Spray-freeze-drying as emerging and substantial quality enhancement technique in food industry

Abstract

Spray freeze drying is an emerging technology in the food industry with numerous applications. Its ability to preserve food quality, maintain nutritional value, and reduce bulk make it an attractive option to food manufacturers. Spray freeze drying can be used to reduce the water content of foods while preserving the shelf life and nutritional value. Spray freeze-drying of food products is a process that involves atomizing food into small droplets and then flash-freezing them. The frozen droplets are then placed in a vacuum chamber and heated, causing the liquid to evaporate and the solid particles to become a dry powder. Spray freeze drying has become a valuable tool for the food industry through its ability to process a wide range of food products. This review’s prime focus is understanding spray freeze-dried approaches and emphasizing their applicability in various products.

Introduction

Almost all foods and bio-products are dried by removing water (moisture) from the substance via the vapor phase. One of the oldest ways to preserve food and produces shelf-stable items that may be maintained in favourable condition at room temperature for months to years. Heat addition and water vapor removal procedures are used to characterize drying systems. According to Adali et al. (2020), in spray drying the food at atmospheric pressure is in direct contact with hot air, or freeze drying the food is frozen by sublimation of water, which can speed up drying. Spray drying (SD) is a popular process for creating products (dried powdered) that involve rapid moisture evaporation from a finely dispersed liquid fed into a spray chamber (Mozaffar et al., 2021). Inside the chamber, warm air flows in a counter-current or co-current path, removing moisture from the finely dispersed particles and modifying the liquid feed into a dry powder result. According to De Mohac et al. (2020), spray drying is useful for producing non-sticky powders with a defined size range of particles at a rapid drying respite the foregoing advantages, the method’s high inlet temperature operation could result in significant losses of bioactive and volatile compounds and degradation of heat-sensitive components due to high temperature. The problem of maintaining and safeguarding volatile components is immediately addressed by freeze-drying (FD), on the other hand. Ice crystals are created by first freezing the substance at a low temperature. Primary drying is the process by which these crystals transcend from the solid to the vapor phase in a chamber having vacuum conditions (Bhatta et al., 2020). Sublimation at the triple point (0.6 kPa and 0.01 °C) of water requires that both the vapor pressure and the temperature. According to Mohammadalinejhad & Kurek (2021), desorption lowers the sorbed water that is still present in the solute matrix (frozen concentrated non-ice phase). Ice crystals that have been sublimated create porous structures, which greatly enhance the ability of powdered substances to rehydrate.

Although freeze-dried particulates generally have superior product quality, their usage is constrained by their lengthy drying times, high vacuum, low temperatures, batch nature, and consequently high operational costs. Many academics have proposed various methods to reduce the costs of traditional freeze-drying (Merivaara et al., 2021; Rockinger et al., 2021; Rostamnezhad et al., 2022). One alternative is to forego the vacuum and instead operate at normal pressure. Researchers have shown that if the partial pressure of the liquid in the dryer is kept low enough, freeze-drying at atmospheric pressure is possible (Rostamnezhad et al., 2022). Because interlock mechanisms are not required, this technology eliminates the need for the vacuum pump and simplifies continuous freeze-drying processes. Then, by closely examining structural changes during freeze-drying, researchers thoroughly analysed the numerous freezing mechanisms. By atomizing fluid meals into a cryogenic fluid (liquid nitrogen) and then sublimating the frozen particle at atmospheric pressure in a fluidized bed, Ishwarya (2022), tried to minimize the drying time. The speed of sublimation was significantly accelerated by the smaller droplets, and each frozen droplet’s ability to dry independently. Figure 1 depicts the three steps of SFD, which include atomizing a liquid or solvent into droplets, hardening by coming into contact with the cold stream, and sublime sublimation at low temperatures and pressure. SFD is thus a one-of-a-kind drying process since it combines spray drying and freeze-drying (Ishwarya, 2022; Merivaara et al., 2021; Rockinger et al., 2021; Rostamnezhad et al., 2022).

Fig. 1.

Schematic representation of spray-freeze drying process

Many authors have looked into the potential uses of SFD in the manufacturing of pharmaceutical and food products. Low-porosity ceramic particles can also be produced via SFD; however, this is not discussed in this paper. SFD has three main uses: (1) drying high-value foods (2) drying pharmaceuticals, and (3) encapsulating active compounds that are environment-sensitive. This method of producing porous particles for pulmonary distribution is especially appealing since it can encapsulate medications with low water solubility and has special aerodynamic features (Ishwarya, 2022; Xi et al., 2023). To adapt the process for various end products, one must have a thorough understanding of every aspect of the SFD technique. The overarching goal of the current paper is to provide an overview of the many SFD approaches, the mechanisms that control these SFD methods, and the use of SFD in the production of consumable and biological products. Furthermore, covered are recent developments in the domain of SFD as well as the scope for additional research.

