Microencapsulation Delivery System in Food Industry—Challenge and the Way Forward

Key Laboratory of Agro-Products Postharvest Handling, Ministry of Agriculture and Rural Affairs, College of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, China Ningbo Research Institute, Zhejiang University, Ningbo 315100, China National-Local Joint Engineering Laboratory of Intelligent Food Technology and Equipment, Zhejiang Key Laboratory for Agro-Food Processing, Zhejiang Engineering Laboratory of Food Technology and Equipment, Zhejiang University, Hangzhou 310058, China


Introduction
Microencapsulation is a physicochemical or mechanical process whereby one substance is embedded in another material, forming particles ranging from a few nanometers to a few millimeters. The global microencapsulation market was expected to expand at a compound annual growth rate of 13.70%, rising to 19.35 billion dollars by 2025 [1]. Microcapsules have been widely used in the drug delivery market. Nowadays, microencapsulation is also highly recommended in food industry because of the benefits provided, such as thermostability enhancement, bioactive compound protection, controlled release, volatiles maintaining, odor shelter, and texture/sense improvement [2]. Figure 1 presents the life of microcapsules, from forming to fading. Core materials are microencapsulated in monolayer or multilayers of wall materials with a variety of molecular interactions, including electrostatic attraction, van der Waals forces, and hydrogen bonding or ionic interaction. The wall materials protect the core material from harsh temperature changes, oxygen, or moisture permeation during processing and storage. Finally, passing through human digestive system, the outer layers of microcapsules might be dissolved in gastric acid at lower pH, then the core substances are released and absorbed in the small intestine. To adapt to different environmental conditions, the wall materials are designed to escort the core to target location.
The preparation of microcapsules requires simple equipment, continuous production, low production cost, and environmental friendliness. In addition, the most important thing for microcapsules applied in the food industry is to ensure that wall and core materials meet food safety standards. Thus, strict requirements must be met including food grade raw and auxiliary materials and sanitary conditions of processing equipment. In the past few decades, the microencapsulation process has been constantly innovated to overcome the problems of easy degradation, low stability, and controlled release of functional components of food ingredients [3,4]. In particular, the modification of particle characteristics was achieved through different equipment, procedures, materials, processing conditions, and other technologies [3][4][5].
In this review, the polymers commonly used in wall materials such as polysaccharides and proteins and their different properties are discussed. Additionally, the latest update of microencapsulation applied in food industry, including plant-derived food, animal-derived food, and additives, is introduced. Moreover, the advantages and limitations of different embedding technologies, as well as the correlation between the selective biopolymer and microencapsulation process, are presented to offer new insight for further development.

Polymers Used in Microencapsulation
The profile of microcapsules with different wall materials and their characteristics is shown in Table 1.

Cellulose and Its Derivatives.
Cellulose is a macromolecular substance composed of glucose and is one of the main components of plant cell wall, the most abundant polysaccharide and widely distributed in nature, accounting for more than 50% of the plant carbon content. In order to improve the solubility, the thermoplasticity, and other properties of products, the hydroxyl group in cellulose polymers is usually artificially esterified or etherified into derivatives, such as methyl cellulose (MC), carboxymethyl cellulose (CMC), hydroxypropyl cellulose (HPC), and hydroxypropyl methyl cellulose (HPMC) [34].
Probiotics Lactobacillus plantarum LAB12 was enclosed in alginate-cellulose derivatives, MC, CMC-Na, or HPMC. Results showed that alginate-MC and alginate-HPMC significantly reduced the vitality loss of LAB12 in comparison with alginate alone [10]. Besides, the microencapsulated LAB12 in alginate-MC and alginate-HPMC exhibited the higher survival rate of 91% under simulated gastric environment, with the maximal release of approximately 100% [10]. With the increase of MC and HPMC concentration, their viscosity increased, the stability and strength of the cementing network also increased, and the matrix and gel system became more compact, reducing the free volume of the matrix, thus blocked the release of probiotics during gastric transport [10]. Furthermore, intermolecular interactions in the gel network strengthened the bonding, while at low pH, the gel network was weakened (molecular interactions were broken), leading to the release of probiotics [10]. Moreover, in order to increase the thermostability of probiotics Bifidobacterium animalis spp. lactis for powdered infant formula (PIF), the double-layered microcapsule with HPMC as an inner layer and HPC as the outer layer was performed to protect it against the high-temperature stress stimuli; and the presence of HPC with higher molecular weight and the thicker coating delayed the bacterial development after exposure to 70°C [35].
Oils, especially those rich in polyunsaturated fatty acids, tend to be oxidized and produce unpleasant odors; thus, oils are often essential to be sheltered in microcapsules. Cellulose derivative microcapsule enhanced the oil stability, and the variance in particle size and the shape affected the mobility, solubility, agglomeration, etc. [11,13,14] For instance, Karim et al. [13] optimized the encapsulation conditions of temperature (60°C), pressure (150 bar), and feed emulsion rate (1.36 mL min -1 ) of the microencapsulated fish oil. Results demonstrated that the higher concentration of HPMC contributed to the increase of particle density, wettability, and powder porosity [13]. In addition to coating effect,   [20] Whey protein isolates and pectin Anthocyanins Freeze-drying Flavonoid entrapment ↑, Thermal stability ↑ Antioxidant [21] 3 Advances in Polymer Technology HPMC promoted agglomeration, which reduced the outer surface of microcapsules, and decreased the oxygen diffusion rate [14].

