Ronald J. Versic, Ph.D., President Ronald T. Dodge Company
The encapsulation of foods, flavors, ftagrances and the like has been attempted and commercialized by many different methods. A comprehensive overview is given of these many methods. The overview describes each method in terms of the basic chemical and/or physical principles involved. The basic literature references, particularly fundamental or basic patents, are included. Each method was generally developed to solve a particular problem encountered by aproduct development or formulation chemist. The relationships among problems, capabilities, andencapsulation methods are shown. The overview concludes with a list of reasons for encapsulation, such as prevention of oxidation, conversion of liquids to solids and detackification.
The use of microcapsules in food is generally that of an additive. By regulatory definition, a food additive is any substance which becomes added to food either intentionally or unintentionally other than food itself This includes both compounds added directly and those that are added indirectly such as migrating from packaging materials. We will limit our discussion here to direct, intentional additives. This means, for example, that the Vitamin C in orange juice is not an additive but the Vitamin C added to orange juice is.
There are several hundred types of microcapsules being used as a food additive in the U.S. today. Most of these are used in the development and production of artificial flavors or natural flavors and spices. Microcapsules as food additives may be added to enhance or alter appearance. Food is not only consumed for its calories or nutrients. It is also part of our cultural experience and it must be appealing in all of its aspects. It is not just a biological necessity, its consumption is a social activity, an aesthetic experience and an expression of cultural and personal experiences. This means that food must not only taste good but it also must have the right color, texture and aroma.
The use of microcapsules can improve or enhance nutrition. The processing required to produce many of the food products used today and the long shelf life needed to provide a variety of foods available far from the place where they were grown often results in the loss of nutritional value. The products are often restored to their natural nutritional values through the addition of vitamins, minerals and in some cases, proteins.
Microcapsules can be used as preservatives. Most food preservation originally was accomplished by curing, smoking or pickling which were primarily effective in changing the moisture content or water activity in foods. Then came canning, pasteurization, freezing and chemical preservatives. Microcapsules can also enhance the convenience of food products. Changing lifestyles and the limited time available for food preparation require an increasing variety of high quality, nutritious and convenient food products today.
Microcapsules represent an extra degree of freedom in the formulation or development of these food products. Many of the reasons or causes for the use of microcapsules are covered in a previous symposium' and a continued updated review on this subject.' The use of microcapsules is one means of achieving controlled release of the core or inner material. The term controlled release actually covers a wide range of technologies and microencapsulation is one way of achieving controlled release. In fact, microencapsulation is the dominant means for achieving controlled release both in product volume and dollar value.
One particular example of controlled release is sustained release. In this form the desired material is continuously released over a period of time at a constant rate. Two timely publications',' cover the general area of controlled release, which can also include the controlled release of agricultural materials and biological materials, for example, pheromones. In using the term microencapsulation in this article, the author intends to refer to capsules in the size range of I micron to 1000 microns. Capsules below I micron in size are frequently refereed to as nanocapsules and they are made by one or more very specialized methods.' The term capsule refers to macro objects in the order of I millimeter or larger. This term of capsule is frequently used in the delivery of pharmaceuticals.
The simplest way of looking at a microcapsule is that of imagining a hen's egg reduced in size. The shell has a number of names depending on the industry and company with which one works. This shell can be referred to as a membrane, a wall, a covering or a coating. The internal core material also goes by a number of terms such as payload, core, encapsulant, fill, active ingredient, internal phase, IP or internal ingredient. For the purposes of the article here we will refer to the terms wall and, core. Often in the making of microcapsules by a chemical process known as coacervation, there is the additional use of the terms external phase or continuous phase.
The architecture of microcapsules is generally divided into several arbitrary and overlapping classifications. One such design is known as matrix encapsulation. In this design the matrix particle resembles that of a peanut cluster. The core material is buried to varying depths inside of the wall material. The most common or well known type of microcapsule is that of a spherical or reservoir design. It is this design that most approaches a hen's egg. It is also possible to design other microcapsules that have multiple cores where the multiple cores may actually be an agglomerate of several different types of microcapsules.
