Encapsulation - American Chemical Society


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Chapter 1

Encapsulation: Overview of Uses and Techniques

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Sara J . Risch Science by Design, P.O. Box 39390, Edina, M N 55439

A variety of encapsulation techniques are used in the food and pharmaceutical industry. These techniques include spray drying, spray chilling and cooling, coacervation, fluidized bed coating, liposome entrapment, rotational suspension separation, extrusion and inclusion complexation. This chapter will provide an overview of these techniques. Encapsulation is used to protect ingredients, to convert liquid components into solid particles and to provide a means for controlled release. Research is continuing to improve the methods used and to find new applications.

Encapsulation is a process by which one material or mixture of materials is coated with or entrapped within another material or sytem. The material that is coated or entrapped is most often a liquid but could be a solid particle or gas and is referred to by various names such as core material, payload, actives, fill or internal phase. The material that forms the coating is referred to as the wall material, carrier, membrane, shell or coating. Encapsulation is used in a number of different industries with a wide variety of techniques or processes available. This overview will highlight techniques commonly used in the food industry. A number of the techniques will be covered in detail in later chapters in this book. Consideration will be given to some techniques which are not currently used on a widespread basis in the food industry but which may have practical applications. A number of reviews on encapsulation have been published in the past few years. Dziezak (7) reviewed the encapsulation of ingredients focusing on methods most often used in the food industry. The review discusses the reasons for 0097-6156/95/0590-0002$12.00/0 © 1995 American Chemical Society In Encapsulation and Controlled Release of Food Ingredients; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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encapsulation and provides a good description of techniques used as well as specific applications. The techniques described in (7) include spray drying, air suspension, extrusion coating, spray cooling/chilling, centrifugal extrusion, rotational suspension separation, coacervation and inclusion complexing. Sparks (2) discussed microencapsulation techniques for food and industrial applications, including criteria for selecting one process over another for various types of core materials. Microencapsulation techniques and applications ranging from graphic arts to pharmaceuticals were reviewed by Thies (3) in Encyclopedia of Polymer Science and Engineering. Another more recent review was published by Jackson and Lee (4) focusing on microencapsulated food ingredients. The review covers properties of microcapsules, potential uses as well as techniques for encapsulation. The reader can refer to these reviews for more details concerning particular techniques as well as the chapters later in this book. Encapsulation provides protection for a flavor or ingredient. With some encapsulation techniques, the product can be designed to either release slowly over time or to release at a certain point. This concept of controlled release is discussed in the chapter by Reineccius entitled "Controlled Release in the Food Industry". The protection provided by encapsulation can be to prevent degradation due to exposure to light or oxygen or to retard evaporation. It can be used to separate components of a flavor that would react with each other, such as acetaldehyde and methyl anthranilate. Encapsulation can be used to separate components within a food system such as oil from egg whites so that the egg whites will yield a larger foam volume when whipped. Virtually any material that needs to be protected, isolated or slowly released can be encapsulated. In food systems, this includes acids, lipids, enzymes, microorganisms, flavors, artificial sweeteners, vitamins, minerals, water, leavening agents, colorants and salts. Spray Drying Traditionally, the most common method of encapsulating food ingredients has been spray drying. Spray drying is still the most economical and widely used method of encapsulation, finding broad use in the flavor industry. Equipment is readily available and production costs are lower than for most other methods of encapsulation. In addition to being an encapsulation process, spray drying is also a dehydration process and is used in the preparation of dried materials such as powdered milk. A complete review of spray drying was written by Reineccius (5)and can be referred to for more detailed information. To prepare materials for spray drying, the carrier or wall material (such as maltodextrin, modified starch, gum or combination of these) is hydrated. The flavor or ingredient to be encapsulated is added to the carrier and homogenized or thoroughly mixed into the system using a similar technique. A typical ratio of carrier to core material is 4:1, however, in some applications higher flavor loads can be used. In Encapsulation and Controlled Release of Food Ingredients; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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The mixture is homogenized to create small droplets of flavor or ingredient within the carrier solution. The creation of a finer emulsion increases the retention of flavor during the drying process (6). Numerous studies have been conducted to evaluate the properties of wall materials, including a comparison of encapsulating agents for artificial flavors by Leahy et al (7) and comparisons of retention of volatiles in systems including combinations of carbohydrate, protein and lipids (8). The core/wall material mixture is fed into a spray dryer where it is atomized through a nozzle or spinning wheel. Hot air flowing in either a co-current or counter-current direction contacts the atomized particles and evaporates the water, producing a dried particle that is a starch or carrier matrix containing small droplets of flavor or core. The dried particles fall to the bottom of the dryer and are collected. A thorough understanding of the core material and intended application is important to select the appropriate wall material and to optomize drying conditions. The chapter later in this book by Kenyon discusses a variety of materials used as carriers for encpasulation techniques, but in particular with applications for spray drying. As mentioned, the advantages of spray drying include low processing costs and readily available equipment. It generally provides good protection to the core material and there is a wide variety of wall materials available. One main disadvantage is that it produces a very fine powder which needs further processing such as agglomeration to instantize the dried material or make it more readily soluble if it is for a liquid application. Due to the heat required for evaporation of water from the system, spray drying is not good for heat sensitive materials. Development of improved carrier materials has been an active area of recent research. Companies such as Colloïdes Naturels and TIC Gums have both investigated processing of gum arabic and as well as using it in combination with various starches to yield higher volatile retention and better shelf life. Chapters in this book by Reineccius, Ward, Whorton and Andon and by Thevent discuss the results of these studies. An earlier study by Risch and Reineccius (9) showed that one particular brand of gum arabic yielded good retention of orange oil and provided good protection against oxidative deterioration. An application of a new type of spray dryer has been proposed (JO). The dryer is a Leaflash spray dryer in which the drying air is at a very high temperature (300 - 400 C) and flows at a very high velocity. It was found that citral and linalyl acetate could be spray dried with little impact on the compounds themselves. An alternative to spray drying was investigated by Zilberboim et al (77) for compounds with low boiling points or that are heat labile. In this method, the emulsion of core material in hydrated carrier, such as gum arabic, is atomized into ethanol which acts as a dehydrating liquid The microcapsules can be separated from the solution by filtration and dried in a vacuum oven at low temperature. Additional work by Zilberboim et al (72) studied the microcapsules produced by this technique in an attempt to determine the effects of different process parameters on retention and shelf life. This method does provide an alternative to the high temperatures In Encapsulation and Controlled Release of Food Ingredients; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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encountered in spray drying, however, the lack of readily available equipment to accomplish it in a continuous flow instead of as a batch operation makes it much more costly than spray drying. There are no significant commercial applications of this procedure, however, it does provide an alternative for expesive, heat labile materials where the additional cost might be justified.