Mechanism of spray freeze drying (SFD)

As the name implies, SFD combines the drying processes of freeze drying (FD) with spray drying (SD). By atomizing a liquid food into a suitable minimum temperature zone initially, this technique aims to shorten the drying time of a traditional FD process (maintained with a cryogen eg: liquid nitrogen) (Mutukuri et al., 2023; Vishali et al., 2019). Once the frozen particles were subjected to air pressure, they melted. Smaller product dimensions improve mass and heat transfer, reducing the time needed for freezing and FD. The typical SFD process, according to Pardeshi et al. (2021), entails three fundamental steps: the dispersion of a high-volume liquid mixture into droplets, the crystal growth of drops by close interaction with a cold stream, and the sublimation of the crystalized droplets at extremely low temperatures and pressure. These three basic steps are depicted in Fig. 2. The key step in the SFD process for dispersing the liquid into fine droplets is atomization. In the atomization process, ultrasonic capabilities, nozzles with two, three, and four-fluid, were used. Among the many nozzles used, the nozzle having ultrasonic effect was found to have strong control over particle size, while the four-fluid nozzle was suitable for medications with undesirable aqueous solubility. Aside from that, the fine dispersion of particles (atomization) is important in estimating the distribution of particle-size droplets that are sprayed. Feed flow rate, atomization energy, feed viscosity, and surface tension are all factors that influence atomization. As a result, large atomization pressures are necessary to obtain smaller droplet sizes (O’Sullivan et al., 2019). After atomization, freezing is carried out at extremely low temperatures with a cryogen, such as liquid nitrogen. The use of low temperatures to reduce drying and enhance the preservation of active ingredients is one of the most prominent benefits. SFD keeps delicate components during processing as opposed to SD and FD. It produces particles that are more stable and purer.

Fig. 2.

Steps involved in spray freeze drying

Moreover, the time of processing is significantly quicker than FD. The SFD particles allow for quick rehydration because they are incredibly permeable. Moreover, compared to FD and SD, SFD particles have a larger surface area. Particles maintain their sphericity generated after atomization due to the rapid freezing procedure used during SFD. In the context of SD and FD, various studies have observed changes in particle structure (Rostamnezhad et al., 2022; Zhang et al., 2023). Particle porosity is significantly dependent on the number of solids in the feed solution. Particle porosity and solid content typically have an inverse relationship. On the other hand, due to the mechanical unreliability of individual particles, a sustained drop in the content of solids would lead to the disintegration of spherical forms. Table 1 illustrates the distinction between FD and SD.

Table 1.

Comparison between spray drying, freeze drying and spray freeze drying

Spray drying	Freeze drying	Spray-freeze drying	References
Drying is done under high pressure	Drying is done below freezing point	Drying is done by combining the two techniques (FD and SD)	Liang et al. (2018), Vishali et al. (2019)
Product and quality losses	Nutrients, color, and flavor retained	The drawbacks of those techniques are covered in this technique	Mozaffar et al. (2021), Rostamnezhad et al. (2022)
Strong shear force	Minimal structure change	Superior quality product	Merivaara et al. (2021), Rockinger et al. (2021)
Thermally stable products	Limited drying of foods		Ishwarya (2022)
Produces products with small particle sizes	Larger particle size product compared to spray drying		Mutukuri et al. (2023), Vishali et al. (2019)

Classification

The SFD technique concludes with FD. In general, FD occurs in three different stages: freezing, sublimation drying (primary drying), and evaporative drying (secondary drying). Spray freezing is followed by allowing the cryogenic liquid to boil off before sieving and collecting or separating the frozen feed droplets suspended in it (Yang et al., 2023). The separated frozen samples are then freeze-dried in a drying module under vacuum, atmospheric, or fluidized conditions, depending on whether the SFD process (Fig. 3) is categorized as:

Fig. 3.

Classification of SFD

Vacuum Spray-freeze drying

Lyophilisation, another name for vacuum freeze-drying (VFD), is a well-known dehydration method used to produce premium-quality food products and pharmaceuticals. Popular vacuum freeze-dried food items include milk powder, coffee, and baby formula. Bioprocessing applications include freeze-drying biological mediums, vaccinations, and living cells. Essentially, during the primary drying process the spray-freezing-produced ice crystals are sublimated (Cao et al., 2020). It takes place in a low-pressure and temperature setting. The porosity microstructure of the finished product is caused by the spaces left over from the sublimation of ice crystals.

