Microencapsulation: An overview on concepts, methods, properties and applications in foods

Correspondence MurlidharMeghwal,Departmentof Food ScienceandTechnology,National Institute of FoodTechnologyEntrepreneurship and Management,Kundli, India. Email:murli.murthi@gmail.com Abstract Microencapsulation is an advanced food processing technology, using which any compound can be encapsulated inside a particular material, making a tiny sphere of diameter ranging from 1 μm to several 100 μm. Microencapsulation is done for protecting the sensitive compounds and, hence, ensuring their safe delivery. The compound or active material which is encapsulated is called the core and the material which is used for encapsulating is called the encapsulant. Encapsulants can be either polymeric or nonpolymeric materials like cellulose, ethylene glycol, and gelatin. There are several techniques used for microencapsulation. Fluidized bed coating, spray cooling, spray drying, extrusion, and coacervation are few to be named. The selection of a particular technique depends upon the properties of the core material, encapsulant, and different properties andmorphology of the capsules desired. The characterization and optimization of efficient and successful encapsulation can be done by studying the encapsulation efficiency and various properties of the capsules like morphology, size, hydrophobicity, hygroscopicity, solubility, surface tension, thermal behavior, and mechanical properties. Microencapsulation is a technology that is extensively used in foods, whether as a fortifying tool or as a mode for the development of a functional food. Based on the fundamental understanding of encapsulation and latest research and findings from literature, this review critically analyses and brings together the utilization of this particular technique in foods, different methods used for encapsulation, different properties of the capsules which result from the different techniques adopted for microencapsulation and different release mechanisms used for delivering the compounds.

safety by inhibiting the growth of the microbes (Hasanvand et al., 2015;Sengupta et al., 2001). Different bioactive compounds, such as omega-3 and omega-6 fatty acids, vitamins, phenolic compounds, and carotenoids are now widely used to develop products with numerous functional properties to meet up the increasing consumer demands.
However, such compounds are highly unstable under certain conditions of light, temperature, pH, and oxygen. Therefore, microencapsulating such compounds is a mode of protecting them from such harsh conditions during processing of foods. Several food constituents which are widely encapsulated include different flavoring agents, lipids, antioxidants, essential oils, pigments, probiotic bacteria, and vitamins (Azeredo, 2005). Different coating materials are used depending on their rheological properties, their ability to disperse the active compound and stabilize it, inertness towards the active compound and their ability to properly hold the active compound. Some coating materials include carbohydrates such as starch, maltodextrin, modified starch, cyclodextrin, cellulose; lipids such as wax, paraffin, beeswax, diacylglycerols; gums such as gum acacia, agar, carrageenan; and proteins such as gluten, casein, and gelatine.
Microencapsulation is a technology that serves as a tool to protect the sensitive and expensive nutrients (Meyers et al., 1998), by providing them with a protective wall, which allows them to get released at a particular site, at a particular time, and under particular conditions. For example, as mentioned by Gudas et al. (2000), in chewing gums, the encapsulated flavors escape only on chewing. In the recent past, complex food formulations have been demonstrated in the food industries like the use of certain volatile flavors in instant mixes, fatty acids in the dairy products, which are highly prone to auto-oxidation. Here, microencapsulation can come to rescue (Gharsallaoui et al., 2012;Khan et al., 2011). Large number of techniques for microencapsulation include spray chilling, spray cooling, fluidized bed coating, liposome entrapment, extrusion, freeze drying, and coacervation.
On the basis of the physical and the chemical properties of the core, composition of the shell material and the microencapsulation method used, various types of capsules are obtained: simple sphere surrounded by the wall material, capsules with irregular core, multiple distinct cores within a continuous coating of wall material, multiwalled microcapsules and core particles embedded within the matrix of wall material. Depending on the kind of coating material used, different techniques are used to produce the microcapsules and these techniques lead to differences in the properties of the capsules like capsule size, morphology, porosity, hygroscopicity, hydrophobicity, surface tension, and thermal behavior. It is very important to learn about these properties of the capsules, so as to understand their behavior in any food system. These properties in turn are closely related to the controlled release of the encapsulated core. The core material needs to be properly protected to be released at a specific time, thus improving the efficacy of the microencapsulation process and leading to the broad range of applications. The major factors influencing core release includes nature of the core material, the ratio of the core and the encapsulant, nature of the encapsulant, and the interaction between the two (Roberts & Taylor, 2000). This review critically analyses all the important concepts associated with microencapsulation including the need of microencapsulation in food, properties of the coating materials, and the type of core material they can be used for. Furthermore, it also includes different techniques of microencapsulation, properties of the microcapsules like physical, mechanical, thermal, and functional and different core release mechanisms. It also showcases the relevance of this technology in food industries.