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Spray Chilling and Spray Cooling Spray chilling and spray cooling are similar to spray drying in that core material is dispersed in a liquified coating or wall material and atomized. However, unlike spray drying, there is generally no water to be evaporated. The core and wall mixture are atomized into either cool of chilled air which causes the wall to solidify around the core. In spray chilling, the coating is typically a fractionated or hydrogenated vegetable oil with a melting point in the range of 32 - 42 C. In spray cooling, the wall is typically a vegetable oil, although other materials can be used. The normal melting point is 45 - 122 C. These two methods, which differ only in the melting point of the wall material used, are most often used to encapsulate solid materials such as vitamins, minerals or acidulants. With the ability to select the melting point of the wall, these methods of encapsulation can be used for controlled release. Extrusion Encapsulation by extrusion involves dispersion of the core material in a molten carbohydrate mass. This mixture is forced through a die into a dehydrating liquid which hardens the coating to trap the core material. The most common liquid used for the dehydration and hardening process is isopropyl alcohol. The strands or filaments of hardened material are broken into small pieces, separated and dried. This method was firtst patented in 1957 (75) with another patent issued 1962 (14). The work which led to this development was accomplished by Schultz et al (75) of the United States Department of Agriculture. They mixed orange oil into a molten carbohydrate mass and allowed it to cool on a stainless steel sheet. When solidified, the material was pulverized. Swisher further developed this idea by extruding the material instead of just pouring it onto a sheet, as revealed in his patents (73,14). Extrusion provides true encapsulation in that the core material is completely surrounded by the wall material. When the material contacts the dehydrating liquid and the wall is hardened, all residual oil or core material is removed from the surface. The absence of residual surface oil and the complete encapsulation gives products manufactured in this manner an excellent shelf life. This method does produce larger particles which can be used when when visible flavor pieces are desirable. More details on this method of encapsulation can be found in (16).