According to Kanwate et al. (2019), the steps of freezing and secondary drying are often finished in a matter of hours. However, depending on the shelf temperature, the product’s critical temperature, and the number of solids in the product, primary drying might take hours or even days to complete. The longer processing time and high production costs of vacuum freeze-drying are caused by the extensive primary drying.

Atmospheric spray-freeze drying

Spray-freezing liquid droplets and drying them at atmospheric pressures are the two steps involved in the process known as atmospheric spray freeze-drying (ASFD). Spraying a liquid solution into a cold chamber causes the droplets to instantly freeze. Over the course of a 7-h operation, frozen droplets are subjected to sublimation with cold drying gas at various temperatures (Ly, 2019). A freeze-drying technique outside of a vacuum environment was initially demonstrated by Meryman (1959), who demonstrated that the sublimation of ice was promoted by a relative vapor pressure gradient at the sample’s surface rather than an absolute pressure gradient. Later, ASFD has been used in food processing applications to dehydrate liquid foods like juices and tea. The reduction in particle size enhanced the surface mass transfer coefficient, which decreased ASFD drying time, according to mathematical models for the ASFD process applied by the researchers in during different studies.

Atmospheric fluidized bed spray-freeze drying

A technique of drying that can apply heat uniformly can take advantage of tiny sample sizes and speed up drying without running the danger of melting and collapsing particles (Ishwarya et al., 2015). One such technique is fluidized bed drying, known for its superior heat-mass transfer due to desirable particle-drying medium interaction. Fluidization is the phenomenon that occurs when fluids flow upward over a bed of particulate at a sufficient speed to maintain the mass of the particles without washing them out in the stream which is fluidized. Additionally, fluidized bed freeze-drying reduces the diffusion distance between the surface of the ice core and the external convective flow compared to lyophilization. Depending on whether the frozen particles are revolving and moving or stationary (packed bed), the drying gas can easily enter and flow around or through the partially porous particles due to the strong convective flow. It has been revealed that incorporating fluidized bed drying into the ASFD process was a successful way to speed up the drying process by forcing the drying gas into convection (Ishwarya, 2022; Teixeira et al., 2017). Due to this, the concept of mixing spray-frozen particles with chilly, in a fluidized bed dry gas has evolved resulting in the development of atmospheric fluidized bed spray-freeze-drying (Fig. 4).

Fig. 4.

The process diagram of vacuum spray-freeze drying

(Adapted from Cao et al., 2020)

Vacuum fluidized bed spray drying

It has been reported that vacuum fluidized bed freeze-drying (VFBFD) is “a dehydration technology for the removal of ice by sublimation so that the product quality is very high and also it requires a lesser amount of cold gas compared to atmospheric conditions Ishwarya (2022). The driving factor for mass transfer is not affected by the drying gas; it is essentially a passive heat flow medium in this process. When a system’s pressure is decreased by an n-fold factor, the amount of gas required for the function is lowered by n times. Hence, the gas density is decreased (Sadiq et al., 2023). Particle elutriation from the bed is prevented by the low density of the gas at a sufficiently low pressure, which lowers the inertial drag forces on the particles. However, it is crucial to understand that viscous forces don’t start to disappear until very low pressures are attained. Elutriation must therefore always be taken into account as a potential processing bottleneck. Moreover, the air’s ability to absorb water is inversely related to the water content at atmospheric pressure. Even at low pressures (applied vacuum), the system’s overall pressures would be significantly greater than the partial pressure of the fluid in the ice phase (Ishwarya et al., 2015). Due to the relationship between air density and total pressure, it was not possible to reduce the air’s velocity when drying. The fluidized bed air velocity can be changed to be 2–1.5 times higher than at air pressure at lower pressures, which leads to faster drying periods.