Chitosan.
Chitosan is the second richest biological polysaccharide in nature, inferior to cellulose, mainly distributes in the shell of shrimp and crab or cell wall of algae and fungi and is one of the most commonly used wall materials in the preparation of microencapsulation. The character of containing free amino group makes the chitosan the only basic polysaccharide among natural polysaccharides. And chitosan has many unique properties such as biodegradability, biocompatibility, and osmotic enhancement effects [36]. Chitosan, nontoxic and biodegradable, is a cationic polymer possessing strong antibacterial and antioxidant properties, thus is widely used in fresh fruit and vegetable, food additive, and even cosmetics, medicals, etc. [37].
For instance, chitosan coating improved the survival of probiotic Lactobacillus casei (only 2.7-2.9 logs reduction) and long-chain inulin (2.7 log reduction) under simulative gastrointestinal solution and better kept the size of micro particles while that of alginate beads was significantly decreased by 0.2 mm [16]. The protection effect might be explained by the electrostatic interactions between chitosan and alginate beads, which blocked gastric juices [16]. Besides, the addition of chitosan into microencapsulated alginate matrix improved the tolerance of Lactobacillus reuteri DSM 17938 against stress conditions during food processing, so  Advances in Polymer Technology as to better maintain its microcapsule morphology and cell vitality after freeze-drying [38]. Lavinia-Florina et al. [39] reviewed in detail the application of chitosan in probiotic embedding products, focusing on cell survival, protection performance, and application. Chew et al. [18] investigated the response of antioxidant activities and biological compounds in microencapsulated kenaf seed oil before and after in vitro simulated digestion, and results showed that the chitosan-coated microcapsule exhibited significantly higher DPPH (145.1%), ABTS (120.9%) scavenging activity, and vitamin E content (32.1%) than that of the noncoated microcapsule. In addition, chitosan can be hydrolyzed/digested by trypsin and lipase (mainly in the small intestine), contributing to the release of encapsulated substances [18]. Similarly, both enhanced antioxidant capacity and less loss of active ingredients were achieved in chitosanmicroencapsulated palm oil, which contained a high content of carotenoid [17,40]. Furthermore, it was evident that the chitosan microsphere-encapsulated P. dioica essential oil presented antibacterial effects against Bacillus cereus, Bacillus subtilis, and Candida utilis due to the antimicrobial activity of chitosan itself [40].

Alginates.
Alginate is a natural polysaccharide and combines with various cations in seawater to form various alginate salts. Alginate is popularly applied in the development of controlled release system and microencapsulation technology due to its great thickening, flocculability, film-forming property, stability, chelation, and biocompatibility, as well as mild reaction conditions, nontoxic and harmless characteristics, simple gelatinization process, and low cost [34].
Sodium alginate is easily soluble in water but forms a gel when meeting the calcium ions, which offers a promising material for microencapsulation. Grape wastes rich in polyphenols were microencapsulated in calcium-alginate beads with optimized vibration nozzle and showed higher stability than nonencapsulated ones [7]. Compared with low molecular weight alginate, microcapsules made of high molecular weight alginate have higher encapsulation efficiency, but they are also bigger in size and release the active constituents much more slowly [7]. Therefore, low molecular weight alginates are more suitable for the occasion when the degradation of active constituents must be avoided, but their rapid release is desirable [7]. Besides, the alginate-based microencapsulated phenolic extracts of Clitoria ternatea (CT) petal flower had smooth surface with the maximal encapsulation efficiency of 84% under 1.5% alginate and 3% CaCl 2 conditions [41]. Higher polyphenol content was maintained, with the improved antioxidant capacity and thermal stability (at 188°C), as well as the inhibited pancreatic α-amylase activity, in this microcapsule after simulated gastrointestinal digestion [41]. A differential scanning calorimetry (DSC) test found that the interaction between alginate and CT enhanced the thermal stability of CT [41]. In addition, the enhanced tolerance in response to severe environmental conditions was achieved by microencapsulated probiotics Lactobacillus reuteri DSM 17938 in freeze-dried skim milk [38], Lactococcus lactis subsp. cremoris LM0230 in functional food [42] and Lactobacillus rhamnosus NRRL 442 exposed to heat stress [9]. A higher concentration of sodium alginate minimized the free volume of the microcapsules, thus reduced the thermal permeability [9].
Fioramonti et al. reported that the emulsion stability was affected by the pH value of solution and the initial sodium alginate concentration due to the electrostatic adsorption between whey protein isolate (charge differently at different pH) and negatively charged sodium alginate [43]. The optimal condition for linseed oil (rich in high unsaturated fatty acids) microencapsulation was 0.25% initial sodium alginate at pH 5.0 where no phase separation and coacervate formation was found [43]. Furthermore, the maximum release rate of spray-dried coriander essential oil embedded in chitosan was at pH 2.5, while that of alginate was at pH 6.5 [6].

