together with energy-efficient field configurations, optimized exposure durations, and intermittent treatment strategies, will be needed to determine whether the structural benefits justify the additional processing requirements.

The limited evidence base also constrains mechanistic comparison with established stabilization approaches such as crosslinking. Crosslinking directly modifies network connectivity through ionic, enzymatic, or molecular interactions, whereas LF-SMF is expected to act indirectly by altering molecular organization and assembly behavior. This distinction may offer an advantage when chemical modification is undesirable, but it also makes the resulting structural response more formulation-dependent and potentially less predictable. Moreover, denser packing is not inherently beneficial: as discussed for crosslinked systems, excessive network compaction could improve gastric resistance while simultaneously restricting hydration, swelling, and intestinal probiotic release. Thus, LF-SMF optimization should target functional barrier properties rather than maximal particle size, viscoelasticity, or structural compactness.

To establish the practical feasibility of LF-SMF-assisted coacervation, future studies should systematically evaluate the effects of field strength, exposure timing, polymer type, and formulation conditions on coacervate assembly. Standardized reporting of particle size, rheology, ζ-potential, and moisture content before and after field treatment will allow cross-study comparison. These measurements should be complemented by parameters that directly connect structural modification to probiotic delivery, including acid and bile permeability, water uptake, swelling or disintegration kinetics, and time-resolved probiotic release. Factorial experiments separating magnetic-field conditions from polymer composition and acidification parameters would further help determine whether observed effects arise from the field itself or from interactions among processing variables. In addition, combining LF-SMF with other stabilization strategies such as ionic or enzymatic crosslinking may create synergistic effects, provided that the resulting changes in network density and release properties are carefully balanced. However, given the antagonistic interactions observed when multiple stabilization mechanisms are combined in other coacervate systems, such combinations should be experimentally demonstrated rather than assumed to be synergistic. Importantly, extending validation to real food matrices and in vivo models will be critical for demonstrating practical benefits beyond laboratory settings.

In summary, LF-SMF offers a clean and innovative lever for physically modulating the assembly of probiotic complex coacervates. By altering protein–polysaccharide interactions and network organization, LF-SMF may enhance microstructural integrity and probiotic survival and may provide opportunities to regulate hydration, permeability, and gastrointestinal release behavior without introducing additional chemical crosslinkers. However, current evidence is too limited to define a universal field–structure–function relationship, and the mechanisms responsible for improved probiotic protection remain partly inferential. Its future value will therefore depend on demonstrating reproducible relationships among field parameters, molecular rearrangement, network structure, and probiotic performance across multiple biopolymer systems, followed by validation under industrially relevant processing conditions. LF-SMF should thus be regarded as a promising but still experimental frontier in probiotic delivery system design.

7. Functional synergy through co-delivery: Co-encapsulation of probiotics with bioactive phytochemicals

To unlock functional synergy through co-delivery, complex coacervation has been increasingly applied to co-encapsulate probiotics with bioactive compounds. This strategy leverages electrostatic interactions between oppositely charged biopolymers to construct a robust protective matrix that shields probiotics from environmental stresses during processing, storage, and gastrointestinal transit, while simultaneously delivering bioactive compounds—such as polyphenols, phytosterols, and omega-3 oils—to the intestinal environment where they exert their health benefits (Ma et al., 2025). This dual protection enhances both probiotic viability and bioactive bioavailability (Bhutto et al., 2025). Successful co-delivery requires not only preserving both components during processing and storage but also achieving coordinated gastrointestinal release. The coacervate matrix should protect probiotics and bioactives during gastric transit while allowing controlled liberation, transformation, and absorption under intestinal conditions. Accordingly, the value of co-delivery should not be judged solely by increased probiotic survival or bioactive encapsulation efficiency. Rather, it depends on whether the two components provide complementary functions without compromising each other's stability, activity, or intestinal availability.

