Microencapsulation is a process of coat­ing of small particles of solid or liquid material (core) with protective coating material (matrix) to produce microcapsules in the micrometer to millimeter range. It is one of the methods of protecting sensitive substances and producing active ingredients with improved properties. Many different active materials like lipids, proteins, vitamins and minerals, enzymes and flavors have been successfully encapsulated. To produce effective encapsulated products, the choice of coating material and method of microencapsulation process are most important and it also depends on the end use of the product and the processing conditions involved. These microcapsules release their contents at desired rate and time by different release mechanisms, depending on the encapsulated products which provide wide application of food ingredients thereby improving the cost effectiveness for the food manufacturer. This review paper highlighted the various microencapsulation methods and its application in the encapsulation of bioactive food ingredients and controlled release mechanisms.

Keywords: microencapsulation, microcapsules, food ingredients, bioactives, controlled release

Now-a-days the demand for healthy and nutritional food products is increasing worldwide. Today foods are intended not only to fulfill the hunger and to provide necessary nutrients for humans. It also intended to prevent nutrition-related diseases and improve physical and mental health. In this regard, functional foods play an outstanding role. Functional foods are foods that enriched with functional ingredients to offer health benefits or to reduce the risk of chronic diseases beyond their basic nutritional functions. Bioactive in food are physiologically active components that provide health benefits beyond their nutritional role. Bioactive ingredients include proteins, vitamins, minerals, lipids, antioxidants, phytochemicals and probiotic bacteria.¹ These bioactives are very sensitive and their application in food is a great challenge to the industry without affecting their properties. Encapsulation technology has proven to be an excellent method to protect the sensitive food ingredients and to develop the novel foods formulations with improved properties.^2,3 Microencapsulation defined as a process of coat¬ing small particles of solids, liquids, or gaseous components, with protective coating material.⁴ Microcapsules or micron size ranged from 2-5000μm. In the food industry, the microencapsulation process can be applied for a various purpose⁵ such as

1. To protect the core material from degradation and to reduce the evaporation rate of the core material to the surrounding environment;
2. To modify the nature of the original material for easier handling;
3. To release the core material slowly over time at the constant rate
4. To prevent unwanted flavor or taste of the core material;
5. To separate the components of the mixture that would react one another. Depends on the consumer needs, microen¬capsulation process has been improved constantly. As a result, it has be¬come an example of a dynamic and technological intensive process method,⁶ characterized by a fast growth of patent in microencapsulation process and its applications, as well as by an increasing number of scientific research articles.

There are separate extensive reviews on microencapsulation techniques used in the food industry. However, there is a need to discuss the different carriers and methods with a particular focus on encapsulating bioactive food ingredients. The objective of this paper is to review the microencapsulation technologies in a three perspectives. First, it focuses on theoretical aspects of different types of microencapsulation techniques and criteria required for encapsulating agents. Next, it dis¬cusses microencapsulation of various bioactive food ingredients such as omega-3 fatty acids, polyphenols, enzymes, protein hydrolysate and peptides, microorganisms, vitamins and minerals and its applications. The third section summarizes controlled release mechanisms of microcapsules.

The material that is encapsulated is called as core material, the active agent, internal phase, or payload phase. The substance or material that is encapsulating the core is called as wall material, coating material, membrane, shell, carrier material, external phase or matrix. Two main types of encapsulates are reservoir type and matrix type.⁷ In reservoir type, the active agents form a core surrounded by an inert barrier. It is also called single-core or mono-core or core-shell type. In matrix type, the active agent is dispersed or dissolved in an inert polymer. Coated matrix type is a combination of first two (Figure 1).

Figure 1 Morphology of microcapsule.

The microcapsules are prepared by a variety of methods. The microencapsulation process can be divided into physical and chemical process. Physical process includes spray drying, spray chilling, rotary disk atomization, fluid bed coating, coextrusion and pan coating. The chemical process includes simple and complex coacervation, interfacial polymerization and phase separation.^8,9 Different types of microencapsulation techniques and their properties, merits and demerits are summarized in (Table 1).