Starch and Its Hydrolysates.
A series of starch hydrolysates can be obtained with acid or amylase treatment. Dextrose equivalent (DE) value was reported to demonstrate the degree of hydrolysis or saccharification of starch and affect the viscosity, browning, and oxidation resistance of starch hydrolysates. Maltodextrin and corn syrup are two commonly used microcapsule wall materials, whose DE values are <20 and >20, respectively [44]. Porous starch is a kind of hollow particle (looks like honeycomb), which have great adsorbability and can contain various substances.
Olive oil microencapsulated with the porous starch prepared from purple sweet potato exhibited higher loading rate and oxidative stability than those of free olive oil, and the best adsorption capacity of porous starch was obtained at 45°C and pH 5.0 with reaction for 12 hours [24]. Maltodextrin-microencapsulated saffron and beetroot pigment extracts showed effective heat protection during storage with the average half-life period of 60.03 and 53.03 weeks, respectively [45].
It is widely known that the natural starch is insoluble in cold water, easy to retrogradation and dehydration, and poor in emulsification, while the modified starch improves its properties to widen its application. Curcumin yellow dye was an antioxidant easily degraded in response to light and oxidation stress, but the ternary mixture of maltodextrin, gum Arabic, and modified starch-encapsulated curcumin yellow dye still maintained a high retention rate after spraydrying or freeze-drying and light exposure [23]. In addition, the probiotics Lactobacillus acidophilus in microcapsule added with 1% resistant starch (Hi-maize) possessed smaller size (78.49 μm) than that in the sole alginate microcapsule (114.51 μm), while Hi-maize offered better protection for the probiotics exposed to the simulated gastrointestinal juice [25]. It was reported that the iron microcapsule embedded in the mixture of gum Arabic, maltodextrin, and modified starch at the ratio of 4 : 1 : 1 had the optimal encapsulation efficiency of 91.58% and particle stability; also, the significantly higher iron bioavailability in vitro was detected in microcapsule than that in the unencapsulated or in iron salt fortified milk [46]. It was reported that 50% modified starch-encapsulated Swiss cheese bioaroma had lower the moisture and water activity, which might be attributed to the reduced water diffusion by increasing viscosity with higher concentration of modified starch.