Recent co-encapsulation studies of probiotics and bioactive compounds based on complex coacervation are summarized in Table 3. Rather than indicating a single universal mechanism of synergy, these studies suggest that bioactives contribute to probiotic protection through at least three interrelated routes: physical barrier reinforcement by hydrophobic components, biochemical protection against cellular stress, and modulation of gastrointestinal release and bioaccessibility. Lipid-based bioactives primarily illustrate the first mechanism. The co-encapsulation of Lactobacillus plantarum with phytosterols using whey protein isolate and gum arabic through complex coacervation significantly improved probiotic viability compared with separate encapsulation methods (Sharifi et al., 2021). Freeze-dried microcapsules exhibited higher encapsulation efficiency and improved survivability during storage, indicating that the hydrophobic component and coacervate matrix can provide complementary physical protection (Sharifi et al., 2021). Similarly, the co-encapsulation of Lactobacillus casei Zhang with triacylglycerol or diacylglycerol oils in gelatin-gum arabic complex coacervates demonstrated superior protective effects on probiotics under freeze-drying, storage, and gastrointestinal digestion conditions (Chen et al., 2023). Diacylglycerol was associated with enhanced cell membrane fluidity, reduced cellular damage, and decreased contact between probiotics and digestive fluids (Chen et al., 2023). Together, these lipid-based systems suggest that protection arises not simply from the nutritional functionality of the co-delivered compound, but also from its ability to modify the local physicochemical environment and mass-transfer barrier surrounding probiotic cells. However, because lipid composition, coacervate formulation, probiotic strain, and drying conditions differ among studies, the relative contribution of the bioactive itself versus the surrounding carrier structure remains difficult to isolate.

Table 3.

Recent co-encapsulation studies of probiotics and bioactive compounds based on complex coacervation.

Probiotic strain	Bioactive compound	Coacervate matrix	Integration mode/Key parameters	Key findings	Reference
Lactobacillus plantarum	Phytosterols	Whey protein isolate + gum arabic	Complex coacervation-based probiotic–phytosterol co-delivery; phytosterol emulsion incorporated into whey protein isolate phase before gum arabic addition; whey protein isolate of 5% (w/v), gum arabic of 2.25% (w/v), pH 3.75; coacervate maturation at 5 °C for 24 h; spray-/freeze-drying applied	There was no significant difference in encapsulation efficiency of probiotics in encapsulation and co-encapsulation; particle size of liquid co-encapsulation coacervates was 366.5 nm; freeze-drying improved encapsulation efficiency (84.62% VS 67.23%) compared to spray-drying; the survival of probiotics in ultra-filtered Iranian white cheese during the storage at 4 °C for 61 days exhibited a decrease of 2.80, 1.30 and 1.09 log CFU/g for free, encapsulated and co-encapsulated probiotics	Sharifi et al. (2021)
Lactobacillus casei Zhang	Triacylglycerol oil or diacylglycerol oil	Gelatin + gum arabic	Complex coacervation-based probiotic–triacylglycerol oil or diacylglycerol oil co-delivery; triacylglycerol oil or diacylglycerol oil incorporated into gelatin phase before gum arabic addition; gelatin and gum arabic solutions of 2.5% (w/v); pH adjusted to 4.0; phase separation by cooling to 10–12 °C and storage at 4 °C overnight	Diacylglycerol-based microcapsules showed superior protective effects; improved survival after freeze-drying (survival rates of 89.24%, 95.51% and 96.24% for gelatin/gum arabic, gelatin/gum arabic/triacylglycerol oil, and gelatin/gum arabic/diacylglycerol oil), thermotolerance at 55 °C for 10 min (reduced by 1.35, 1.38, 1.28 and 1.15 log CFU/g for free cells, gelatin/gum Arabic, gelatin/gum arabic/triacylglycerol oil, and gelatin/gum arabic/diacylglycerol oil), and simulated gastrointestinal conditions (reduced by 3.82, 3.76, 3.08 and 2.71 log CFU/g for free cells, gelatin/gum arabic, gelatin/gum arabic/triacylglycerol oil, and gelatin/gum arabic/diacylglycerol oil)	Chen et al. (2023)
Lactobacillus casei	Polyphenols	Zein + chitosan	Zein–chitosan complex coacervation-based probiotic–polyphenol co-delivery; zein nanoparticles prepared by antisolvent method and mixed with chitosan (1:7); bioactives and probiotics incorporated during coacervation; pH adjusted to 7.0	Polyphenols at appropriate concentrations (quercetin, 0.05%) improved probiotic survival (over 6.23 × 109 CFU/mL) and encapsulation efficiency (over 19.3%); higher concentrations were detrimental to probiotic viability	Ma et al. (2024)
Lactobacillus plantarum	Algal oil	Gelatin + gum arabic + soy lecithin	Complex coacervation-based probiotic–algal oil co-delivery; algal oil incorporated into gelatin phase containing probiotics, followed by gum arabic-induced coacervation; gelatin and gum arabic = 30 g/kg, with soy lecithin added for stabilization	The addition of soy lecithin improved freeze-drying survival of probiotics (90.06%), encapsulation efficiency (75.74%), release rate (79.54%), lipolysis degree (74.63%), and docosahexaenoic acid lipolysis efficiency (64.8%) of algal oil during digestion	Zhang et al. (2023)
Lacticaseibacillus paracasei BGP-1 and Bifidobacterium animalis subsp. lactis BLC-1	Guaraná extracts	Gelatin + gum arabic	Gelatin–gum arabic complex coacervation for probiotic–guaraná extract co-delivery; Guaraná extracts incorporated into gelatin phase before gum arabic addition; gelatin/gum arabic = 2.5% (w/w), pH 4.2, cooling to 10 °C, storage at 7 °C overnight, followed by freeze-drying	Co-encapsulation enhanced antioxidant activity and promoted probiotic growth; improved survival (7 log CFU/mL) in simulated intestinal fluid	Silva et al. (2022)