Protection and improved delivery
Many components, like the essential oils, having numerous benefits like antimicrobial and antioxidant properties, the microorganisms which are used in fermentation and the probiotics, can widely be used in the preparation of functional foods. But components like these are highly susceptible to oxidation when exposed to high temperature, light or oxygen atmosphere. So, encapsulation has proved to be one stop solution for such problems. Piletti et al. (2019)  and calcium alginate gel, using extrusion, which resulted in increasing survival rate of the strains in Iranian cheese, even after storing for 6 months. Yuliani et al. (2006) demonstrated that encapsulation of limonene within β-cyclodextrin using extrusion proved to be an effective way to protect limonene from getting oxidized. Hence, these studies prove that microencapsulation is an effective method for protecting different sensitive components.
Improved delivery of the components being encapsulated means that these components are being delivered completely upon their controlled release and this is based on choosing the right coating mate-

Controlled release
Encapsulated functional components like certain vitamins, flavors, or essential oils when incorporated in the food matrix are of importance

Masking of flavor and odor
Microencapsulation is used for masking the undesirable flavor and aroma of certain compounds before incorporating them into any food.
For instance, fish oil and certain bitter tasting compounds can all be used in foods without rendering the food with unpleasant taste and smell, all thanks to microencapsulation. Breternitz et al. (2017) in their study showed that the bitter taste of mussel protein hydrolysate could be successfully masked by encapsulating it by spray drying using modified starch and maltodextrin as the carrier agents. Similarly, unpleasant odor and taste of isoflavones were masked by microencapsulating them using maltodextrin and inulin (Wyspianska et al., 2019).

COATING MATERIAL
The coating material or the wall material used in microencapsulation should be such that it is able to form a cohesive film on the core, stabilize it, and provide strength to the capsules, inert, so that it has no reaction with the core material, does not provide any specific taste to the product, impermeable and with ability to release the core at a specific time and place, upon specific treatment. Oleoresin was encapsulated within a mixture of maltodextrin, modified starch, and gum Arabic using spray drying and the results showed increased protection of oleoresins (Krishnan et al., 2005). Bayram et al. (2005) reported successful encapsulation of sumac flavor in sodium chloride using spray drying.
Although spray drying is one of the most extensively used methods for microencapsulation and has many stated advantages also, some studies have portrayed certain drawbacks of the technique. Fang et al. (2006) reported that when hot air is used as a drying medium for encapsulation of omega-3 fatty acids, dried powder has particles with highly porous structure, making the powder more prone to oxidation, thus, reducing the shelf life. Similar results were reported by Kolanowski (2005), while developing spray dried fish oil powder. Thus, it can be said that the same method can be effective for encapsulating one kind of material, while having drawbacks for some other kind of materials.