Applications of SFD

A healthy diet must include fruits, vegetables, beans, seeds, spices, and other plant-based foods. Consuming these items regularly and in sufficient amounts may help prevent serious illnesses including cancer and cardiovascular conditions, among others. The greatest way to maintain a product’s nutritional value is to keep it fresh, yet most storage methods need low temperatures, which are challenging to maintain throughout the distribution chain. On the other side, drying is a good substitute for post-harvest management, particularly in nations like India with shoddy low-temperature distribution and handling infrastructure (Herman et al., 2022; Kaimal et al., 2022). Changes in food quality occur as a result of drying. There are three categories of food quality: physical, chemical, and nutritional. Some of the key characteristics that drying techniques can affect in foods are color, flavor, texture, rehydration capacity, bulk properties, flow capacity, water activity, and preservation of nutrients and volatile compounds (Bhatta et al., 2020; Rezvankhah et al., 2020). Regarding nutritional qualities, oxygen, extreme heat, and cell degeneration typically work against the retention of bioactive components during processing. Thus, the stability of the advantageous compounds in plant-based diets may be altered by dehydration. The activity of polyphenol oxidase suggests that phenolic substances may be sensitive to enzymatic breakdown (Hou et al., 2022; Shaik and Chakraborty, 2022). SFD is an unusual freeze-drying method that yields distinctive powdered goods while retaining the advantages of products that have been traditionally freeze-dried. SFD can be used in high-value products, as illustrated in Fig. 5, due to its advantage over traditional techniques of drying in terms of product structure, integrity, quality, and the persistence of bioactive and volatile compounds. While conventional drying techniques are unable to produce these product attributes, SFD performs admirably despite its high cost and complexity. The applications of SFD (Table 2) are discussed below:

Fig. 5.

The application of spray freeze drying

Table 2.

Different applications of SFD

Type of product	Parameters	Discussion	References
Nanosuspension of cefixime	Production of stable microparticles	SFD of nanosuspensions can be used as a base for the pulmonary absorption of cefixime molecules that are not highly water soluble. The introduction of raffinose and trehalose with a lower NP-to-carrier ratio and a higher level of leucine would be advantageous for the continued administration of cefixime through the respiratory system	Haghighi et al. (2022)
Micellar casein powder	Structural modification and rehydration characteristics	Serum Ca2+ and PO43− were released from the micellar structure in greater than 50% of cases when SFD powders with smaller droplet sizes were utilised. This powder’s extraordinarily high porosity (92%) and spherical form significantly reduced the amount of time it took for them to become wet	Ren et al. (2022)
Encapsulation of flaxseed oil	Proven to be an efficient approach	Although SFD has a lower encapsulation efficiency, but it offered better flow characteristics	Elik et al. (2021)
Nasal powder	Enhanced resveratrol dissolubility	SFD microparticles had a bigger size of ~ 173 µm that was near the actual droplet size	Di et al. (2021)
Bromelain aerosol	Porous nature & suitable for inhalation	Between 413.73 and 462 mg/g of total bromelain were present, and between 333.22 and 404.64 CDU−1 (casein digestion units−1) were measured as activity. As a result, this study demonstrated that SFD can create aerosols with the appropriate characteristics, and the method may be utilized to ingest food bioactive through the lungs while retaining and activating them well	Lavanya et al. (2021)
Instant coffee	More stable froth & nanobubbles	In comparison to SD and FD powders, the coffee powder created with SFD formed a froth with greater stability and nanobubbles. SFD foam’s FE-SEM examination revealed the existence of nanobubbles between 100 and 200 nm in size	Deotale et al. (2020)
Whole milk powder	High porous microstructure & easily dissolvable	Due to the contained inert particles and the milk powder’s increased porosity microstructure, drying time can be reduced slightly	Zhang et al. (2019)
Nanostructure composite material	Efficient catalyst	Stable TiO2 and SiO2 sols were subjected to colloidal hetero-coagulation, and then spray-freeze-drying was used to create uniform porous mixed oxides with a very high surface area	Lolli et al. (2018)
Skim milk powder	Increasing S. cerevisiae stability and survival rates	S. cerevisiae implanted with 11% SMP has a viable bacteria rate of 76.36%, which is greater than that at other concentrations. Photographs taken with an SEM (scanning electron microscope) show that the SMP concentration has a noticeable effect on the surface structure of microencapsulation	Cao et al. (2020)
Plant extract polyphenol	Solid state stability of particle	The findings demonstrated that the primary prenylated compounds of Bd are susceptible to drying at subfreezing temperatures, whereas d-mannitol had superb cryoprotectant activity, reducing the loss of all indicators. The various powders produced in the fluidized bed ASFD also displayed outstanding process yields, adequate structure, moisture, and excellent pharmaco-technical characteristics	Teixeira et al. (2017)
Soluble coffee production	More volatile present compared to other drying techniques	Excellent flavor quality was attained by keeping the typical low-boiling coffee volatile components, a porous microstructure, instantaneous solubility, excellent flowability, high bulk density, and favorable packaging and transportation properties	Ishwarya et al. (2017)
Microencapsulation of vitamin E	Porous structure & good dilution	A viable method to increase the oral bioavailability of bioactive substances like vitamin E that are weakly water-soluble. The highest encapsulation efficiencies for SFD vitamin E microcapsules were 89.3 2.56	Parthsarathi and Anandaramkrishnan (2016)
Milk powder	Change in particle size	The powder composition had a greater impact on its mobility and wetting characteristics than other physicochemical characteristics including the size of the particles and microstructure. High-fat powders had poor mobility and wetting characteristics	Silva and O’Mahony (2017)
Encapsulation of whey protein & $\beta -$ cyclodextrin	Stable microencapsulation	Compared to SD and FD microencapsulated samples, the vanillin + Whey protein isolate sample that was SFD showed improved thermal stability	Hundre et al. (2015)