Polyphenol-containing systems illustrate a different and more concentration-dependent interaction. Zein-chitosan complex coacervates containing polyphenols improved probiotic survival and encapsulation performance (Ma et al., 2024). However, higher polyphenol concentrations reduced probiotic viability (Ma et al., 2024). This biphasic response is particularly important for co-delivery design because it demonstrates that a bioactive compound can shift from being protective to inhibitory as its local concentration increases. Thus, maximizing bioactive loading is not necessarily compatible with maximizing probiotic survival, and an optimal co-delivery formulation must balance functional dose against microbial tolerance. Similarly, co-encapsulation of probiotics with guaraná extracts using gelatin and gum arabic improved probiotic survival during digestion, while the coacervate structure also influenced the release of phenolic compounds from the extract (Silva et al., 2022). This finding further shows that the carrier regulates both sides of the co-delivery system: it determines the extent of probiotic protection while simultaneously controlling whether the bioactive remains accessible under gastrointestinal conditions.

Omega-3-containing systems provide additional evidence that probiotic protection and bioactive delivery cannot be optimized independently. Soy lecithin was used in a study to co-encapsulate Lactobacillus plantarum and algal oil in gelatin and gum arabic systems, significantly improving freeze-drying survival of probiotics together with encapsulation efficiency, release, and lipolysis of omega-3 oils (Zhang et al., 2023). The simultaneous improvement of probiotic survival and lipid digestion is encouraging because it extends the evaluation beyond a single cargo. Nevertheless, such outcomes should not automatically be interpreted as biological synergy. Demonstrating true synergy requires comparison with appropriate single-cargo controls and, ideally, evidence that simultaneous delivery produces functional outcomes greater than those obtained from the independently delivered components. Taken together, the available studies therefore support the feasibility of multifunctional co-delivery more strongly than they establish universal probiotic–bioactive synergy.

Beyond conventional co-encapsulation within coacervate matrices, layer-by-layer (LbL) assembly has recently emerged as an alternative surface-engineering strategy for probiotic–bioactive co-delivery. Unlike bulk encapsulation approaches, LbL assembly enables the sequential deposition of oppositely charged polymers, proteins, polysaccharides, or bioactive-loaded layers onto probiotic carriers, allowing precise control over interfacial properties and spatial organization of functional components (Teng et al., 2026). This multilayer architecture provides opportunities to incorporate bioactive phytochemicals, such as polyphenols and other antioxidant compounds, into specific coating layers while maintaining probiotics within the core structure. Such spatial organization may enhance probiotic protection during gastrointestinal transit and enable coordinated delivery of probiotics and bioactive compounds (Virk et al., 2025). Conceptually, this spatial organization offers an advantage over conventional bulk co-encapsulation because the probiotic and bioactive components need not occupy the same microenvironment, potentially reducing unfavorable direct interactions while allowing different layers to perform distinct protective or release functions. However, compared with conventional coacervation-based systems, LbL approaches remain limited by multistep fabrication procedures, coating uniformity, scalability, and potential effects of dense surface layers on probiotic activity and release behavior. Moreover, direct evidence integrating LbL architectures with complex-coacervate probiotic carriers remains limited. LbL should therefore currently be regarded as a complementary surface-engineering concept rather than an established extension of complex coacervation. Therefore, integrating LbL nanocoating with coacervate-based carriers represents a promising future direction for developing multifunctional probiotic delivery systems.