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Also, the concentration of modified starch was positively correlated with the bulk density and average particle diameter [22].
2.5. Pectin. Natural pectin, widely exists in plant roots, stems, leaves, and fruits, is one of the components of the plant cell wall. The main component of pectin is partially methylated α-1,4-D-polygalacturonic acid, whose residual carboxyl units existed in the form of free acids or salts of ammonium, potassium, sodium, and calcium. Pectin has a good gelatinization and emulsification stability, widely applied in the food industry, such as ice cream, jam, and fruit juice gelatinization, and the gelatinization mechanism varies with different degree of esterification of pectin to match various characteristics (pH sugar concentration, etc.) of food gels [47].
Sanguansri et al. [19] reported that both the addition of pectin and heat treatment at pH 3 could reduce the surface oil and free oil of spray-dried fish oil powder and also lessen the oil leakage during powder compression, because a protective layer can be formed around the positively charged oil-water interface by adding the negatively charged pectin to a heated, positively charged stable emulsion. However, additional heat treatment of pectin-free emulsion at low pH resulted in greater loss of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in powder, since heating facilitated proteolysis and increased polydispersity [19]. Additionally, by using the layer-by-layer technology, the pectin was attached to the protein monolayer emulsions of fish oil microcapsule through the electrostatic interactions and reinforced the microencapsulation efficiency to 95.2% [20]. Although the addition of pectin increased the thickness of interface layer, the antioxidant effect and stability of the microcapsule was enhanced because of the inhibition of lipid oxidation [20]. Besides, pectin, as the wall material, protected the multicomponent carriers of microencapsulated hydrophobic and hydrophilic active substances from oxidation or degradation by oxygen or water [48]. Stănciuc et al. [21] embedded the grape anthocyanin extract with whey protein isolates and two different polysaccharides (acacia gum and pectin), and the encapsulation efficiency was up to 94%-99% after freeze-drying. The laser confocal scanning microscope observation showed that the addition of pectin contributed to the smaller particles, and the variant Cu pectin retained more flavonoids in microcapsules, leading to higher antioxidant activities; and results of the thermal stability study indicated that the pectin had a better protective effect against the anthocyanin degradation [21].
2.6. Protein. Protein widely holds processing properties such as solubility and emulsification, as well as the physiological activities of oxidation resistance, good biocompatibility and biodegradability. In addition, the susceptibility to pH rendered the protein in a great role in strict pH-controlled release conditions [49]. Therefore, protein is another widely used wall material of microcapsules to meet particular situations.
It was reported that microcapsule with sodium caseinate as wall material had a 13.93% higher retention rate of vitamin A than that with milk protein concentrate, which might be attributed to the excellent emulsification performance and the molecular flexibility of sodium caseinate [26]. Moreover, the combination of sodium caseinate and pea protein isolates as wall materials led to smaller droplet size, better emulsion thermal stability, and the optimal microencapsulation efficiency (96.08%) [26]. Higher denaturation temperature of pea protein isolates (83.8°C) contributed to its great thermal stability, while the denaturation temperatures of whey protein and soy protein were only 70°C and 68-88°C, respectively [26]. Oxidation kinetics and thermodynamic analysis indicated that water within the a w range of 0.614-0.654 acted as the plasticizer in the polymer matrix, preventing oxygen from passing through the pores, thus delaying the oxidation process to improve the stability of the microcapsule in a whey protein concentrate-polysaccharide matrix and to extend the shelf life [28].
Compared with animal protein, which costs more natural resources, plant protein that is more environmental friendly and meets the needs of vegetarians and people who want to lose weight, gradually enters the market [50]. Studies showed that the application of pea protein, soy protein, zein, or other plant protein well maintained the bioactivities of core materials and improved the solubility, dispersibility, stability, and so on [27,29]. For example, the microcapsule encapsulated with pea protein isolates exhibited smaller size by preventing droplet aggregation, and higher oxidative stability, which possibly resulted from its antioxidant properties [26]. Besides, the hydrogen bonding between the amino group of protein and the hydroxyl groups of the polyphenolic compounds strengthened the compactness of the wall and the core materials [29].
2.7. Composite Materials. The application of a single material is usually limited to its own nature; therefore, it is necessary to add other ingredients to make composite materials to enhance advantages and weaken disadvantages, so as to satisfy various environmental requirements. Santana et al. (2016) found that the microcapsule made from the ternary formula of gum Arabic and modified starch, together with either whey protein concentrate or soy protein isolate, exhibited higher process yield, solubility, anthocyanin retention rate, and encapsulation efficiency, as well as lower moisture content than that made from pure or binary formulas [32]. The addition of the ternary mixture of gum Arabic, modified starch, and whey protein concentrate with a high molecular weight increased the glass-transition temperature of the powder and decreased its viscosity, so that fewer solids would paste on the dryer chamber wall, which improved the process yield [32]. In addition, cell survival was increased by nearly 40% in encapsulated microcapsule composite of 1.06% alginate, 0.55% pectin, and 0.39% gelatin, in comparison to the 50.36% survival of free cells [30]. At high pH, the chain repulsion between the negatively charged deprotonated carboxyl groups in alginate and pectin produced osmotic pressure, leading to an increase in the swelling rate [30]. But the strong hydrogen bonding between the protonated amino group in gelatin and deprotonated carboxyl groups in alginate and pectin well balanced the intermolecular forces to avoid destruction [30]. Higher thermal resistance and 6 Advances in Polymer Technology crystallinity degree were detected in (ovalbumin, pectin, and xanthan gum) microencapsulated sacha inchi oil, indicating better heat stability and structure organization to protect ω-3 in oil [51]. The composition and proportion of microcapsule materials were adjusted to shape different properties, applying in personalized customization. For example, in the sweet orange essential oil microcapsule, the presence of cellulose nanofibrils helped reduce the droplet size and improve the encapsulation efficiency by increasing the emulsion viscosity, but promoted more essential oils release at 25°C due to its favorable permeability to liquid, in comparison to the formulation without cellulose nanofibrils [33]. Chitosan/carboxymethyl cellulose encapsulated carotenoid microcapsule showed low release in water and gastric juice which avoided degradation, and also low release in intestinal juice (adverse for absorption). In contrast, the microcapsules encapsulated in chitosan/sodium tripolyphosphate exhibited high release rate in water and gastric juice, and intestinal juice [31]. Similar research was performed on the microencapsulation of palm oil and the release curve revealed that the chitosan/xanthan wall material was more suitable for yoghurt system than chitosan/pectin [17].
FTIR spectra is widely used to investigate the characteristics of molecular structure and is also used to determine the intermolecular forces, including the bonds between wall and core materials, or those among composite materials of microcapsule, such as hydrogen and Van der Waals bonds [30], electrostatic interactions [52], and Ca 2+ complexation [8]. However, the lack of structure-activity relationship studies remains to be a major barrier in microcapsule application. That is why there were lots of research on microencapsulation with composite materials, but much fewer were actually applied in market production. It might be a promising solution to design the microparticles according to corresponding delivery process in desperate need of further study on molecular mechanism. Moreover, it is of great significance to establish a database to sort out and summarize the current researches on microcapsules, including the properties of various materials and the interactions and effects of these materials in different occasions. Then, the researchers can customize the microcapsules according to their use to match the suitable wall material and give full play to the advantages of different ingredients.

