Recent Developments in Microencapsulation of Food Ingredients Kashappa Goud H. Desai and Hyun Jin Park* Graduate School of Biotechnology, Korea University, Sungbuk-ku, Seoul, South Korea Abstract: Microencapsulation involves the incorporation of food ingredients, enzymes, cells, or other materials in small capsules. Microcapsules offer food pro- cessors a means with which to protect sensitive food components, ensure against nutritional loss, utilize otherwise sensitive ingredients, incorporate unusual or time-release mechanisms into the formulation, mask or preserve flavors and aro- mas, and transform liquids into easily handled solid ingredients. Various techni- ques are employed to form microcapsules, including spray drying, spray chilling or spray cooling, extrusion coating, fluidized-bed coating, liposome entrapment, coacervation, inclusion complexation, centrifugal extrusion, and rotational suspension separation. Recent developments in each of these techniques are discussed in this review. Controlled release of food ingredients at the right place and the right time is a key functionality that can be provided by microencapsulation. A timely and targeted release improves the effectiveness of food additives, broadens the application range of food ingredients, and ensures optimal dosage, thereby improving the cost effectiveness for the food manufacturer. Reactive, sensitive, or volatile additives (vitamins, cultures, flavors, etc.) can be turned into stable ingre- dients through microencapsulation. With carefully fine-tuned controlled-release properties, microencapsulation is no longer just an added-value technique, but the source of totally new ingredients with matchless properties. Keywords: Microencapsulation Food ingredients Controlled release Spray drying Microcapsules INTRODUCTION Microencapsulation is defined as a technology of packaging solids, liquids, or gaseous materials in miniature, sealed capsules that can release Correspondence: Hyun Jin Park, Graduate School of Biotechnology, Korea University, 1, 5-Ka, Anam-Dong, Sungbuk-ku, Seoul 136���701, South Korea Tel.: 82-2-3290-3450 Fax: 82-2-953-5892 E-mail: hjpark@korea.ac.kr Drying Technology, 23: 1361���1394, 2005 Copyright Q 2005 Taylor & Francis, Inc. ISSN: 0737-3937 print/1532-2300 online DOI: 10.1081/DRT-200063478

their contents at controlled rates under specific conditions.[1���6] The microencapsulation technology has been used by the food industry for more than 60 years. In a broad sense, encapsulation technology in food processing includes the coating of minute particles of ingredients (e.g., acidulants, fats, and flavors) as well as whole ingredients (e.g., raisins, nuts, and confectionary products), which may be accomplished by micro- encapsulation and macro-coating techniques, respectively.[7] More specifically, the microcapsule has the ability to preserve a substance in the finely divided state and to release it as occasion demands.[8] These microcapsules may range from submicrometer to several millimeters in size and have a multitude of different shapes, depending on the materials and methods used to prepare them. The food industry applies micro- encapsulation process for a variety of reasons: (1) encapsulation= entrapment can protect the core material from degradation by reducing its reactivity to its outside environment (e.g., heat, moisture, air, and light), (2) evaporation or transfer rate of the core material to the outside environment is decreased=retarded, (3) the physical characteristics of the original material can be modified and made easier to handle, (4) the pro- duct can be tailor to either release slowly over time or at a certain point (i.e., to control the release of the core material to achieve the property delay until the right stimulus), (5) the flavor of the core material can be masked, (6) the core material can be diluted when only very small amounts are required, yet still achieve a uniform dispersion in the host material, and (7) it can be employed to separate components within a mixture that would otherwise react with one another.[9���14] Various properties of microcapsules that may be changed to suit spe- cific ingredient applications include composition, mechanism of release, particle size, final physical form, and cost. The architecture of microcap- sules is generally divided into several arbitrary and overlapping classifica- tions (Fig. 1). One such classification is known matrix encapsulation. This is the simplest structure, in which a sphere is surrounded by a wall or membrane of uniform thickness resembling that of a hen���s egg. In this design, the core material is buried to varying depths inside the shell. This microcapsule has been termed a single-particle structure (Fig. 1A). It is also possible to design microcapsules that have several distinct cores within the same microcapsule or, more commonly, number numerous core particles embedded in a continuous matrix of wall material. This type of design is termed the aggregate structure (Fig. 1B). In order to improve the properties of food ingredients, immobiliza- tion of food ingredients onto a suitable polymer or addition of antimicro- bial agents are common practices in the food industres.[15���17] For example, an important bacteria used in the food industry, lactic acid bac- teria, was first immobilized in 1975 on Berl saddles and Lactobacillus lactis was encapsulated in alginate gel beads years later.[18] Seiss and Davis suggested that immobilized lactic acid bacteria could be used to 1362 Desai and Park

