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Coacervation is a term borrowed from colloid chemistry to describe the basic process of capsule wall formation. The encapsulation process was discovered and developed by Barrett K Green of the National Cash Register Corporation (NCR) in the 1940's and 1950's. Actually, coacervative encapsulation (or microencapsulation) is a three part process: particle or droplet formation; coacervative wall formation; and, capsule isolation. Each step involves a distinct technology in the area of physical chemistry. The first coacervative capsules were made using gelatin as a wall in an "oil- in-water " system. Later developments produced "water-in-oil" systems for highly polar and water soluble cores. The capabilities and limitations of coacervative encapsulation are presented along with the basic literature references. The basic process for using one form of coacervation, also known as aqueous phase separation, was described in an early patent by B.K. Green and L. Schleicher'. The patent relates to oil-containing microcapsules of a complex hydrophilic colloid wall material and a method of making them. B.K. Green's attempts began in a Dayton, Ohio laboratory. In the late 1930's B.K. Green, a young chemist just out of school, was intrigued by the dearth of information in the colloid field of liquids dispersed in solids. He had earlier recognized the usefulness of such disperse systems in photographic applications. When his company needed a product that would give multiple paper copies without carbon paper. B.K. Green turned to his ideas on dispersions. By 1940, the first working No-Carbon-Required (NCR) paper was prepared, but this was only the beginning. His breakthrough came in 1942 when he was investigating Bungenberg de Jong' s coacervation studies'. One paper mentioned the preparation of solid gelatin spheres, while another dealt with the inclusion of an oil phase within a gelatin coacervate. B.K. Green used both concepts and prepared the first gelatin microcapsules. From this beginning it was nine long years to the development of a marketable product. The new printing system was triggered by including a colorless dye-base in the oil droplets (coated back, CB) and coating the second sheet of paper (coated front, CF) with acidic clay which could react with the dye-base to produce a color. Coacervation and Microencapsulation Coacervation is a colloid phenomenon. If one starts with a solution of a colloid in an appropriate solvent, then according to the nature of the colloid, various changes can bring about a reduction of the solubility of the colloid. As a result of this reduction a large part of the colloid can be separated out into a new phase. The original one phase system becomes two phases. One is rich and the other is poor in colloid concentration. The colloid-rich phase in a dispersed state appears as amorphous liquid droplets called coacervate droplets. Upon standing these coalesce into one clear homogenous colloid-rich liquid layer, known as the coacervate layer which can be deposited so as to produce the wall material of the resultant capsules. Coacervation may be initiated in a number of different ways. Examples are changing the temperature, changing the pH or adding a second substance such as a concentrated aqueous ionic salt solution or a non-solvent. As the coacervate forms, it must wet the suspended core particles or core droplets and coalesce into a continuous coating for the process of microencapsulation to occur. The final step for microencapsulation is the hardening of the coacervate wall and the isolation of the microcapsules, usually the most difficult step in the total process. Simple Coacervation
Simple coacervation involves the
use of either a second more-water soluble polymer or an aqueous non-solvent for
the gelatin. This produces the partial dehydration/desolvation of the gelatin
molecules at a temperature above the gelling point. This results in the
separation of a liquid gelatin-rich phase in association with an equilibrium
liquid (gelatin-poor) which under optimum separation conditions can be almost
completely devoid of gelatin. Simple coacervation can be
effected either by mixing two colloidal dispersions, one having a high affinity
for water, or it can be induced by adding a strongly hydrophilic substance such
as alcohol or sodium sulfate. The water soluble polymer is concentrated in water
by the action of a water miscible, non-solvent for the emerging polymer
(gelatin) phase. Ethanol, acetone, dioxane, isopropanol and propanol have been
used to cause separation of coacervate of gelatin, polyvinyl alcohol and methyl
cellulose. Phase separation can be effected by the addition of an electrolyte
such as an inorganic salt to an aqueous solution of a polymer such as gelatin,
polyvinyl alcohol or carboxymethyl cellulose. A typical simple coacervation
using gelatin colloid is as follows: to a 1 0 percent dispersion of gelatin in
water, the core material is added with continuous stirring and at a temperature
of 40'C. Then a 20 percent sodium sulfate solution or ethanol is added at 50 to
60 percent by final total volume, in order to induce the coacervation. This
system is cooled to 50'C; then, it is necessary to insolubilize the coacervate
capsules suspended in the equilibrium liquid by the addition of a hardening
agent such as glutaraldehyde and adjusting the pH. The resulting microcapsules
are washed, dried and collected. Complex Coacervation
Complex coacervation' can be
induced in systems having two dispersed hydrophilic colloids of opposite