Yeast Cells.
Yeast cells, which are light in color and smell, disperse well in water, have strong adaptability to environment, and can be cultured on a large scale. Their natural eukaryotic cell structure makes them potential for material embedding. These characteristics of yeast cells offer advantages that other microcapsule wall materials cannot match: (1) the natural double-layer cyst structure formed from the outer cell wall and inner cell membrane of yeast cells can avoid volatilization loss of aromatic substances and oxidative deterioration from environmental light or oxygen invasion [53]. (2) No requirement of any other additives during the preparation of microcapsules (only yeast cells, core substances to be embedded, and solvents are needed to contact at high frequency) [14]. (3) Easy release of core substances: once meeting the wet mucosa such as tongue or nose mucosa, flavor substances or other active components can be released without breaking the wall. Yeast cells' natural biological adhesion provided a long-term release for flavor substances. (4) The nonthermoelastic cavity structure ensures that the wrapped core material will not be damaged by heat extrusion, roasting, frying, or boiling in the food processing. The β-glucan, which supports the physical strength of the cell wall, is difficult to break down and protects the core substance from pressure heating or freezing treatments [54]. (5) Yeast cells, safe and nontoxic, are easy to culture, thus have become an economical and eco-friendly microcapsule wall material.
For example, two flavors (D-limonene and ethyl hexanoate) in the yeast retained almost 85% after 1 h dry heating at 140°C, and thermogravimetric analysis suggested that the yeast cell wall would not break until a temperature above 260°C [54]. However, flavors were rapidly released once water was added to the powder, because hard β-glucans became soft and soluble under wet conditions and turned into a gelatinized solution [54].

Applications in Plant-Derived Food.
With the improvement of people's living standard and the pursuit of healthy life quality, health-promoting and environmental friendly plant-derived food will have a larger m4arket. At present, there have been a few studies on the application of microcapsules in baking, fruit/vegetable juice, or other plant foods to enhance the nutrition.
Umesha et al. found that the addition of microencapsulated garden cress seed oil protected α-linolenic acid against the oxidation in biscuits to prolong its shelf life [55]. The Garcinia cowa fruit extract rich in hydroxycitric acid, which is health-promoting but hygroscopic and thermosensitive, was protected by microencapsulated powder using whey protein concentrate and was incorporated into bread baking [56]. Among three microcapsules with different wall materials (whey protein isolate, maltodextrin, and a composite of both), the whey protein isolate exhibited higher encapsulation efficiency during the baking process. Besides, the bread with whey protein isolate-encapsulated Garcinia cowa fruit extract showed softer, lighter-texture, more desirable color and organoleptic properties, and higher free hydroxycitric acid concentration [56]. Furthermore, the pasta with microencapsulated hydroxycitric acid by spray-drying had higher antioxidant ability and sensory characteristics [57].
It was reported that passion fruit juice encapsulated in n-octenylsuccinate-derivatised starch retained over 70% of vitamin C after 77 days of storage at 7 or 25°C [58]. Alginate microbeads were effective in reducing the acidification and improving the sensory properties of fruit-based foods during storage [59]. In addition, the microencapsulation of allyl isothiocyanate (resistant to pathogenic fungi but irritating) effectively controlled its release rate to relieve irritation. And its application on fresh tomato significantly reduced the decay rate and weight loss, thus prolonged the shelf life of fresh products [60]. Microencapsulation was also a promising alternative to improve the stability 7 Advances in Polymer Technology of polyphenols, pigments, and nutrients in fruit-based food [61][62][63]. It was evident that the microcapsule of maltodextrin and Arabic gum maintained the anthocyanin content up to 150 mg 100 g -1 , over 80% of the initial concentration in juçara fruit pulp [63]. Interestingly, the microencapsulated alginate beads (Lactobacillus plantarum HER1325) prevented bacteriophage infections during vegetable fermentation [64].