Spray cooling
Spray cooling method of encapsulation is very similar to spray drying in operation, the major difference being the use of cold air in it.
Here, a mixture of core material and wall material is atomized to form a mist inside a chamber, inside which cold air flows. The low temperature within the chamber results in solidification of the micro droplets, leading to the formation of microencapsulated powder. This technique also has a huge potential in scaling up. Some successful implementations of this technique in encapsulation includes, microencapsulation of tocopherols within lipid matrix, with encapsulation efficiency as high as 90% (Gamboa et al., 2011), encapsulation of iron, iodine, and vitamin A within hydrogenated palm oil to fortify salt, where in the microcapsules formed were highly stable (Wegmüller et al., 2006).
However, this method also has some documented drawbacks. Some studies have shown that the microcapsules formed by spray cooling are not very stable and it leads to expulsion of the core material during storage (Jenning et al., 2000;Müller et al., 2002).

Coacervation
Coacervation is a simple technique which involves formation of a homogeneous layer of the polymeric wall material around the core material. This is achieved by altering the physicochemical properties of the wall material by change in temperature, pH, or ionic strength. Here, the core material and the wall material are mixed to form an immiscible solution. Then, phase separation is carried out by changing the ionic strength, pH, or temperature to form coacervates, which are tiny liquid droplets, consisting of polymer-rich dense phase. These coacervates then surround the core material, forming the microcapsules. Electrostatic interaction between two aqueous media is responsible for liquid to gel transition, that is, ionic gelation, hence, leading to the for-mation of coacervates. This technique is basically used for encapsulating hydrophilic molecules. Such coacervation, which involves only one polymeric material is called simple coacervation. One example of such a polymer can be sodium alginate. In simple coacervation, sodium alginate is dissolved in water and the active compound that needs to be encapsulated, which is usually an oil, is mixed into it and the emulsion formed is released in drops into a gel-forming media like calcium chloride. Ionic interaction between sodium alginate and calcium chlo-

Extrusion
Extrusion technology for microencapsulation can be used for producing highly dense microcapsules. To use this method, the core and the

Solids and liquids
Cyclodextrin Inclusion Hydrophobic interaction between the cyclodextrin surface and the guest compounds.
Solids and liquids wall material should be immiscible. Here, the core and the wall materials are passed in such a way that the wall material surrounds the core and they are passed through concentric nozzles, thus, forming droplets containing the core surrounded by the wall material. Then solidification is done either by cooling or using an appropriate gelling bath wherein the droplets fall and solidify due to formation of complex. The encapsulates formed using this method are relatively larger in size than formed using any other method and also, this technology is useful with limited wall materials.

Emulsification
Encapsulation using emulsification technique is done by dispersing the core in an organic solvent, containing the wall material. The dispersion is then emulsified in the oil or water, to which emulsion stabilizer is added. Encapsulation of the core occurs by formation of a compact polymer layer around it, by evaporation of the organic solvent. This is one of the frequently used techniques of encapsulation as the procedures involved are very simple. This technique is widely used for encapsulating enzymes and microorganisms. Song et al. (2013) reported encapsulation of probiotics in alginate-chitosan using emulsification and demonstrated better resistance of the probiotics under stimulated gastrointestinal conditions.

Cyclodextrin inclusion
Cyclodextrins are cyclic oligosaccharides, capable of forming inclu- Some of these techniques, along with their principles and type of core material they are used for are represented in Table 2  Particle size and morphology of microcapsules The particle size of the microcapsules depends on the different techniques which are used to produce the microcapsules. Table 3 shows the variation in the particle sizes due to different techniques used.
Morphology of the microcapsules refers to the internal as well as the external structure of the capsules which largely depend on the oper-ating conditions that are used to produce the microcapsules as well as the wall materials used. Different types of capsules can be obtainedsimple sphere surrounded by the wall material, capsules with irregular core, multiple distinct cores within a continuous coating of wall material, multiwalled microcapsules and core particles embedded within the matrix of wall material, as shown in Figure 2.
Morphology can be determined by Scanning Electron Microscopy (SEM). By using SEM, both internal as well as external microstructures can be obtained. According to Carneiro et al. (2013), while observing the external morphology of the microcapsules containing flaxseed oil surrounded by different shell materials like gum arabic, obtained using spray drying technique, the particles showed varied sizes, a TA B L E 3 Relative size of microcapsules formed corresponding to microencapsulation process