continuously produce yogurt.[19] However, the alginate gel beads leaked large quantities of cells. The use of microencapsulated food ingredients allows food ingredi- ents to be carefully tailored to the specific release site through the choice and microencapsulation variables, specifically, the method and food ingredients-polymer ratio.[7] The total amount of ingestion and the kinetics of release are variables that can be manipulated to achieve the desired result.[7,9,14] Using innovative microencapsulation technologies, and varying the copolymer ratio, molecular weight of the polymer, etc., microcapsules can be developed into an optimal food ingredient device.[7] Microcapsule-based systems increases the life span of food ingredients and control the release of food ingredients. Various properties of microcapsules that may be changed to suit specific ingredient applications include composition, mechanism of release, particle size, final physical form, and cost. Before considering the properties desired in encapsulated products, the purpose of encapsu- lation must be clear. In designing the encapsulation process, the following questions are taken into consideration: 1. What functionality should the encapsulated ingredients provide the final product? 2. What kind of coating material should be selected? 3. What processing conditions must the encapsulated ingredient survive before releasing its content? 4. What is optimal concentration of the active ingredient in the microcapsule? 5. By what mechanism the ingredient be released from the microcapsules? 6. What are the particle size, density, and stability requirements for the encapsulated ingredient? 7. What are the cost constraints of the encapsulated ingredient? Figure 1. Schematic diagram of two representative types of microcapsules. Microencapsulation of Food Ingredients 1363

Controlled release may be defined as a method by which one or more active agents or ingredients are made available at a desired site and time at a specific rate. With the emergence of controlled-release technology, some heat-, temperature-, or pH-sensitive additives can be used very conveniently in food systems. Such additives are introduced into the food system mostly in the form of microcapsules. The additive present in the microcapsule is released under the influence of a specific stimulus at a specified stage. For example, flavors and nutrients may be released upon consumption, whereas sweeteners that are susceptible to heat may be released toward the end of baking, thus preventing undesirable caramelization in the baked pro- duct.[20���30] Although quite a number of reviews are published on the micro- encapsulation of food ingredients, we have made an attempt here to update the recent developments in the microencapsulation of food ingredients. MICROENCAPSULATION TECHNIQUES Encapsulation of food ingredients into coating materials can be achieved by several methods. The selection of the microencapsulation process is governed by the properties (physical and chemical) of core and coating materials and the intended application of food ingredients. However, the microencapsulation processes that are used to encapsulate food ingre- dients are given in Table 1, which outlines various methods used for the preparation of microencapsulated food systems. Sophisticated shell mate- rials and technologies have been developed and an extremely wide variety of functionalities can now be achieved through microencapsulation. Any kind of trigger can be used to prompt the release of the encapsulated ingredient, such as pH change (enteric and anti-enteric coating), mechan- ical stress, temperature, enzymatic activity, time, osmotic force, etc. How- ever, cost considerations in the food industry are much more stringent than in, for instance, the pharmaceutical or cosmetic industries. The selection of microencapsulation method and coating materials are inter- dependent. Based on the coating material or method applied, the appro- priate method or coating material is selected. Coating materials, which are basically film-forming materials, can be selected from a wide variety of natural or synthetic polymers, depending on the material to be coated and characteristics desired in the final microcapsules. The composition of the coating material is the main determinant of the functional properties of the microcapsule and of how it may be used to improve the performance of a particular ingredient. An ideal coating material should exhibit the following characteristics: 1. Good rheological properties at high concentration and easy work- ability during encapsulation. 2. The ability to disperse or emulsify the active material and stabilize the emulsion produced. 1364 Desai and Park

Table 1. Various microencapsulation techniques and the processes involved in each technique No Microencapsulation technique Major steps in encapsulation 1 Spray-drying a. Preparation of the dispersion b. Homogenization of the dispersion c. Atomization of the infeed dispersion d. Dehydration of the atomized particles 2 Spray-cooling a. Preparation of the dispersion b. Homogenization of the dispersion c. Atomization of the infeed dispersion 3 Spray-chilling a. Preparation of the dispersion b. Homogenization of the dispersion c. Atomization of the infeed dispersion 4 Fluidized-bed coating a. Preparation of coating solution b. Fluidization of core particles. c. Coating of core particles 5 Extrusion a. Preparation of molten coating solution b. Dispersion of core into molten polymer c. Cooling or passing of core-coat mixture through dehydrating liquid 6 Centrifugal extrusion a. Preparation of core solution b. Preparation of coating material solution c. Co-extrusion of core and coat solution through nozzles 7 Lyophilization a. Mixing of core in a coating solution b. Freeze-drying of the mixture 8 Coacervation a. Formation of a three-immiscible chemical phases b. Deposition of the coating c. Solidification of the coating 9 Centrifugal suspension separation a. Mixing of core in a coating material b. Pour the mixture over a rotating disc to obtain encapsulated tiny particles c. Drying 10 Cocrystallization a. Preparation of supersaturated sucrose solution b. Adding of core into supersaturated solution c. Emission of substantial heat after solution reaches the sucrose crystallization temperature (Continued) Microencapsulation of Food Ingredients 1365