Applications in Animal-Derived Food.
Compared with plant-derived food, microencapsulation technology is more applied in animal-derived food, especially in antiseptic and antimicrobial properties of meat products and probiotics maintenance in dairy products.
Microencapsulated clove oil might be an alternative preservative with antimicrobial effect, and only 0.070% of addition could reduce the disease index and mold spore variation rate, which made it effective for cooked meat products [65]. Although nisin has antibacterial activity, but no free nisin was available after 28 days of storage at 4°C. It was reported that the nisin microencapsulated in alginate-cellulose beads was evident to maintain half of the initial concentration of nisin (63 μg mL -1 ), which significantly reduced the number of Listeria monocytogenes in ready-to-eat ham and did not change the pH and the color [66]. Furthermore, lower fat content and energy value and higher protein concentration were detected in sausage, partially replaced with microencapsulated fish oil [67]. Besides, better preserved EPA and DHA, oxidation protection, and healthier polyunsaturated/saturated fat ratio were achieved by microencapsulation which reduced the atherogenicity and thrombogenicity traits [68][69][70].
Microencapsulation technology was also widely used in dairy products, especially in yogurt, in order to improve the vitality of probiotics in lower pH environment [71], resist gastric juice [72], inhibit postacidification [73], and release microbial cells in the intestinal environment to increase the bioavailability [74,75]. Lactobacillus paracasei subsp. paracasei and Lactobacillus paraplantarum microencapsulated with whey protein isolate and gum Arabic by complex coacervation exhibited significantly higher viability in simulated gastric juice (from 19% to 73%) and higher survival rate (from 59% to 86%) after 60 days of storage at 4°C than nonencapsulated cells [74]. Results demonstrated that the polyphenol extract microcapsule obtained by freeze-drying method had higher stability than that by spray-drying method, and the pH, titratable acid, viscosity, or other physiochemical properties of the supplemented yogurt were less affected [76]. Penhasi microencapsulated Bifidobacterium animalis spp. lactis in a double-layer capsule with the smart coating of hydroxypropyl cellulose and hydroxypropyl methyl cellulose for powdered infant formula [35]. The polymer formed a gel structure around the bacterial core to prevent heat and humidity from reaching the core materials and to protect the bacteria [35].
Excitingly, a research confirmed the specific interactions between the bacteria and whey protein, and that the pilus played a crucial role in the localization of bacteria in the microparticles. On the contrary, the encapsulation efficiency of the mutant lacking pili decreased significantly [77]. This discovery provided a new insight into the molecular mechanism of the embedding of probiotics [77].

Additives.
Except for plant-derived food and animalderived food, microencapsulation is also widely applied in food additives to offer food attractive appearance and fragrance to satisfy market requirement. Aromatic substances (D-limonene and ethyl hexanoate) coated with yeast powder had higher oxidation stability than that coated with maltodextrin [54]. Moreover, the presence of cyclodextrin entrapped volatiles and well maintained the strawberry flavor in response to the environment stress stimuli [78]. Estevinho and his colleagues produced flavor microparticles encapsulated in water-soluble chitosan by spray-drying, and the particle less than 100 μm in size with smooth spherical surface was obtained [79]. The mussel protein hydrolysate had bitter taste because of the hydrophobic amino acids produced during protein hydrolysis, but the bitterness was well covered up by microencapsulation to improve the sensory acceptance [80]. Similarly, unpleasant taste and odor of isoflavone added in the beverages were masked by being microencapsulated in inulin and maltodextrin [81]. Then, the microencapsulated isoflavone was gradually released during simulated digestion [81]. In addition, hibiscus extract rich in anthocyanins was a natural colorant but sensitive to light, heat, and oxygen. Microencapsulation through dripping-extrusion or atomization effectively increased the stability of anthocyanin during food processing and storage [82]. Furthermore, the controlled release achieved by microencapsulation offered a long duration of flavor in chewing gum [83]. Due to the high cost and technical constraints, the application of microcapsule technology in everyday condiments is still under research in the laboratory stage. But microcapsule technology has already commonly appeared in the fortified food market, such as Capsulae