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Other Techniques A number of other techniques are also finding applications in the food industry. Fluidized bed coating, also referred to as air suspension coating or the Wurster process is typically used to coat solid particles. In simpllified terms, the particles to be coated are circulated through a chamber with high velocity air. As they circulate, the coating material is atomized into the particle stream and deposited on the surface. The amount of coating applied can be controlled by controlling the length of time that the particles are in the chamber. The chapter by Zimmermann discusses the method in detail while the chapter by deZarn presents applications for this technique. Liposome entrapment, which found initial applications in the pharmaceutical industry and is now being investigated for the food industry, is discussed in a chapter by Reineccius. Liposomes consist of an aqeous phase that is completely surrounded by a phospholipid-based membrane. When phospholipids are dispersed in an aqueous media, the liposomes will form spontaneously. It is possible to have either aqueous or lipid soluble material enclosed in the liposome. The only application that is not possible is for any materials that are soluble in both aqueous and lipid phases which limits the use of liposomes for most flavor compounds. Coacervation was patented by National Cash Register Company in the 1950's for carbonless paper. This technique is often regarded as the original and true method of encapsulation. A liquid phase of the coating material is separated from a polymeric solution and surrounds the suspended core material. The coating is then solidified. Until recently, this method was not used for food ingredients due to the fact that the materials available for hardening the wall materials were not food grade. This technique will be discussed in detail in the chapter entitled "Coacervation of Food Ingredients" by Risch. Inclusion complexation is the only method of encapsulation that takes place on a molecular level. It is accomplished using cyclodextrins, typically B-cyclodextrin which consists of 7 glucose units linked 1-4. It has a hollow, hydrophobic center with a hydrophilic outer surface. When in solution, molecules that are less polar will replace the water molecule that is held in the center of the cyclodextrin. This complex becomes less soluble and will precipitate out of solution. The development and applications of cyclodextrins are discussed in detail by Hedges later in this book. Rotational suspension separation is discussed in chapters by Sparks and Schlameus. It involves the suspension of the core material in the selected wall material. This mixture is introduced onto a rotating disk. The encapsulated particles are spun off the disk and then dried or chilled. Other chapter include research on factors influencing retention during spray drying and a specific application for spray dried flavors. There are ongoing research efforts to determine new and better ways to protect food ingredients. Much of this work is confidential and protected by trade secrets. Some of the work is patented and one chapter later in the book will review some of the significant patents that have In Encapsulation and Controlled Release of Food Ingredients; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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been issued in the past few years. We will continue to see more technology developed for specialized applications of encapsulated and controlled release products. Literature Cited 1. Dziezak, J.D. Food Technol. 1988, vol.42, pp. 136 - 151. Downloaded by INDIANA UNIV PURDUE UNIV AT IN on June 24, 2013 | http://pubs.acs.org Publication Date: March 24, 1995 | doi: 10.1021/bk-1995-0590.ch001

2. Sparks, R.E. Encylopedia of Chemical Technology, 3rd edition; John Wiley and Sons, Inc.: New York, N Y , 1981; Vol. 15, pp. 470 - 493. 3. Thies, C. Encyclopedia of Polymer Science and Engineering; John Wiley & Sons, Inc.: New York, N Y , 1987; Vol. 9, 2nd ed., pp. 724 - 745. 4. Jackson, L.S.; Lee, K. Lebensmittel-Wissenschaft und - Technologie. 1991, Vol. 24 (4), pp. 289 - 297. 5. Reineccius, G.A. In Flavor Encapsulation; Risch, S.J. and Reineccius, G.A., Eds.; ACS Symposium Series 370; American Chemical Society: Washington, D.C., 1988, pp. 55 - 66. 6. Risch, S.J.; Reineccius, G.A. In Flavor Encapsulation; Risch, S.J.; Reineccius, G.A.; Ed.; ACS Symposium Series 370; American Chemical Society: Washington, D.C., 1988; pp. 67 - 77. 7. Leahy, M . M . ; Anandaraman, S.; Bangs, W.E.; Reineccius, G.A. Perfum. & Flavorist. 1983, Vol. 8, pp. 49 - 56. 8. Bangs, W.E.; Reineccius, G.A. J. Fd. Sci. 1981, Vol. 47, pp. 254 -259. 9. Risch, S.J.; Reineccius, G.A. Perfum. & Flavorist. 1990, Vol. 15, pp. 55 - 58. 10. Bhandari, B.R.; Dumoulin, H.M.J.; Noleau, R.I.; Lebert, A . M . J. Fd. Sci. 1992, vol. 57, pp. 217 - 221. 11. Zilberboim,R.; Kopelman, I.J.; Talmon, Y. J. Fd. Sci. 1986, vol. 51, pp. 1301 - 1306. 12. Zilberboim, R.; Kopelman, I.J.; Talmon, Y. J. Fd. Sci. 1986, vol. 51, pp. 1307 - 1310. 13. Swisher, H.E. U.S. Patent 2,809,895, 1957. 14. Swisher, H.E. U.S. Patent 3,041,180, 1962. 15. Schultz, T.H.; Dimick, K.P.; Makower, B. Food Tech. 1956, vol. 10, pp. 57 - 60. 16. Risch, S.J. In Flavor Encapsulation, Risch, S.J.; Reineccius, G.A., Eds.; ACS Symposium Series 370; American Chemical Society: Washington, D.C., 1988, pp. 103 - 109. RECEIVED October 11, 1994

In Encapsulation and Controlled Release of Food Ingredients; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.