There are numerous methods are used for microencapsulation of bioactive ingredients. But no single encapsulation process is applicable to all core materials or active agent. Microencapsulation methods used for various bioactive ingredients are discussed below:

Encapsulation of omega-3 fatty acids

Omega-3 fatty acids are belongs to the family of polyunsaturated fatty acids that the body cannot synthesize, but are essential for multiple function in human health. Biochemically, omega-3 fatty acids which have their first double bond (unsaturated) in the third carbon from the methyl end. The most important omega-3 fatty acids are alpha linolenic acid (ALA, 18:3 n-3), eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaneoic acid (DHA, 22:6n-3). Due to its unsaturated nature, they are susceptible to oxidation and also produce hydro peroxides and off-flavours which are objectionable by consumers. To overcome the above mentioned problems, the utilization of microencapsulation technique has been studied by various researchers.^11-14 Different methods used for microencapsulation of omega-3 fatty acids are given in (Table 2).

Encapsulation of polyphenols/flavors

Flavor plays an important role in food products which influences further consumption of foods and provide consumer satisfaction. The market for flavors is focused in using aromatic materials coming from natural sources to replace the use of synthetic flavors in the food products.¹⁵ These aroma compounds are not only delicate and volatile, but also very expensive.¹⁰ Commercially available food flavors in liquid forms are difficult to handle or incorporate into food systems. However, many flavor constituents are very sensitive to oxygen, light, and heat. These problems can be solved by encapsulation. Encapsulation provides an effective method to protect flavor compounds from degradation, oxidation and migration from food. Essential oils (Eos) are volatile, complex mixtures of compounds characterized by a strong odor, and they are formed by aromatic plants as secondary metabolites. Several essential oils such as ginger, garlic, cinnamon, coriander, clove, peppermint, citrus peel, oregano, thyme, rosemary basil, eucalyptus and have been demonstrated various biological properties activities, including antioxidant, antimicrobial, antiviral and anti-inflammatory functions.^16,17 Several researchers reported that plant polyphenols can slow the progression of cancers, diabetes, and osteoporosis and reduce the risks of cardiovascular disease.^18,19 Due their instability and unpleasant taste (astringency) which needs to be protected or masked before incorporation into food products.²⁰ Different methods used for encapsulation of polyphenols are given in (Table 3).

Encapsulation of vitamins and minerals

Fat-soluble (e.g. A, D, E, K) and water-soluble (e.g. ascorbic acid) vitamins can be encapsulated by microencapsulation.²³ Iron is one of the most important elements and plays a major role in human health and its inadequate consumption leads to iron deficiency. One of the ways to prevent this problem is fortification of food with iron. But, the bioavailability of iron is affected by interactions of iron with the food ingredients such as tannins, phytates and polyphenols. Moreover, iron catalyses oxidative processes in fatty acids, vitamins and amino acids, which results in loss of sensory features and decrease in nutritional value of the food. Microencapsulation can be used to prevent these reactions. Microencapsulation methods used for vitamins and minerals are given in (Table 4).

Encapsulation of calcium

Soya milk contains much less calcium (12mg/100 g) than cow’s milk (120mg/100 g), which is undesirable from a nutritional point of view. By encapsulating the Ca salt (calcium lactate) in a lecithin liposome, provides possible to fortify 100g soya milk with calcium up to 110 mg for obtaining calcium levels equivalent to those in normal cow’s milk.²⁶

Encapsulation of enzymes

Enzymes are biomacromolecules or in other words complex protein molecules with specific catalytic functions and they regulate the chemical reactions needed for the human body. Because of their enormous catalytic power in aqueous solution at normal temperatures and pressures, enzymes are of great commercial and industrial importance. In the microencapsulation method, the enzyme is entrapped within a semi permeable membrane so that the activity of an enzyme is not affected (Table 5). But the movement of the substrate to the active site may be restricted by the diffusional limitations especially when large molecules like starch and proteins are used, which can have an adverse effect on the enzyme kinetics.²⁷

Encapsulation of microorganism

Probiotic bacteria are the live microorganisms that are confer a beneficial physiological effect on the host (humans or animals). These bioactive ingredients have been at the forefront of the development of functional foods, particularly in dairy products.³⁰ There are five microencapsulation methods have been applied to probiotics such as spray-coating (fluid bed coating), spray-drying, extrusion, emulsion and gel particle technologies (which include spray-chilling). Among these spray-coating and gel-particle technologies are most often used for microencapsulation of probiotics.³¹ Different wall materials used for microencapsulation of microorganisms are given in (Table 6).