Microencapsulation Techniques
The publications of commonly used microcapsule embedding technologies from 2010 to the end of 2019 are shown in Figure 2. Apparently, "spray-drying" came first by a landslide, accounting for almost a third of all publications, followed by "emulsification," "freeze-drying," and "coacervation." Next, the advantages and disadvantages of several top published embedding techniques are presented below.
4.1. Spray-Drying Technique. Spray-drying microencapsulation is to atomize the emulsion of wall material and core material in the dry and high-temperature environment, which evaporates the moisture via heat exchange between the droplets and the drying medium and solidifies the shell of droplet quickly to wrap the core material. This coating method, the most widely used embedding technology, is characterized by low cost (30-50 times lower than freezedrying), simple operation, continuous production, and 8 Advances in Polymer Technology suitability for mass production [4]. However, during the preparation process, the core material should be in the high-temperature airflow, where the active substances are easy to be inactivated, resulting in lower embedding rate and coating efficiency [4]. Besides, spray-drying method requires that wall materials have good water solubility, low viscosity, and good fluidity, so only few wall materials can be used for spray-drying, such as Arabic gum and modified starch, which limit the application of spray-drying [3]. Moreover, low water evaporation brings about agglomeration or hardening, whereas the excessive water evaporation leads to cracks in the wall material and reduced compactness. Several studies combining spray-drying with vacuumdrying or freeze-drying improved the embedding efficiency of probiotics or other heat-sensitive materials but also increased the production cost [85].

Emulsification Technique.
Emulsification is a chemical embedding method in which the mixture of core material and wall material (dispersed phase) is added into a large number of vegetable oil (continuous phase), containing the emulsifier to form a stable emulsion and microencapsulate under the action of cross-linking agent. Although the high survival rate and simplicity of encapsulated probiotics, the feasible preparation process was achieved by this method, and the production cost was usually very high because of the large amount of vegetable oil required [5].

Freeze-Drying Technique.
Freeze-drying is a method of sublimating the ice into vapor under high-vacuum condition after quick freeze. The ice sublimation removes heat and keeps the whole process cool, which preserves the activity of some biological samples, such as proteins. However, the formation of ice crystals during freezing process and the high osmotic pressure during dehydration might destroy the integrity of the microbial cell membrane; therefore, the hydrophilic substance is usually added into the system as the cryoprotectant [86]. Due to the high-cost limitation, freeze-drying technique is more used for heat-sensitive food with high value.

Coacervation
Technique. The core material is emulsified or suspended in the solution of wall material, and then another substance or solvent is added to reduce the solubility of the wall material, which is evenly aggregated and surrounded by the core material to form microcapsules. The coacervation technique includes the complex condensation and the single condensation. In the complex coacervation, two materials with opposite charges are used as wall materials, and the core materials are emulsified and dispersed in the wall materials solution. By regulating the pH, temperature, or concentration of aqueous solution of the system, the two wall materials are aggregated with the core material through the interaction between the opposite charges to form microcapsules [87]. Complex coacervation is a commonly used technique for embedding fat-soluble food ingredients with advantages of no need of special equipment, mild process conditions, less damage to core material quality, and higher product encapsulation efficiency, as well as the better antioxidation and release controlling properties. Gelatin and gum Arabic wall materials are more applied in the microcapsule preparation by coacervation method [88,89].
The main disadvantages of coacervation technique are high cost, a lot of coagulants consumed, difficult to control the conditions of coacervation reaction, fewer coagulants available for wall materials, and easily produced chemical residue during processing [3]. Due to these shortcomings, this technique is still at the experimental stage and has not been widely applied in food industry. However, it is still a potential technique due to the great release control ability.

4.5.
Layer-by-Layer (LBL) Assembly Technique. Layer-bylayer (LBL) self-assembly is a process in which layers are spontaneously attached with each other to form the stable molecular aggregates or supramolecular structures which possess specific functions or performances by noncovalent interactions, like electrostatic attraction, hydrogen bond, and coordination bond. LBL assembly technology does well in controlling the size, shape, composition, thickness, and structure of the capsule on the nanometer scale accurately [90,91]. Therefore, LBL assembly technology is a promising method for the preparation of multilayer microcapsules matching variable environment [90]. However, the preparation process of LBL self-assembly, where multiple wall materials with different charges are needed, takes a long time; therefore, this technique is not suitable in mass rapid production yet. Moreover, the insufficient stability of LBL assembly that resulted from the weak interactions may impact the product quality.