The concept of synergy in co-delivery should extend beyond the protective effects of bioactive compounds on probiotics to consider reciprocal probiotic–bioactive interactions. Emerging evidence indicates that probiotics can, in turn, improve the stability and bioavailability of co-delivered polyphenols (Ma et al., 2025, Ma et al., 2025). In particular, probiotic-associated enzymatic activities can transform polyphenols through hydrolysis, deglycosylation, and demethylation, generating smaller phenolic metabolites with altered bioactivity and potentially greater bioavailability (Williamson, 2025). Such microbial biotransformation therefore represents an important mechanism through which viable probiotics may influence the gastrointestinal fate and functional efficacy of co-delivered bioactives. This reciprocal interaction distinguishes functional co-delivery from simple simultaneous encapsulation: the bioactive may influence probiotic stress tolerance, while the viable microorganism may subsequently alter the chemical form and biological accessibility of the bioactive. However, these interactions are highly strain- and compound-dependent, and microbial metabolism may also accelerate the depletion or conversion of the parent bioactive compounds. More importantly, direct quantitative evidence demonstrating how probiotic viability and metabolic activity affect the stability and release kinetics of bioactives within complex-coacervate systems remains scarce. Therefore, true co-delivery synergy should be evaluated bidirectionally by simultaneously monitoring probiotic viability and metabolic activity together with bioactive retention, transformation, release kinetics, bioaccessibility, and bioavailability. Studies should also distinguish physical co-release from biological interaction: synchronized liberation of two cargos from a carrier does not by itself demonstrate that the released probiotic and bioactive act synergistically. In this context, intestinal targeting should be considered as a coordinated process involving carrier degradation, probiotic activation, and bioactive liberation, rather than merely the delivery of maximum amounts of encapsulated compounds.

While the co-encapsulation of probiotics with bioactive compounds through complex coacervation offers clear advantages, several challenges remain. One of the most pressing issues is the optimization of bioactive compound concentration. As mentioned earlier, higher concentrations of bioactive compounds like polyphenols may have detrimental effects on probiotic viability. Achieving the optimal concentration of bioactives is essential to maximize their protective effects while preventing potential toxicity to the probiotics. Similarly, the release kinetics of both probiotics and bioactives must be optimized to ensure that they are both delivered to the gut in a timely manner, with sufficient gastric protection and appropriate intestinal release synchronization rather than premature leakage in the stomach. Additionally, long-term stability remains a significant challenge. The stability of both probiotics and bioactive compounds during long-term storage depends on several factors, including water activity, moisture sensitivity, and oxidation stability. Proper packaging to minimize moisture and oxygen exposure is crucial to maintaining the efficacy of co-encapsulated systems, especially for bioactives like polyphenols and omega-3 oils, which are sensitive to degradation. Lastly, scalability remains an important issue for industrial applications. While lab-scale studies have demonstrated the potential of co-encapsulation systems, translating these findings to large-scale production requires careful attention to processing conditions, consistency, and cost-effectiveness. Additionally, packaging plays a critical role in maintaining the stability of the co-encapsulated system during storage. Proper moisture and oxygen barrier properties are essential to protect both probiotics and bioactive compounds from degradation.

Looking ahead, future research should focus on several key areas to advance co-encapsulation technologies. First, further studies on the mechanisms of bioactive-probiotic interactions in co-encapsulation systems are needed. Understanding how bioactives like polyphenols, phytosterols, and omega-3 oils influence probiotic survival and functionality will help optimize the design of these systems for specific health benefits. Additionally, the controlled release of probiotics and bioactives needs to be optimized to ensure the compounds are delivered effectively to the right part of the digestive system. Future studies should therefore move beyond measuring encapsulation efficiency and survival rate toward establishing structure–release–function relationships that connect coacervate properties with intestinal delivery outcomes. Innovations in novel biopolymers, advanced coacervation methods, and the use of nanomaterials could also play a significant role in improving encapsulation efficiency, stability, and controlled release. Future studies should focus on in vivo evaluations to validate the long-term health benefits of these systems, particularly in terms of gut health, immune modulation, and metabolic health.