Microencapsulation process
Approximate particle size (µm)  Trindade and Grosso (2000). However, sometimes capsules may also develop roughness on their surface during spray drying.
And such imperfections are developed when the film formation process during drying of atomized droplets slows down. In a similar way, internal morphology was analysed and it was observed that the microcapsules obtained were hollow and the core material was stuck onto the surface, which is also a characteristics of particles obtained using spray drying. Differences in wall material also affected the topography of the microcapsules formed. Similarly, Dong et al. (2011), while studying about the microcapsules with peppermint oil as the core material, produced by complex coacervation method reported that multinuclear microcapsules were formed with many emulsion droplets being encapsulated by coacervates. SEM results also showed that microcapsules were spherical, with their surface being smooth and continuous. Likewise, on the basis of the technique and the core and the wall material used, morphologies of microcapsules differ.

Porosity
Porosity of the microcapsules, formed using any technique, is one of the most important properties of the microcapsules, responsible for their function in a particular food matrix. And this property is greatly dependent on the composition of the wall material of the microcapsule and the technique which is used to produce the microcapsule.
Wall matrix, which holds the core is designed in such a way so as to direct the mass transfer between the environment and core (Rosenberg et al., 1985;Jackson & Lee, 1991;Shahidi & Han, 1993). The porosity of the wall material plays a great role in controlling the permeation of volatiles within the capsule (Arshady, 1993;Dziezak, 1988).
It also determines the oxidative stability of the core of the microcapsule by controlling the permeation of oxygen through it. In case of volatile cores, wall permeability is a major factor for increasing the chances of core loss during storage (Rosenberg et al., 1985). Moreau and Rosenberg (1999) examined the porosity of the spray-dried microcapsules, encapsulating anhydrous milk fat within the mixture of lactose and whey protein as wall system by using gas displacement pycnometry. In this study, helium and nitrogen were used as the permeating gases. Results showed the differences in the way of penetration of helium and nitrogen through the microcapsules. It was seen that helium could fill all the accessible volume very fast, while penetration of nitrogen was comparatively slower. Overall, the results of gas-displacement pycnometry indicated the presence of pores characterizing both, anhydrous milk fat containing and core-free microcapsules (Rosenberg et al., 1985). The microcapsules containing milk fat were found to be more porous than the ones, which were free from the core material. Similar characteristics were obtained for the microcapsules encapsulating fish oil with dextrin and sodium caseinate wall materials. Those microcapsules also exhibited molecular-sieve kind of porosity, that is, with pores, which are minute enough to pre-  (2002) and Sultana et al. (2000). TEM analysis also showed the similar results. Hence, for determining the porosity of microcapsules, gas displacement pycnometry and electron microscopy, both can prove to be very important tools for analyzing the microstructure of the capsules, useful for designing the carrier substances for a particular core material.

Surface hydrophobicity
Surface hydrophobicity can be defined as a physical property of a molecule that is repelled by water. This is a property which is largely based on the core material to be encapsulated and the wall material. In a study by Mendanha et al. (2009)