Fruit and Vegetable products

Fresh food continues to draw more consumer interest. But maintaining the freshness of fresh fruit continues to be a major problem. Fresh food has a less shelf life due to its perishable nature and moisture content and seasonal availability (Bhatta et al., 2020; Martínez-Navarrete et al., 2019; Rezvankhah et al., 2020). Dehydration and drying are one of the most often used techniques for increasing the shelf life of fresh food, which keeps it always available. Fresh fruit is also lighter and smaller after drying, making it simpler to handle and transport. Nevertheless, bioactive compounds and thermolabile minerals, such as vitamins, may be damaged during high-temperature processing, resulting in fresh produce losing some of its nutritive benefits and health benefits. Hence, freeze-drying is a process that has the potential for thermally sensitive drying materials, such as fresh vegetables (da Fonseca Machado et al., 2018; Djekic et al., 2018; Zhang et al., 2020). A dehydration method called freeze drying has minimal impact on food product quality. Several applications presently use freeze-drying to treat fresh vegetables. The addition of biopolymers with freeze-drying properties, such as 5 g of gum arabic and 1 g of an isolate of whey protein in mandarin juice (100 g), is advantageous, according to a new analysis by Martínez-Navarrete et al. (2019). Moreover, it reduces the processing time while maintaining the vitamin content and sensory stimulation of the final product. To investigate the impact of pulsed electrostatic potential and freeze drying on macro- and microstructure characteristics, total phenolic content, color, and antioxidant activity, Lammerskitten et al. (2019) processed apples at various temperatures (40 and 60 °C) and pressures (1, 0.25 and 0.1 mbar). PEF was conducted with electric current strengths of 1.07 kV/cm and specific embodied energy of 5, 1, and 0.5 kJ/kg. PEF pre-treated samples demonstrated a total phenol increase of up to 47% when compared to control samples. However, the antioxidant activity of apples treated with PEF was decreased by up to 60% when compared to reference samples. These results support the need for these process combinations to be optimized. In a study on the stability of a microencapsulated strawberry flavor, three drying methods that are freeze drying, spray drying, and fluid bed—were contrasted (Pellicer et al., 2019). The mixes were successively lyophilized and frozen at 80 °C for 24 h to produce a powder. The freeze-drying method gave the maximum yield, but the spray-drying method generated powder with the lowest water content. However, spray drying yielded the best results for benzyl alcohol, fraistone, and ethyl acetoacetate stability, followed by freeze-drying. Carotenoids were extracted from the waste of carrot processing using sunflower oil, according to Šeregelj et al. (2021). A simplex CMD (centroid mixture design) was utilized to optimize the wall material compositions (whey protein/inulin/maltodextrin) for FD and SD of carrot waste extract. The freeze-dried encapsulate exhibited the maximum hygroscopicity, color characteristics, and oxidation reaction stability, whereas the encapsulate that was spray-dried had the lowest moisture, particle size, and water activity. The morphological qualities of the best encapsulation were analyzed. The tested technologies and formulations show tremendous promise for developing functional foods with improved nutritional, aesthetic, and bioactive properties. From the above discussion, both methods have some positive and negative aspects in fruit and vegetable drying. By combining both methods superior quality dried fruits and vegetables can be produced. Thus, SFD has a great future in terms of research for fruit and vegetable drying.

Dairy products

Dried dairy products are an essential component of the dairy value chain. Milk’s high-water content (88–90%) is responsible for its perishability. A significant amount of temperature is necessary to evaporate the extra water in milk. This places a significant energy strain on commonly employed milk drying processes, as well as causes nutritional depletion in powdered milk. Despite this, spray drying has maintained its monopoly in the dairy business for almost seven decades due to its industry-friendly functioning and commercial sustainability for bulk manufacture (Ishwarya, 2022). Instant powdered milk, whey protein isolate, and whey protein concentrate are among the spray-dried dairy foods in the company’s portfolio. Dairy products can also be dried using drum or roller drying or freeze-drying. Drum-dried infant milk formula and freeze-dried probiotic milk cultures are well-known dairy products. Vincenzetti et al. (2018), investigated the effects of FD and SD on the levels of lysozyme and b-lactoglobulin in donkey milk. The SD process significantly reduced lysozyme enzymatic reactions and b-lactoglobulin content in mg/ml (6.43 in milk vs. 5.51 in spray-dried milk) due to the use of the high degree of temperature to which the donkey milk.