electric charges. Neutralization of the overall positive charges on one of the
colloids by the negative charge on the other is used to bring about separation
of the polymer-rich complex coacervate phase. The gelatin-gum arabic (gum
acacia) system is the most studied complex coacervation system. Complex
coacervation is possible only at pH values below the isoelectric point of
gelatin. It is at these pH values that gelatin becomes positively charged, but
gum arabic continues to be negatively charged. A typical complex coacervation
process using gelatin and gum arabic colloids is as follows: The core material
is emulsified or suspended either in the gelatin or gum arabic solution. The
aqueous solution of both the gelatin and gum arabic should each be below 3
percent by weight. Then, the gelatin or the gum arabic solution (whichever was
not previously used to suspend the core material) is added into the system. The
temperature of the system must be higher than the gel point of an aqueous
gelatin solution (greater than 35'C). The pH is adjusted to 3.8-4.3 and
continuous mixing is maintained throughout the whole process. The system is
cooled to 50'C and the gelled coacervate capsule walls are insolubilized by
either adding glutaraldehyde or another hardening agent and adjusting the pH.
The microcapsules are washed, dried and collected. Aqueous Phase Separation
The term aqueous phase separation is often more simply described as "oil-in-water" microencapsulation. The two encapsulation processes described above are examples of this "oil-in-water" encapsulation. In this process the core material is the oil and it should be immiscible in the continuous phase, namely water. A commercial example of aqueous phase separation would be the microencapsulation of an oily flavor such as sour cream with a gelatin wall. These microcapsules would then be dispersed in a dry cake mix. The mechanism of release would be during the moist baking cycle of the cake, moist-heat causing the capsule walls to first swell and then rupture. Organic Phase Separation The term organic phase separation' is sometimes more simply referred to as "water-in-oil" microencapsulation. In this case the polar core is dispersed into an oily or non-polar continuous medium. The wall material is then dissolved in this continuous medium. A simple technique for encapsulation consists of dissolving ethyl cellulose in cyclohexane at a temperature of 50'C with continuing mixing. Only one phase is present. The cyclohexane is the oily, continuous phase and the ethyl cellulose will later form the coacervative wall. The temperature is elevated to 70'C over a period of 20 to 30 minutes. The core material is added and the temperature raised to 80'C over a period of time and is held at that temperature for one hour. The system is allowed to cool rapidly to 20-40'C. Upon cooling, the ethyl cellulose will gradually emerge as a separated coacervate phase which will then gradually solidify by the time 20'C is reached (unlike hot cyclohexane, the cold material is a non-solvent). The capsules are washed, filtered and air dried. It should be noted that ethyl cellulose is generally approved for use in the pharmaceutical industry. However, for its use in the food industry the "Code of Federal Regulations" should be consulted under the categories of both microencapsulation and ethyl cellulose. Another ethyl cellulose wall encapsulation system involves the addition of polyisobutylene to the ethyl cellulose/cyclohexane. The procedure begins with the addition of ethyl cellulose to a mixture of cyclohexane and polyisobutylene at room temperature. Polyisobutylene, which is more soluble in cyclohexane than is ethyl cellulose, is more effective in causing the latter to emerge as a separate liquid coacervate than would be the case with merely cooling the cyclohexane. The solution is then heated to 80'C and the core material added. The system is cooled to 40'C in 60 minutes and cooled quickly to 20-25'C. Microcapsules are filtered, washed and dried. A modification of this system involves the following procedure: The polyisobutylene is dissolved in cyclohexane at a temperature of 700C and with continuous stiffing. After cooling the system to 40'C, ethyl cellulose is added and dissolved. The core is dispersed in this solution and the system again heated to 78'C and held for 10 minutes at this temperature. Then it is slowly cooled to room temperature followed by cooling to 10'C over two hours. Besides polyisobutylene, other coacervation inducing agents have been used such as polyethylene and butyl rubber. Designing the Process--Process and
Material Selection Coacervation is a very complicated physical phenomenon. And, many factors affect the properties of the resulting microcapsules. Coacervation and phase separation from organic and aqueous media involve many properties, materials and processes such as: phase inducing agents, stirring rates, core to wall ratios, polymer characteristics, core characteristics (wettability, solubility), cooling rates and rates of addition. The basic production of
microcapsules actually involves three distinct steps as discuss above. The
hardening of the microcapsules sufficient for isolation of them into a dry
free-flowing powder remains as a persistent problem. One of the earliest
attempts is the gelatin-tannin reaction'. Tannic acid is recognized as a means
for "hardening" gelatin walled capsules. It should be noted that a particular
encapsulation system, such as would be described in any one of the patents in
the literature, is not necessarily effective in encapsulating a given flavor.