Flow properties
Flow properties of the microencapsulated powders include bulk density, tapped density, porosity, and compressibility. Analysis of bulk density and tapped density of the capsules is important in order to obtain the capacity of the powder formed, in packaging, storage, and the distribution process. Bulk density is dependent on the particle density, size, shape, and water content of the microcapsule (Shamaei et al., 2017), which in turn depends on the coating material and technique used for encapsulation. Dependence of bulk density on the coating material has been explained by Mehyar et al. (2014) in a study, where cardamom essential oil was encapsulated using whey protein isolate (WPI), guar gum and carrageenan by freeze drying. Bulk density of the microcapsules was highest when WPI was used alone as the coating material as compared to combination of WPI with guar gum and carrageen. In a study by Hermanto et al. (2016), bulk density of cinnamon oil microcapsule, encapsulated within a coating of maltodextrin and gum arabic at different concentrations, was measured using a very common method of tapping, which is also reported in Chinta et al. (2009). And it was found that if the size of the microcapsule formed is small, it would lead to larger mass density of the capsule due to decrease in the cavities between the particles. Also, higher levels of water leads to increase in the weight of the material in the container, volume remaining the same, thus, causing an elevation in the bulk density (Prabowo, 2010 with the decrease in feed atomization pressure and increase in the inlet air temperature, the bulk density of the microcapsules decreased. This was because, with the increase in inlet air temperature, microcapsules with hollow structure and high sphericity were formed. Similar results were also reported by Tonon et al. (2011).
Flowability of microencapsulated powder formed is determined by using two parameters, percent compressability or Carr's Index and the Hausner Ratio (HR), as reported by Turchiuli et al. (2005). Similar methods were adopted by Xue et al. (2013) for determining the flowability of the lycopene microcapsules. The higher value of HR attributed to the fact that the powder was cohesive, indicating high powder viscosity and was restricted to free-flow. Flowability largely depends on the particle size and particle size distribution of the microcapsules, which in turn is influenced by other factors like shape and surface roughness of the microcapsules. Thus, it is very crucial to choose a particular wall material and a particular technique of encapsulation to obtain the flow properties of the capsules as desired.

Micromechanical properties
Mechanical trigger of the microcapsules depends upon their micromechanical properties. It is a fundamental need to study the mechanical properties of the microcapsules once they are produced, so as to ensure that the release of the core material takes place at a specific target and at a specific time and not before that. More specifically in many food applications, a very adjustable kind of mechanical strength is desired in the microcapsules. The mechanical properties of the microcapsules include elastic modulus, rupturing force that is required to rupture the capsule and nominal rupture stress, which is equal to the rupture force divided by the initial cross-sectional area of the microcapsule (Sagis, 2015). And these largely depend on the core to shell ratio in capsule and also on the preparation and processing conditions, involves microcompression of a single particle (Sagis, 2015). Herein, the capsule under consideration is allowed to adsorb onto a substrate, which is then deformed by a colloidal probe particle attached to a cantilever AFM, as shown in Figure 3. The probe particle compresses the microcapsule by itself getting displaced in a vertical direction over a distance d. The force required for displacement of the probe particle over a distance d can be calculated from the known force constant of the cantilever and the deflection it undergoes, which is determined by a laser. So, a force displacement curve can be generated, which in turn can be analyzed by using an appropriate model to determine the mechanical properties of the capsule (Fery & Weinkamer, 2007;Neubauer et al., 2014).

b. Fluid-mechanics-based determination of mechanical properties
Mechanical properties of the microcapsules can be determined by applying extensional and shear force on them in flowing fluid (Sagis, 2015). Rotating shear devices are used for this purpose, as shown in Figure 4. These rotating shear devices have transparent concentric cylinders with narrow gap between them, in which a very dilute dispersion of the microcapsules is being poured. The microcapsules in the dispersion are deformed in such a way that they form an angle with the flow direction, once the dispersion gets exposed to a steady shear field.
The degree of deformation as well as the orientation of the microcapsules gives their mechanical characterization.

c. Osmotic swelling method
For microcapsules having water permeable shell and aqueous core, their mechanical characterization can be done by using this method (Sagis, 2015). In this method, the microcapsules are made to come in F I G U R E 4 Rotating shear cell contact with an aqueous high molecular weight polymer with a continuous phase. Difference in osmotic pressure between the interior and the exterior phase is created due to difference in the chemical potential of water between the two phases. And when the concentration of the external polymer is very high, migration of water starts taking place from the interior aqueous phase to the exterior one, until equal chemical potential is reached on both the sides. As a result of this movement of water, there is a shrinkage in the capsule, leading to the crumpling of the shell, as shown in Figure 5. In order to observe such a phenomena, laser microscopy is used. The critical osmotic pressure required for crumpling is determined and the shell characterization is done by plotting this critical pressure versus the wall thickness of capsule.