According to Baldelli et al. (2022), SD is a popular method for making milk powders. In this study, the SFD technique was employed to increase the re-dispersibility of milk powders in solution form. Because of lower tapped density and increased porosity, milk powder generated by spray freeze drying had 15% greater re-dispersibility than spray-dried powders only after 20 s of agitation. Spray freeze-dried dairy powder re-dispersibility was limited by the higher percentage of fat and the addition of the component maltodextrin. A 6% increase in fat deposition resulted in a 30% reduction in redissolved powder. Similarly, raising the maltodextrin content by 5% reduced re-dispersibility by 5%. They discovered that the spray freeze-drying method can produce high-quality milk and other milk powders with high nutritional retention. Cao et al. (2020) proposed that the effect of skimmed milk powder (SMP) concentrations on the biochemical properties of vacuum-spray-freeze-drying (VSFD) microencapsulated S. cerevisiae be examined. S. cerevisiae embedded in 11% SMP had a higher live bacteria rate (76.36%) than other concentrations. SEM (Scanning electron microscopy) images reveal that the SMP content has a significant impact on the surface structure of microencapsulation. Additionally, microparticles containing 11% SMP were the most stable in both high and low temperatures. This paper presents the experimental results of Zhang et al. (2019), which suggested that SFD of whole milk in a combined spray-freeze and vacuum freeze-drying equipment. Based on the size of the steel balls utilized as inert particles, the impacts were assessed in terms of SFD drying time and powdered milk quality. They showed that the presence of inert particles can shorten drying to some extent, and the powdered milk of reduced size has a highly porous microstructure. SFD is an advanced drying technique used in the production of high-value foods and pharmaceutical drugs (Poursina et al., 2016). However, for SFD applications, the prolonged drying time is still a concern. This constraint could be bypassed by using inert particles.

Production of coffee

One of the most significant products in the world is coffee. The most popular coffee beverage is “soluble coffee,” which has a pleasant aroma, the ability to instantly rehydrate, and long shelf life. The dried soluble portion of roasted and ground coffee that is marketed to consumers as either powder or granules is known as soluble or instant coffee. The initial steps in producing soluble coffee involve the same sorting and grading of green beans of coffee, roasting, and crushing as those used to produce roasted coffee. In the final stages, soluble solids are extracted, concentrated (through evaporation or freeze-concentration), and dried (by spray drying or freeze-drying), with optional pre- and post-processing procedures for aromatic preservation and agglomeration. The spray-dried coffee powder can be further transformed into granules via steam agglomeration or by contacting the granules with finely atomized water for immediate rehydration. The agglomerates are subsequently dried using cold input air on a belt conveyor. In contrast, freeze-drying is employed to produce high-quality soluble coffee. The term “quality” addresses the physiochemical, and organoleptic characteristics of coffee. Freeze-dried coffee maintains 17–20% more low boiling point aromatic components and 75% more boiling point volatile chemicals than spray-dried soluble coffee (Ishwarya et al., 2015).

According to Deotale et al. (2020), SFD is known to be more successful than traditional freeze-drying (FD) and spray-drying (SD) processes for the manufacturing of instant coffee. However, its effectiveness in producing nanobubbles has not been investigated. To address the issue, SFD was used in the study to manufacture instant coffee, and the results were compared to SD and FD. About SD and FD powders, SFD powder produced froth with more stability and bubbles of nano-size. FE-SEM investigation of SFD foam revealed the presence of nanobubbles in the 100–200 nm range. When the coffee was prepared, the SFD coffee dissolved in hot water at 90 °C generating incredible froth. Superior aroma quality is obtained by distinguishing spray-freeze-dried solubilized coffee from conventionally processed counterparts by retaining coffee’s characteristic low-boiling aromatics, high density and porosity, immediate solubility, porous microstructure, great flowability, and good packaging and transportation characteristics. Despite research showing that spray-freeze-dried soluble coffee has a higher quality, efforts to industrialize the technology have yet to begin. The spray-freeze-drying technology’s operational restrictions and the monopolies of spray-drying and freeze-drying in the instant coffee manufacturing industry are barriers to establishing it as a process industry for soluble coffee production.