Co-encapsulation of probiotics and bioactive compounds using complex coacervation represents a promising approach to enhance both probiotic survival and bioactive compound efficacy. However, optimizing the balance between bioactive concentration, release control, and long-term stability remains crucial to the success of these formulations. With continued advancements in formulation strategies, co-encapsulation can play a key role in developing functional foods and nutraceuticals that provide synchronized delivery of probiotics and bioactive compounds to the gut through controlled protection, intestinal release, and enhanced functional accessibility.

8. Translating engineered systems to commercial reality: bottlenecks and strategic roadmaps

Despite the remarkable progress summarized in preceding sections, translating complex coacervation-based probiotic delivery systems from laboratory-scale successes into industrially viable products remains a formidable challenge. This section critically examines the key translational bottlenecks that currently separate proof-of-concept studies from commercial reality, and proposes targeted strategic roadmaps to bridge these gaps.

8.1. Standardization deficit: a hidden barrier to reproducibility

One of the most pervasive yet underappreciated challenges is the absence of standardized characterization protocols. Across the literature surveyed in this review, critical parameters—residual moisture and water activity trajectories during drying, droplet-size distributions pre- and post-coacervation, interfacial viscoelasticity, crosslinking degree and network density, tablet porosity and disintegration profiles, and field intensity-exposure time matrices for magnetic-field treatments—are inconsistently reported or entirely omitted. This methodological heterogeneity precludes meaningful cross-study comparison and impedes the establishment of reliable structure-function correlations.

The field would benefit from a minimum reporting standard framework for complex coacervation-based probiotic encapsulation, analogous to those established in nanomedicine and biomaterials research. Such a framework should mandate reporting of (1) coacervation yield and zeta potential at each formulation step; (2) water activity and moisture content of final dried products; (3) key performance indicators including encapsulation efficiency, viability retention, gastrointestinal release behavior, and release kinetics under standardized gastrointestinal models; and (4) critical quality attributes of integrated technologies (e.g., tablet hardness and friability; emulsion droplet-size distribution; crosslinking degree; magnetic field parameters). Adoption of such standards would transform the current empirical recipe collection into a predictive design discipline, while also enabling the rational selection and comparison of coacervate systems according to specific processing conditions, storage requirements, intestinal delivery objectives, and final application scenarios. Importantly, standardized evaluation should move beyond survival-based assessment by incorporating site-specific release parameters, such as gastric retention, intestinal release profiles, and bioactive accessibility, to establish meaningful relationships between carrier structure and delivery performance.

8.2. The paradox of combinatorial complexity: when more is less

A cross-technology comparison of the preceding sections reveals a common design principle: additional structural or functional complexity is beneficial only when the integrated components remain mechanistically compatible. While the integration of multiple technologies can provide hierarchical protection, it simultaneously introduces nonlinear interactions that can produce counterintuitive outcomes. As exemplified by Hu et al. (2023), combining coacervation with enzymatic crosslinking unexpectedly weakened probiotic protection due to interference at the molecular level. Similarly, excessive bioactive concentrations in co-encapsulation systems can shift from protective to detrimental, while overly dense crosslinked networks may safeguard cells during storage but fail to release them at the intended intestinal site. Together, these observations indicate that technological combinations should be evaluated according to the compatibility and interaction of their underlying mechanisms rather than the number or apparent strength of the individual protective components.

Future research should adopt factorial design methodologies to systematically deconstruct additive, synergistic, and antagonistic effects among integrated technologies. High-throughput screening platforms coupled with machine learning could accelerate the identification of optimal formulation-process combinations without exhaustive empirical iteration. Moreover, release kinetics should be evaluated not as a single end-point measurement but as a dynamic profile correlated with intestinal site-specific requirements and functional delivery objectives. Accordingly, optimization should focus on identifying application-specific combinations that provide sufficient protection while preserving the required release and functional performance, rather than maximizing structural complexity itself. Such application-driven optimization is essential because the optimal combination of coacervation and complementary technologies may differ depending on the intended use environment, processing conditions, and functional requirements of the final product.