Extrusion Technique.
Extrusion is a physical embedding method for forming microcapsules by squeezing core material and colloid mixture into the hardening bath in the form of the liquid drops through the needle tube under pressure.

Advances in Polymer Technology
The cost is more than twice as much as the spray-drying, but this technique could effectively protect oil from volatilization and oxygen to significantly extend the shelf life of products due to its small surface area of micropores [3]. However, the low production rate could not meet the demand of large-scale production in industrial application, and the large particle size affects its taste [47]. And carbohydrates that could form glassy structures are commonly used as wall materials [92]. Extrusion technique is commonly used for embedding all kinds of volatile, vitamin, and pigment compounds or other heat-sensitive materials. 4.7. Supercritical Technique. The nonvolatile substance is dissolved in the supercritical fluid, which could rapidly expand in a very short time when decompressed through the pore capillary, so that the solute oversaturates and a large number of fine particles are formed. By controlling the experimental conditions, the hollow microcapsules with a certain particle size could be precipitated and separated [93]. Then, the generated hollow microcapsules and the core material collide with each other frequently and evenly wrapped together. Afterwards, the microcapsule product could be obtained after removing the unembedded core material [93]. Among the supercritical fluid, the supercritical CO 2 is most widely applied because of the low critical temperature, low viscosity, high solvent, high dispersion, high mass transfer, and nontoxicity. Usually, the supercritical CO 2 to prepare microcapsules requires almost no organic solvents, green and environmentally friendly, and the resulting product has a small particle size and is suitable for heat-sensitive substances [93]. In addition, hydrophobic or hydrophilic materials that can be dissolved in CO 2 can be used as microcapsule wall materials [94]. However, the solubility of CO 2 to different solutes varies greatly, and materials with low boiling point, low polarity, or low molecular weight are generally more suitable [94].

Electrospray
Technique. Electrospray is to decompose the polymer fluid transported by the conductive capillary pump into the fine droplets through a high-voltage electric field [95]. After the solvent evaporates, the polymer particles are generated and collected on the metal collector to obtain microcapsules [95]. This electrospray technique does not require additional reaction solvent and could be realized in one-step and eco-friendly [96]. Microcapsules prepared by electrostatic spray are homogeneous and nanometer in size, which has attracted more and more attention [5]. The electrospray technology has widely been applied in the pharmaceutical industry and has great potential in the encapsulation of bioactive substances, volatile compounds, sustained-release preservatives, and functional foods in food industry [5].
4.9. Nanocapsulation Technique. With the development of microcapsule technology, the particle size of microcapsule can reach the nanoscale, that is, the nanocapsule. Nanocapsule is characterized by a small particle size, large specific surface area, and easy formation of uniform and stable colloidal solution. Besides inheriting the advantages of ordinary microcapsules, nanocapsules are smaller in size, which can increase the adhesion of active substances to tissues, so they effectively improve the bioavailability of nutrients in functional foods [97]. Nanocapsules can also penetrate through capillaries, penetrate into tissues, and be absorbed by cells, thus enabling more precise targeting of the core materials [98]. Therefore, nanocapsule has become an emerging direction in the microcapsule research field.
Compared with ordinary microcapsules with particle size in microns, nanocapsules have higher requirements on particle size; thus, nanocapsules need additional special treatments, usually high energy, based on the preparation of ordinary capsules, such as ultrasound, high pressure, or intense mechanical agitation [5,93].

Conclusions
This paper offers an overview on the recent development of microencapsulation in food industry, especially the advances in polymer wall materials, food applications, and embedding technologies. Microencapsulation technology is mainly applied in the delivery of functional ingredients, which contributed to the extension of shelf life, the enhancement of thermal stability, the inhibition of pathogenic microorganism development, the enrichment of nutrition, the controlled release, etc. A variety of materials were used to prepare microcapsules, including polysaccharides, proteins, and other rich natural sources. And the composite materials were increasingly attractive to meet the needs of different delivery systems in response to various environmental stress stimuli. A database can be built based on the characteristics of different wall materials and various microencapsulation techniques to help/guide researchers to design and produce suitable microcapsules. Moreover, microencapsulation was widely used in animal-derived food, including dairy and meat, and plant-derived food, especially the preservation of fresh fruits and vegetables or color-protection in baking. Besides, additives with microencapsulation got further progressed and wider application. Among the microencapsulation technologies, the spray-drying technique was ranked first for the low cost and suitability for large-scale production. However, other novel techniques free from environmental pollution, such as the LBL self-assembly and electrospray, are potential and likely to be new directions in the future.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.