d. Method based on thermal expansion
This method is used for mechanical characterization of the microcapsules with an oily core. It is valid only when the coefficient of thermal expansion of the core material is more than that of the shell material as well as the continuous phase in which the capsules are dispersed.
In this technique, capsules are dispersed in a continuous phase and they F I G U R E 5 Osmotic collapse of the microcapsule: (a) core moving out, (b) collapsed capsule

F I G U R E 6 Thermal expansion of microcapsules
are observed under a microscope with a thermal stage. The dispersion is subjected to a temperature that is continuously increasing at a fixed rate. The core of the capsule expands more than that of the shell and the continuous phase, which leads to the building up of stress inside the capsule. This continues until the shells burst. This process is shown in Figure 6. Once the 90% of the capsules are burst, temperature as well as the time required for bursting is recorded. Use of this technique is reported by Humblet-Hua, van der Linden, and Sagis (2012) for shell characterization of the microcapsules having shells reinforced with lysozyme fibrils, which were produced by using LBL adsorption.
No such studies have been reported in the food microcapsules.

Thermal properties
Thermal properties of microcapsules is one of the crucial properties to be studied so as to determine their storage stability as well as the release rates. These can be obtained by a technique called Differential respectively, which showed that the vitamin A microcapsules had the storage and heating stability. It can be clearly observed from the above mentioned study that determination of thermal properties of the capsules is crucial for indication of correct storage temperature and also the indication of the temperature at which the food containing the microcapsules needs to be processed.

Functional properties
In addition to the physical, mechanical, and thermal properties of the microcapsules, functional properties are also very important, especially while using the microcapsules to develop a new product with added functional properties. Following are some of the important functional properties of the capsules. Different functional properties of various microcapsules, encapsulating different compounds are shown in Table 4.

Solubility
Solubility evaluation of the microcapsules is basically done to determine the behavior of microcapsules in water or any other medium, that is, whether the core material is released in that medium or not. Solubility is a property of microcapsules, which is attributed to the type of wall material used for encapsulation as well as the technique used for production of the microcapsules. In a study performed by Mendanha et al. (2009), solubility of the microcapsules, containing casein hydrolysate within Soybean Protein Isolate (SPI) and pectin was very low, even after soaking in water for 24 h. This confirmed that the method i.e.
coacervation for producing the microcapsules as well as the encapsulating agents used led to the production of the microcapsules, which are very stable in the aqueous medium as well as which have favorable controlled release properties. The findings in this paper leads to the conclusion that solubility also depends on the concentration of the core material that is used for encapsulation as here, the solubil- to the surface and unfolded, and then they rearranged to form multiple layers. Hence, the wall materials used did not act as effective surfactants. From the above study, it is clear that surface tension of the microcapsules is largely a function of the wall material used.

Hygroscopicity
Microcapsules when exposed to an environment with a high relative humidity, tend to absorb moisture from the environment and this property is called hygroscopicity. It decides the stability of the core material. Hygroscopicity of a microcapsule largely depends on the type of wall material which is used to hold the core material, that is, how hygroscopic it is. For example, for microencapsulation of oils and certain flavors, a wall material which is less hygroscopic is used, such as WPI. This property of the microcapsules during storage can be determined by using the sorption isotherms. Frascareli et al. (2012) determined the critical storage conditions of the microencapsules encapsulating coffee oil by obtaining the sorption isotherms of the microcapsules. The microcapsules were produced using spray drying technique and gum Arabic, WPI and mixtures of maltodextrin (MD) and WPI, at three different proportions (3:1, 1:1, and 1:3) were used as encapsulating agents. Sorption isotherms of the microcapsules were determined by the gravimetric static method (Tonon et al., 2009).
Results revealed that the microcapsules produced with gum arabic showed significantly higher water adsorption, while the microcapsules produced with WPI showed lowest hygroscopicity. Such property could be attributed to the different chemical structures of the wall materials used. Gum arabic has more number of ramifications with hydrophilic groups that can bind to water molecules easily, whereas whey protein has less number of such groups to interact with water, leading to lower hygroscopicity of the microcapsules. So, selection of wall material will significantly effect the hygroscopicity of the microcapsules obtained.