Encapsulation of protein hydrolysate and peptide

Food protein hydrolysates and peptides are considered as a promising functional food ingredients. However, food application of protein hydrolysates and peptides can be inhibited by their bitter taste, hygroscopicity and interaction with the food matrix. These problems can be solved by encapsulation.³³ Proteins, polysaccharides and lipids based carrier systems used for protein hydrolysates and peptide encapsulation (Table 7). The protein and polysaccharide based carried used for masking the bitter taste and reducing the hygroscopicity of protein hydrolysates, whereas the lipid-based carriers are intended for enhancing the bioavailability and biostability of encapsulated peptides.

Application of microencapsulated bioactive ingredients in food industry

Microencapsulation offers numerous benefits to the materials being encapsulated. Some of the encapsulated food ingredients and their applications are summarized in (Table 8).

Controlled release has been defined as a method by which one or more active agents are occurs at the target site and at the desirable rate and time.³⁶ The major objectives of controlled release are to decrease the loss of target compound such as vitamins and minerals during the processing and storage, to optimize the absorption and to increase of effective use. The advantages of controlled release are; the active ingredients are released at controlled rates over prolonged periods of time.³⁷ The most commonly used methods for controlled release includes thermal and moisture release.³⁸ The major mechanisms involved in the core release are pH, temperature, use of solvent, diffusion, degradation and swelling or osmotic pressure activated release. Normally, a combination of more than one mechanism is used for release of core material.⁵

In this method, core or active material is re¬leased by diffusion through the polymer (reservoir system) or through the pores existing in the poly¬mer (matrix systems).

Reservoir systems: The release of an active agent by this method is carried out by diffusion of the active agent within the reservoir; dissolution of the active agent between the reservoir carrier fluid and the barrier. The release rate from a reservoir system depends on the permeability, area and thickness of the barrier.³⁹

Matrix systems: The active agent is released by this method is carried out by diffusion of the core material to the surface of the coating material; dissolution of the active agent between the carrier and the surrounding medium. The rate release depends on the percentage of active agent, coating material and the geometry of the system.³⁹

Swelling controlled release: In this method, when the polymer matrix is placed in a thermodynamically com¬patible medium, the polymer swells which leads to absorption of fluid from the medium. The active agent in the swollen part of the matrix then diffuses out.⁴⁰

Release of active agent by degradation: Degradation type of release occurs when enzymes such as proteases and lipases are degraded in to proteins or lipids, respectively.⁴¹ An example of release of active agent by degradation is reducing the time required for the ripening of cheddar cheese by 50% compared to conventional ripening process.⁴²

Solvent-activatedrelease: The active agent is released when the food material comes in contact with a solvent, resulting in swelling of the microcapsule. For example, microencapsulated of coffee flavors is released upon contact with water.⁴³

pH-controlled release: The active agent is released at a specific pH. For example, microencapsulated probiotic microorganisms will resist in the acidic pH of the stomach and it will be released in the alkaline pH of the intestine.⁴⁴

Temperature-sensitive release: The active agent is released according to the change of temperature. Examples are, aromas for tea and baking are based on the effect of melting of the matrix; encapsulated cheese flavor used in microwave popcorn release the flavor when the temperature rises to 57-90˚C.⁴⁵

Pressure-activated release: In this method, the active agent is released when the pressure is applied on the matrix. For example, release of sweetener and/or flavor in chewing gum when chewed.⁴⁶

Microencapsulation process provides an effective protection for active agent against oxidation, evaporation or migration in food. It plays a major role in development high quality functional food ingredients with improved physical and functional properties in order to make superior products. To produce effective encapsulated products, the choice of coating material and method of microencapsulation process are most important. Despite the wide range of application of encapsulated products in pharmaceutical and cosmetic industries, microencapsulated product has found a comparatively much smaller market in the food industry. The microencapsulation technology is yet to become a conventional tool for food industry to develop the healthy and novel food products which can be achieved by multidisciplinary based research approach and consideration of industrial requirements and constraints.

None.

The author declares no conflict of interest.

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