8.3. Bridging the laboratory-to-industry chasm

Three industrial realities are conspicuously underrepresented in current literature. First, cost-economics: many biopolymers (e.g., shellac, chitosan, specialized pectins) and crosslinkers (e.g., transglutaminase, vanillin) that perform excellently at laboratory scale may be economically prohibitive for large-scale production. Second, process robustness: pilot-scale spray-dryers with different nozzle geometries, atomization shear regimes, and feed rheology conditions produce survival outcomes that can deviate substantially from bench-scale results. Third, regulatory acceptance: chemical crosslinkers face increasing consumer skepticism and regulatory scrutiny in food and nutraceutical applications, favoring clean-label and naturally derived alternatives.

Industrial translation requires parallel investment in (1) life-cycle cost assessment to identify cost-effective yet high-performance material combinations; (2) pilot-scale demonstration studies that systematically map processing parameter-sensitivity windows; and (3) proactive engagement with regulatory frameworks to pre-qualify GRAS (generally recognized as safe) biopolymers and crosslinkers. Partnerships between academic researchers and food industry stakeholders should be formalized earlier in the development pipeline to ensure that scalability considerations inform, rather than follow, formulation design. In particular, future industrial validation should evaluate not only probiotic survival performance but also compatibility with real product matrices, including processing stability, storage behavior, sensory acceptance, and regulatory requirements. Furthermore, scale-up studies should verify whether engineered release characteristics, including gastrointestinal responsiveness and intestinal delivery performance, are retained after industrial processing and prolonged storage.

8.4. Beyond CFU: the need for mechanistic and in vivo evidence

The vast majority of studies assessed in this review rely on CFU as the sole measure of probiotic protection. While CFU provides a pragmatic readout, it offers limited insight into mechanistic pathways: How do coacervate shells modulate probiotic membrane integrity, metabolic activity, or stress-protein expression? Does co-encapsulation with polyphenols induce synergistic or antagonistic effects at the transcriptomic level? Furthermore, in vitro gastrointestinal models, although widely adopted, cannot fully replicate the dynamic complexity of the human gut ecosystem or fully predict the site-specific release and functional availability of encapsulated probiotics and bioactive compounds.

Future investigations should incorporate mechanistic readouts including membrane fluidity, intracellular ATP levels, reactive oxygen species generation, and stress gene expression (e.g., via qPCR or transcriptomics) to delineate how different integrated technologies protect probiotics at the cellular and molecular levels. Parallel in vivo studies in animal models and, ultimately, well-designed human trials are essential to validate that enhanced in vitro survival translates into genuine gut colonization and health benefits. In addition, these studies should evaluate whether structural advantages of coacervate-based carriers, such as acid resistance, bile tolerance, and controlled disintegration, effectively translate into gastrointestinal-responsive delivery.

Although CFU remains the predominant indicator for evaluating probiotic protection in complex coacervation systems, several studies have begun to incorporate complementary mechanistic analyses to better understand cellular responses beyond survival. For example, fluorescence-based membrane integrity assays, intracellular oxidative stress measurements, and metabolic activity evaluations have been applied in probiotic encapsulation studies to distinguish whether improved viability results from physical shielding or preservation of cellular functionality (Arroyo-Moreno et al., 2025). In addition, molecular approaches such as qPCR and transcriptomic analysis have revealed that probiotic stress adaptation involves regulation of pathways associated with membrane repair, oxidative stress defense, and cellular homeostasis (Bustos et al., 2025). However, within complex coacervation-based delivery systems specifically, the systematic application of these advanced biological readouts remains limited. Most current studies still emphasize survival outcomes rather than linking carrier structure, cellular responses, and functional recovery through mechanistic evidence.

To address this limitation, a standardized multi-level analytical framework is proposed for future investigations of coacervation-based probiotic delivery systems. At the carrier level, structural and physicochemical properties should be characterized, including particle size, morphology, porosity, crosslinking degree, permeability, water activity, and moisture content. At the cellular level, probiotic functionality should be evaluated using complementary indicators, including membrane integrity, membrane fluidity, intracellular ATP content, reactive oxygen species generation, and metabolic activity. At the molecular level, stress-related gene expression, proteomic analysis, or transcriptomic profiling should be considered to identify cellular pathways involved in protection or injury. Furthermore, the fate of co-encapsulated bioactive compounds should be assessed through release kinetics, retention, transformation, and bioaccessibility analyses. Finally, in vivo validation using appropriate animal models and, when possible, human intervention studies should be integrated to determine whether enhanced in vitro survival corresponds to improved intestinal colonization and health benefits. Such evaluations should establish structure–release–function relationships, connecting coacervate architecture with gastrointestinal targeting, probiotic functionality, and bioactive efficacy. Such a hierarchical analytical strategy would facilitate a transition from empirical optimization based mainly on CFU measurements toward mechanism-driven design and evaluation of next-generation complex coacervation systems, thereby improving the predictability of their performance under different processing, storage, and gastrointestinal conditions.