Encapsulation

Encapsulation is the process of encasing an active molecule into a stable, protected material to create encapsulates with varying sizes and functional qualities. The encased ingredient is known as the core, while the carrier material is known as the wall (Kandasamy & Naveen, 2022). A food component (flavors specialized lipids, vitamins, and nutraceuticals) or an active medicinal substance can serve as the core (API). Similarly, the wall material can be classified as either carbohydrate (starch, cellulose derivatives, and gums), proteins (whey protein, gluten, casein, gelatin, and soy protein isolate), or lipids (glycerol, fatty acids, waxes, and phospholipids) (Buljeta et al., 2022). The finished product is known as encapsulating. Encapsulates are classed as microcapsules (> 5000 μm), microcapsules (0.2–5000 μm), and nanocapsules (2000 Å–0.2 μm) based on their size. According to Elik et al. (2021), Spray freeze drying, and spray drying have been used to encapsulate flaxseed oil. For the first time, maltodextrin, pectin, and wax were employed in an encapsulation method. Spray freeze drying proved to be an efficient approach for encapsulating unsaturated oils. Enrichment with carotenoids improved the oxidative stability of oils. Smaniotto et al. (2020) has investigated that freeze-drying did not affect the active release behaviour of nanoparticle-containing alginate fluid gels (AFG). When AFG was made from microparticles, however, freeze drying affected active release behaviour, explaining the size-dependent influence on release behaviour. The freeze-drying approach resulted in the lowest loss in cell survival during storage, according to Moayyedi et al. (2018). Furthermore, the results revealed that freeze-dried microcapsules outperformed live L. rhamnosus in digestive system settings.

Ishwarya and Anandharamakrishnan evaluated the SFD technique for its suitability for soluble coffee processing (2015). The attributes of final product have been compared with that of its SD and FD equivalents. According to the electronic nose study, the flavour profiles of SFD and FD coffee granules were equivalent. According to GC–MS analysis, SFD had higher volatile recovered (93%) over FD (77%) and SD (57%). Because of its highly porous nature, SFD coffee demonstrated rapid solubility in morphological studies. Vitamin E (α-tocopherol) is an antioxidant-rich fat-soluble vitamin. It is an essential nutrient for the immune system, eyesight, and bone health (Reddy & Jialal, 2020). However, the hydrophobic nature of Vitamin E (low aqueous solubility) and its instability under processing and storage conditions make it difficult to incorporate in food products (Gonçalves et al., 2016; Tomas & Jafari, 2018). Furthermore, its low water solubility restricts vitamin E absorption in the gastrointestinal tract, reducing its oral bioavailability. Encapsulating vitamin E in hydrophilic carrier matrices is a well-established method of protecting it against oxidative stress. However, acquiring stable vitamin E encapsulated in powder form involves challenges such as decreasing its vulnerability to chemical degradation at high temperatures, oxygen levels, and light exposures (Loewen et al., 2018; Moeller et al., 2018; Saboti et al., 2017). As a result, spray-freeze drying is seen as a process that overcomes complications.

Production of nanomaterial

The use of nanoparticles as carriers for targeted drug delivery and bioactive has gained popularity in recent years. Nanoparticles are materials with any exterior nanoscale dimension or with internal nanoscale surface structure, according to the International Organization for Standardization (ISO, 2017). As a result, their size ranges from 1 to 1000 nm. In comparison to the original bulk material, the changed surface properties of nanoparticles are essential for their distinctive properties (for example, different colors, lower melting points, and increased solubility) (Surface energy, 2021). According to Luo et al. (2022), the approach employs NH₄OH as a temporary protectant, which successfully aids graphene oxide in resisting folding. A unique spray-freeze-drying process ensures that Pt and Cu precursors are monodispersed on GO. High mass loading PtCu₃/rGO exhibits significantly increased ORR performance due to its ultra-small size and ordered structure of nanoparticles. This approach is generalizable, as proved by the synthesis of additional Pt-M nanoparticles. Another researcher Haghighi et al. (2022), suggested that SFD with various carbohydrates and leucine was used to create a dried nanoemulsion of cefixime with an enhanced dissolving profile, good dissolution rate, and great breathing performance. The SFD of nanosuspensions can be constructed as a foundation for the pulmonary administration of weakly water-soluble cefixime molecules. Trehalose and raffinose formulations with a lower NP-to-carrier ratio and a higher dose of leucine could be offered as promising candidates for future cefixime respiratory delivery. SFD is undoubtedly a promising method for dealing with poorly water-soluble medicines and producing inhalable nanoaggregates and nanocomposite nanoparticles. Ali and Lamprecht (2017) suggested SFD as a viable alternative for dispersion nanoparticle lyophilization. Polymeric and fatty nanoparticles of diverse sizes and shapes were created and described. After that, the samples were freeze-dried in a section of cold air with a predetermined concentration of various cryoprotectants, and the spherules frozen were retrieved for further freeze-drying. Similar samples were created as controls using the standard freeze-drying process. SFD was utilized to make rapidly dissolving, round, and porous composite nanomaterials with exceptional flowability. Unlike freeze-dried samples, the studied polymerically and lipid nanoparticles were completely reconstituted after SFD. SFD is shown to be a good platform for boosting the long-term stability of colloidal nanoparticles. The comparatively expensive process cost of SFD for nanoparticle manufacturing is justified by the final product’s distinctive structure, high stability, aerosolization efficiency, and increased dissolution and dispersibility. However, one limitation of SFD that may be addressed is the modest increase in the size of nanomaterials after redispersion. Since the involvement of atomization and freezing in the aforementioned gap has been ruled out, additional research into the influence of the freeze-drying process on the aggregation of SFD nanoparticles can be done.