Consequently it is often necessary to develop or modify an encapsulation system
for each flavor. This is particularly true when one looks at the multitude of
requirements related to storage and release of the microcapsule core. In developing or modifying an
encapsulation system, it is very helpful to look at the available wall materials
that one may have for use. A reasonably comprehensive list is available'. This
list should be used in conjunction with materials also listed in the "Code of
Federal Regulations", Title 21, particularly parts I to 199'. The following
materials should be useful in developing a microencapsulation system using
coacervation: Acacia (Gun Arabic) Polyisobutylene Butadiene-Styrene 75/25 Rubber Polyvinyl Acetate Butadiene-Styrene 50150 Rubber Polyvinylpolypyrrolidone Butyl Rubber Potassium Alginate Carob Bean Gum Potassium Citrate Carrageenan Potassium Polymetaphosphate Citric Acid Potassium Tripolyphosphate Dextrin Povidone Dimethylpolysiloxane PVP Dimethyl Silicone Refined Paraffin Wax Ethyl cellulose Shellac, Bleached Food Starch, Modified Sodium Alginate Guar Gum Sodium Carboxymethylcellulose Hydroxypropyl Cellulose Sodium Citrate Hydroxypropyl Methyl cellulose Sodium Ferrocyanide Isobutylene-Isoprene Copolymer Sodium Polyphosphates, Glassy Locust Bean Gum (Sodium hexametaphosphate) Methyl cellulose Sodium Trimetaphosphate Methyl Ethyl Cellulose Sodium Tripolyphosphate Microcrystalline Wax Synthetic Wax (Ethylene Polymer) Paraffin, Synthetic Tannic Acid Petroleum Wax Terpene Resin, Natural Petroleum Wax, Synthetic Tragacanth Poloxamer White Shellac Polyethylene Xanthan Gum Polyethylene Glycols Conclusions A number of encapsulation systems such as spray drying appear to be superior to coacervation because of cost and availability of materials. There are times, however, when coacervation is absolutely necessary. This generally occurs when reservoir microcapsules of a small size, say 10 to 70 microns, are needed or when the core material is a polar liquid and capsules can only be made by organic phase separation. With oily cores it is generally best to start with a gelatin system and modify it accordingly. The glutaraldehyde hardening of gelatin should be judiciously used in accordance with the restrictions stated in the "Code of Federal Regulations". There yet remain considerable amounts of art to coacervative microencapsulation. Here, art is best described as a phenomenon awaiting a scientific explanation. In coacervation the kind of addition and the rate of and order of addition are extremely critical. In general, the slower the process the better it is for coacervative encapsulation. It is the intuitive feel that encapsulators practice that is frequently termed "art." Literature Cited 1 Green, B.K.; Schleicher, L. U.S. Patent 2 800 457, 1957. 2 Bungenburg de Jong, H.G., Proc. Acad. Sci, Amsterdam, 41 p. 646 (1938). 3 Fogle, M.V.; Horger, G. U.S. patent 3 697 437, 1972. 4· Rowe, E.L. U.S. Patent 3 336 155 1967 5 Thomas , A.W., Frieden, A. Industrial and Engineering Chemistry (The Gelatin-Tannin Reaction), 15, p. 839-841, (1923). 6 Food Chemicals Codex; 3rd edition, National Academy Press, Washington, DC 1981 7 Title 21, Code of Federal Regulations, Superintendent of Office, Washington, D.C. 20402. Reprinted from ACS Symposium Series No. 370 Flavor Encapsulation S.J. Risch and G. A. Reineccius, Editors Copyright (D 1 988 by the American Chemical Society Reprinted by permission of the
copyright owner
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