8.5. Toward predictive engineering: the role of computational tools

Currently, formulation design for complex coacervation-based systems remains largely empirical. Predictive modeling of biopolymer electrostatic interactions, phase behavior, drying-induced glass transition, and release kinetics could substantially accelerate development cycles and reduce reliance on trial-and-error approaches. Importantly, predictive engineering should extend beyond optimizing laboratory-scale formulations and incorporate application-oriented factors, including processing stability, storage performance, and compatibility with specific product matrices.

Collaborative efforts between experimental food scientists and computational materials engineers should develop: (1) coarse-grained and molecular dynamics models of protein-polysaccharide coacervation to predict optimal pH and charge-ratio windows; (2) transport models correlating microcapsule porosity, water activity, and oxygen transmission with probiotic survival during storage; and (3) gastrointestinal release models integrating coacervate shell degradation, swelling, permeability changes, pH responsiveness, and intestinal environmental conditions to predict site-specific delivery behavior. Such digital design tools would represent a transformative leap from “what works” to “why it works and how to make it better”, providing a pathway toward application-specific probiotic delivery systems rather than universal trial-and-error formulations.

9. Conclusion

This review has presented a systematic engineering framework for next-generation probiotic delivery via complex coacervation, demonstrating that its full potential is unlocked not through coacervation alone, but through synergistic integration with complementary technologies. We have shown that by combining complex coacervation with drying, tableting, emulsification, crosslinking, and low-frequency static magnetic fields, a hierarchical protection architecture can be achieved—encompassing powder vitrification, macroscopic shielding, multi-compartment interfacial design, mechanical shell reinforcement, and clean-label physical modulation. When extended to co-encapsulation with bioactive phytochemicals, this platform further delivers dual functional benefits: enhanced probiotic survival and improved bioavailability of co-delivered polyphenols, omega-3 fatty acids, and phytosterols, offering synergistic gut-health outcomes. Beyond preservation, these integrated strategies provide opportunities to engineer gastrointestinal-responsive delivery by controlling barrier properties, release timing, and interactions between probiotics and co-delivered bioactive compounds. Looking forward, the path from laboratory design to commercial products demands more than incremental optimization—it requires a paradigm shift in how we approach probiotic encapsulation. The bottlenecks identified in this review—standardization gaps, combinatorial complexity, industrial translation hurdles, and limited mechanistic understanding—are not insurmountable, but they must be addressed through deliberate, coordinated efforts. The proposed strategic roadmaps, including minimum reporting standards, factorial design methodologies, pilot-scale validation, mechanistic readouts, and computational modeling, collectively chart a course toward predictive, rather than empirical, engineering of coacervate-based delivery systems.

Beyond technological optimization, successful translation of coacervation-based probiotic systems requires tailoring the engineering design to specific product formats. For liquid beverages, including fermented drinks, fruit-based beverages, and functional drinks, key considerations include maintaining probiotic viability under acidic environments, preventing oxidation, minimizing impacts on sensory properties, and ensuring stability during storage. For solid foods, such as probiotic powders, nutrition bars, and dried functional foods, moisture control, water activity regulation, and compatibility with processing conditions are critical factors for preserving probiotic activity and extending shelf life. For dietary supplements, including capsules and sachet formulations, concentrated delivery, long-term storage stability, gastrointestinal-responsive release, and regulatory requirements represent major design priorities. Therefore, future development should move from universal formulation strategies toward application-specific engineering approaches that integrate coacervate structure, processing conditions, and final product requirements.

With continued interdisciplinary innovation spanning food science, materials engineering, and gut microbiology, complex coacervation-based systems are poised to fundamentally reshape probiotic delivery across functional foods, nutraceuticals, and pharmaceuticals. By moving beyond empirical formulation toward rationally designed platforms, these systems promise consumers safer, more effective, and better-targeted probiotic products—through coordinated protection, predictable gastrointestinal release, and improved delivery of probiotics and bioactive compounds to their intended intestinal sites—ultimately translating the health benefits of probiotics from laboratory promise into everyday reality.

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.