Future approaches and challenges

Even though numerous studies have shown the positives and unique qualities of SFD, the method, like any other, includes limitations. Because of the low temperature and pressure requirements, the process is expensive in terms of both capital and operational expenditures. The majority of established SFD units are batch-type and are not suited for commercial-scale application in the scale-up field (Liang et al., 2018; Vishali et al., 2019). This is due to the vacuum requirement’s energy-intensive operation, as well as the batch mode procedure, which incurs additional expenses. Although atmospheric freeze drying is thought to be a solution to this issue, the drying temperature determines the cost per kilogram of dried goods. As a result, the cost of feed materials with a low eutectic (Te) or glass transition temperature (Tg) necessitates a low drying temperature increase (Ishwarya et al., 2015). Additionally, due to the unpredictable Te and Tg values of most food products, the prototypes of dryers (SFD dryers) mentioned in the literature are much too large for highly valuable pharmaceutical items yet too low for freeze-drying foods.

Besides the economics and difficulties of handling and building a scaled-up liquid nitrogen operation, there are additional process issues with SFD. For protein adsorption and unfolding, the atomization stage in ambient gas creates a large contact area between gas and liquid. The SFV/L technique has a disadvantage, especially when used for biological products, in that protein solubility is lost during the atomization step. Additionally, the rapid freezing of droplets creates a large ice-liquid interaction, which has been demonstrated to denature proteins. When the fluid freezes during SFV/L, the components (active ingredient) become supersaturated in the defrosted sections of the atomized droplet, allowing AI crystals to nucleate and grow, according to researchers (Vishali et al., 2019). This slows down the freezing process and promotes particle formation, resulting in greater sizes of particles and lower interfacial areas. Similar observations were obtained during the SFD of active pharmaceutical components. As previously stated, the requirement of a vacuum in standard freeze drying, as well as the considerable amounts of dry cold gas in the ASFD process, jeopardize the economics of SFD operations. Processing expenses for SFD are 30–50 times higher than for regular SD, according to Ishwarya et al. (2015). Elutriation of microscopic particles is also an issue.

The potential of SFD as a technique formulating potential shelf-stable biologically active food products of a better quality than conventional drying technologies has been explored in this review. According to the previous studies discussed in this study, several advancements have been augmented to this technique to make it more precise and useful in terms of product quality and characteristics. It is still necessary to have a deep insight of the process factors that influence particle geometry during the SD and FD processes in diverse food applications, and this holds to a wide variety of compounds used in foods and bioproducts. Further research on SFD is yet possible. According to the literature, because of the extremely low economic viability of the process in the food industry, SFD has gained more importance in the pharmaceutical industry than in the food sector. The method can be fine-tuned to generate a tailor-made final product with a specific particle size that offers the appropriate functional attributes, which is highly desirable in the case of high-value food products containing bioactive compounds or volatile molecules. Further research is highly recommended necessary to pave way for a better understanding of the spray freezing process in terms of possible factors that affect freezing rates and droplet shape and size during spray freezing, as well as particle structure during the final step of FD. The economic sustainability of the process on an industrial scale remains uncertain due to the vacuum conditions required for typical SFD procedures, substantial requirements for dried gas in the case of ASFD, and particulate elutriation and pollution on the wall with ASFBFD. As a result, a further in-depth study needs to be carried out to overcome the above-mentioned grey areas and make SFD a promising technology in food processing sector, like other drying technologies.

Declarations

Conflict of interest

There is no conflict of interest between the authors.