If the core material is an irregular material, such as occurs with a ground particle, then the wall will somewhat follow the contour of the irregular particle and one achieves an irregular microcapsule. The last well known design for a microcapsule is that of a multiple wall. In this case the multiple walls are placed around a core to achieve multiple purposes related to the manufacture of the capsules, their subsequent storage and controlled release.
In a discussion of microencapsulation technology, particularly when one is talking quantities and cost, it is necessary to understand that encapsulation is a volume process, independent of the density or value of the core material. Thus microencapsulators frequently state that it is just as expensive on a volume basis to encapsulate diamond as graphite. Likewise, on a volume basis it is just as expensive to encapsulate paraffin wax as tungsten metal. When experimenting with or acquiring microcapsules, it should be emphasized that it is necessary to use common terminology because preference to discuss microcapsules in terms of the core material, particularly when one is discussing the cost of production.
One should also keep in view that the total process of microencapsulation actually covers three separate processes on a time scale. The first process consists of forming a wall around the core material. The second process involves keeping the core inside the wall material so that it does not release. Also, the wall material must prevent the entrance of undesirable materials that may harm the core. And finally, it is necessary to get the core material out beginning at the right time and at the right rate. There is a good reference material covering the current work in the area of microencapsulation' and a review article that is continually updated.'
The uses of microcapsules since the initial coacervation work in the 1940's are many and varied. A good early review of these uses that also includes pharmaceuticals and agricultural materials is contained in reference.' The uses of microcapsules that are of interest here include the following:
1. Reduce the reactivity of the core with regard to the outside environment, for example oxygen and water;
2. Decrease the evaporation or transfer rate of the core material to the outside environment;
3. Promote the ease of handling of the core material;
a. Prevent lumping;
b. Position the core material more uniformity through a mix by giving it a size and outside surface like the remainder of the materials in the mix
C. Convert a liquid to a solid form; and,
d. Promote the easy mixing of the core material.
4. Control the release of the core material so as to achieve the proper delay until the right stimulus;
5. Taste mask the core; and,
6. Dilute the core material when it is only used in very small amounts; but, achieve uniform dispersion in the host material.
A variety of release mechanisms have been proposed for microcapsules; but in fact, the number that have actually been achieved and are of interest here are rather limited. These are as follows:
I . A compressive force in terms of a 2 point or a 12 point force breaks open the capsule by mechanical means;
2. The capsule is broken open in a shear mode such as that in a Waring blender or a Z-blade type mixer;
3 . The wall is dissolved away from around the core such as when a liquid flavoring oil is used in a dry powdered beverage mix;
4. The wall melts away from the core releasing the core in an environment such as that occurring during baking; and,
5. The core diffuses through the wall at a slow rate due to the influence of an exterior fluid such as water or by an elevated temperature.
The release rates that are achievable from a single microcapsule are generally "O" order, 1/2 order or I st order. "O" order occurs when the core is a pure material and releases through the wall of a reservoir microcapsule as a pure material. Half order release generally occurs with matrix particles. I st order release occurs when the core material is actually a solution. As the solute material releases from the capsule the concentration of solute material in the solvent decreases and a I st order release is achieved. Please note that these types of release rates occur from a given single microcapsule. A mixture of microcapsules will include a distribution of capsules varying in size and wall thickness. The effect, therefore, is to produce a release rate different from "O", t' 1/2" or " 1 " because of the ensemble of microcapsules. It is therefore very desirable to carefully examine on an experimental basis the release rate from an ensemble of microcapsules and to recognize that the deviation from theory is due to the distribution in size and wall thickness.
The general technology for forming microcapsules is divided into two classifications known as physical methods and chemical methods. The physical methods are generally divided into the following:
I. Spray coating.
Pan coating. This is a mature, well established technology initially patented by a pharmacist in the 19th century by the name of Upjohn. It generally requires large core particles and produces the coated tablets that we are familiar with;
Fluid bed coating. One version of this coating is known as Wurster coating and was developed in the 1950's and 60's.' The Wurster coater relies upon a bottom positioned nozzle spraying the wall material up into a fluidized bed of core particles. Another version sprays the wall material down into the core particles.