Encapsulation efficiency
Encapsulation efficiency is defined as the amount of core material that is encapsulated within a wall material, against the concentration of the core that was used for encapsulation, using a particular technique.
It depends on the concentration of the core material that is used for encapsulation. With the increasing levels of the core material, encapsulation efficiency tends to decrease. In a study by Mendanha et al. (2009), encapsulation efficiency was determined by centrifuging the microcapsules and estimating the protein content (casein hydrolysate content) by using biureto-spectrophotometric method (Gornall et al., 1949). Formula for determining encapsulation efficiency is (Equation 1): Total free hydrolysate is the protein (g) in the supernatant, which does not take part in the formation of microcapsules. And total hydrolysate is the amount of protein (g) in the formulation. Another method of determining encapsulation efficiency of encapsulating certain oils, as mentioned by Bae and Lee (2008), involves use of solvents like hexane. Herein, the powder is mixed with a solvent like hexane for extracting the free oil, which is then filtered through a whatman no. 1 filter paper. Once the powder is obtained on the filter paper, it is washed numerous times with hexane and kept for drying at room temperature, then at 60 C for evaporating the solvent. Once a constant weight is obtained, the surface oil is determined by subtracting the weight of the initial clean glass container from that, which contains the extracted oil residue (Jafari et al., 2008). Similar methods were reported by Varavinit et al. (2001). And encapsulation efficiency is found out by Equation (2): where EE is encapsulation efficiency, TO is total oil, and SO is surface Results showed that after 8 h of exposure to the sun light, concentration of raw resveratrol decreased to 20%, as compared to 57% drop in resveratrol dispersion. Thus, light stability was found to be improved by encapsulation.

DIFFERENT METHODS FOR CONTROLLED RELEASE OF THE CORE MATERIAL
Microencapsulation is considered to be effective only if the core material is protected until its release is desired. There are different mechanisms which are used for releasing the core material. These include degradation, diffusion, dissolution, application of pressure and change in temperature and pH. Mechanisms used are based on the properties of the core and the wall material.
Some of the methods are described below: a. Diffusion: Diffusion of the core normally occurs when the wall of the microcapsule is intact and a fluid penetrates through the wall, dissolves in it, the core material and disperses out through the pores.

b. Dissolution:
Here the release of the core depends upon the solubility of the wall material into the dissolution fluid. The wall remains no longer intact when comes in contact with the fluid, it solubilizes and releases the core. The rate of release depends upon the properties of the wall material and dissolution fluid and the thickness of the wall. c. Osmosis: In osmotic release of the core material, the wall of the microcapsule behaves as a semipermeable membrane, which allows development of osmotic pressure difference on either side of the wall. Due to this pressure difference core material moves from inside of the capsule to outside.

d. Degradation:
In this method of core release, enzymes such as, proteases and lipases, are used to degrade the proteins and lipids in the wall material, hence breaking of the wall and releasing the core. e. Change in pH: Change in pH can lead to release of core as it can affect the solubility of the wall material. A wall material may be such that it remains intact under acidic conditions, while gets solubilized by altering the pH leading to alkaline conditions. f. Changes in temperature: Core release can be promoted by changing the temperature to which the microcapsules are exposed. For temperature-mediated core release, two different mechanisms are involved: (a) One is called temperature-sensitive release, where the wall material is such that it collapses when it gets exposed to a temperature known as the critical temperature, beyond which it cannot withstand and expands. (b) The other mechanism is called fusion-activated release, in which the wall material starts melting when it is exposed to an increased temperature and, hence, releases the core.