2. Annular jet. This technology was developed by the Southwest Research Institute and has not been extensively used in the food industry." It relies upon two concentric jets. The innerjet contains the liquid core material. The outer jet contains the liquid wall material, generally molten, that solidifies upon exiting the jet. This dual fluid stream breaks into droplets much as water does upon exiting a spray nozzle;
3. Spinning disk. A new method was developed by Professor Robert E. Sparks at Washington University in St. Louis. This method relies upon a spinning disk and the simultaneous motion of core material and wall material exiting from that disk in droplet form." The capsules and particles of wall material are collected below the disk. The capsules are separated from the wall particles (chaff) by a sizing operation;
4. Spray cooling. It is a method of spray cooling a molten matrix material containing minute droplets of the core materials." This method is well known because of its practice by the Sunkist Company;
5. Spray drying. It is a method to be discussed at length in the following papers; and,
6. Spray chilling. It is a process of spray chilling the wall around an atomized core." The resulting capsules move countercurrent to a flow of tempered air and are collected in a large container below the spray nozzle. It is practiced currently by the Durkee Company.
The number of methods for chemical encapsulation is actually far less. They are necessary because they are very effective in encapsulating liquids and small core sizes. In particular, it is possible to encapsulate flavors and fragrances down to 10 microns in size. Two methods are known as water-in-oil and oil-in-water. The oil-in-water process is known as complex coacervation. This process relies, for example, upon a solution of gelatin and the complex it forms with a solution of gum arabic. Complex coacervation will be described in a later paper by the author. The other method of encapsulating water soluble cores within an oil medium is generally not used in the food industry.
In looking at the need for encapuslation in a product it should be emphasized that microcapsules are rarely sold and consumed by themselves. Microcapsules are generally additives to a larger system and must function within that system. Consequently there are a number of performance requirements placed on microcapsules when a limited number of encapsulating materials and methods exist. Consequently it is necessary to make a number of trade-offs and compromises to incorporate microcapsules into a food product as an additive. In contrast to this, though, there is the continual development of new materials for encapsulation, particularly in the Wurster method. Fortunately a number of the patents have now expired and so it is possible for the encapsulator to use a number of methods without fear of patent infringement.
Whitten, J. A., Food Additives in the 1980's: New Demands, New Products, New Opportunities. Food Additives Symposium, American Chemical Society April 1986.
2 Food; Encapsulation, 1972 - November 1986 PB87-850335/CAW, U.S. Dept. of Commerce, National Technical Information Service, Springfield, VA, 1987.
3 J. Controlled Release: Controlled Release Society, Elsevier: New York.
4 CA Selects. Controlled Release Technology, Chemical Abstracts Service, Columbus, OH.
5 Cook, E. J.; Lagace, A. P. U.S. Patent 4 533 254, 1985.
6 J. Microencapsulation: Taylor and Francis Inc.; Philadelphia.
7 Kirk-Othmer, Encyclopedia of Chemical Technology, John Wiley and Sons; New York, 198 1; Vol. 15, p. 470-493.
8 Fanger, G. 0. Chemtech, 1974, p. 398-405.
9 Wurster, D. E. U.S. Patent 2 799 241, 1957; 3 089 834, 1963; 3 117 027, 1964; 3 196 927, 1965; 3 207 824, 1965; 3 241 520, 1966; 3 253 944, 1966.
10 Somerville, G., U.S. Patent 3 015 128, 1962; 3 310 612, 1967; 3 389 194, 1968.
11 Sparks, R. E. U.S. Patent 4 675 140, 1987.
12 Swisher, H. E. U.S. Patent 3 041 180, 1962.
13 Johnson, L. A.; Waiters, L. A. U.S. Patent 3 976 794, 1976; Johnson, L. A.; Beyn, E. J. U.S. Patent 3 949 094, 1976; Johnson, L. A.; Beyn, E. J. U.S.Patent 3 949 096, 1976.
Reprinted from ACS Symposium Series No. 370
S.J. Risch and G. A. Reineccius, Editors
Copyright (D 1988 by the American Chemical Society
Reprinted by permission of the copyright owner