DEGRADATION OF CORE IN THE MICROCAPSULES
To study the stability of the microcapsules under various conditions of temperature and pH and to determine how effective is the coating material and the encapsulation method in protecting the core during storage, degradation kinetic studies are performed for the microcapsules. While performing such studies, known amount of the microcapsules are stored under controlled conditions and taken out at regular intervals for determining the degradation in the core content. Table 5 shows comparison in degradation kinetics of various encapsulated compounds produced using different methods and coating materials.

APPLICATION OF MICROENCAPSULATION WITH RESPECT TO FOOD INDUSTRIES
Microencapsulation has proven and is further considered to prove as an effective tool in creating novel food products with numerous functional properties, when looked from industrial point of view. Microencapsulation technology has been widely used in certain commercial food products like juices, chocolates, meat and poultry products, etc.
Marcial-Coba, Saaby, Knøchel, and Nielsen (2018) reported the com-mercial availability of dark chocolates, which serve as a stable carrier of microencapsulated probiotic strains, namely, Akkermansia muciniphila and L. casei. These strains were encapsulated within xanthan gum matrix and then embedded in dark chocolate. Microencapsulated A. muciniphila showed better survival in the chocolate matrix under simulated gastric condition of pH 3, with an increase of 1.80 log CFU/mL and for L. casei, 0.8 logCFU/mL, when compared to the naked cells. Various other food products are also available commercially. Danone Research and Micopharma Inc. Canada have developed fermented milks with microencapsulated Lactobacillus reuteri incorporated in them. In Mexico, A company named Yoplait Inc. has commercialized a yoghurt containing encapsulated bifidobacteria, called "Bificapsulas." Another company, Belgo & Bellas, Boisbriand, QC, Canada has marketed a product named, Yogactive®, which is a ready-to-eat cereal containing "pearls" of probiotic bacteria (Champagne & Kailasapathy, 2011). In bakery products, for production of short dough biscuits, vegetable shortenings are encapsulated and converted into oxidatively stable powders (O'Brien et al., 2003). In other food products also, like certain beverages, microcapsules of curcumin and catechin, prepared by water-inoil-in-water emulsion are incorporated too serve as functional foods.
Also, microencapsulation is used as an effective tool in transferring micronutrients into certain staples like rice and wheat for fortifying them. Microencapsulation technology has full potential to expand further in near future leading to the development of many more functional food products, thus, contributing to the growing food industry. However, production of microcapsules for applications in the food industries is very challenging, owing to many reasons like difficulty in scaling up of a process and dramatically increased production costs, which acts as a limitation in the economic viability of the process. Also, despite its immense scope to be used in food, complexity always prevails as food itself is a complex matrix. While encapsulating food ingredients, there are several limitations in choosing the wall materials, as they need to be food grade or generally recognized as safe (GRAS). When a core material is encapsulated, its interaction with the wall material, stability in various food matrices where it is used as an ingredient, during processing of food, needs to be properly understood. Another significant challenge is the release of core material at the appropriate site during digestion and it requires thorough understanding of the breakdown mechanisms of the food and encapsulated material inside the human digestive tract. And all these challenges need to be overcome without compromising the sensory qualities of the foods. So, careful consideration should be taken while choosing a coating material for encapsulation, the ratio of core to coating material, the method of encapsulation.
Also, thorough evaluation of the properties of the microcapsules needs to be carried out before incorporation into any food matrix. Only then it will lead to successful implementation of the technology.

CONCLUDING REMARKS
Different properties of the microcapsules and their method of determination have been discussed in this critical review. Effects of different techniques of microcapsule production, type of the core, and the TA B L E 5 Comparison in degradation kinetics of various encapsulated compounds produced using different methods and coating materials