Chapter 14
Coacervation for Flavor Encapsulation Ronald J. Versic
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Ronald T. Dodge Company, 55 Westpark Road, Dayton, OH 45459 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. There is some discussion on the art of coacervation. The basic process for using one form of coacervation, also known as aqueous phase separation, was described i n an early patent by B.K. Green and L. Schleicher (1). The patent relates to o i l containing microcapsules of a complex hydrophilic c o l l o i d wall material and a method of making them. B.K. Green's attempts began in a Dayton, Ohio laboratory. In the l a t e 1930's B.K. Green, a young chemist just out of school, was intrigued by the dearth of information i n the c o l l o i d f i e l d of l i q u i d s dispersed i n s o l i d s . He had e a r l i e r 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 f i r s t working No-Carbon-Required (NCR) paper was prepared, but this was only the beginning. His breakthrough came i n 1942 when he was investigating Bungenberg de Jong's coacervation studies (2). One paper mentioned the preparation of s o l i d g e l a t i n spheres, while another dealt with the inclusion of an o i l phase within a g e l a t i n coacervate. B.K. Green used both concepts and prepared the f i r s t g e l a t i n microcapsules. From this beginning i t was nine long years
0097-6156/88/0370-0126$06.00/0 ® 1988 American Chemical Society
In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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to the development of a marketable product. The new printing system was triggered by including a colorless dye-base i n the o i l droplets (coated back, CB) and coating the second sheet of paper (coated front, CF) with a c i d i c clay which could react with the dye-base to produce a color. Coacervation and Microencapsulation Coacervation i s a c o l l o i d phenomenon. If one starts with a solution of a c o l l o i d i n an appropriate solvent, then according to the nature of the c o l l o i d , various changes can bring about a reduction of the s o l u b i b i l i t y of the c o l l o i d . As a r e s u l t of this reduction a large part of the c o l l o i d can be separated out into a new phase. The o r i g i n a l one phase system becomes two phases. One i s r i c h and the other i s poor i n c o l l o i d concentration. The c o l l o i d - r i c h phase i n a dispersed state appears as amorphous l i q u i d droplets called coacervate droplets. Upon standing these coalesce into one clear homogenous c o l l o i d - r i c h l i q u i d layer, known as the coacervate layer which can be deposited so as to produce the wall material of the resultant capsules. Coacervation may be i n i t i a t e d i n a number of d i f f e r e n t 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, i t must wet the suspended core p a r t i c l e s or core droplets and coalesce into a continuous coating for the process of microencapsulation to occur. The f i n a l step for microencapsulation i s the hardening of the coacervate wall and the i s o l a t i o n of the microcapsules, usually the most d i f f i c u l t step i n the t o t a l process. Simple Coacervation Simple coacervation involves the use of either a second more-water soluble polymer or an aqueous non-solvent for the g e l a t i n . This produces the p a r t i a l dehydration/desolvation of the gelatin molecules at a temperature above the g e l l i n g point. This results i n the separation of a l i q u i d g e l a t i n - r i c h phase i n assocation with an equilibrium l i q u i d (gelatin-poor) which under optimum separation conditions can be almost completely devoid of g e l a t i n . Simple coacervation can be effected either by mixing two c o l l o i d a l dispersions, one having a high a f f i n i t y for water, or i t can be induced by adding a strongly hydrophilic substance such as alcohol or sodium sulfate. The water soluble polymer i s concentrated i n water by the action of a water miscible, non-solvent for the emerging polymer (gelatin) phase. Ethanol, acetone, dioxone, isopropanol and propanol have been used to cause separation of coacervates of g e l a t i n , polyvinyl alcohol and methylcellulose. Phase separation can be effected by the addition of an e l e c t r o l y t e such as an inorganic s a l t to an aqueous solution of a polymer such as g e l a t i n , polyvinyl alcohol or carboxymethyl c e l l u l o s e .
In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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A t y p i c a l simple coacervation using g e l a t i n c o l l o i d i s as follows: to a 10 percent dispersion of g e l a t i n i n water, the core material i s added with continous s t i r r i n g and at a temperature of 40° C. Then a 20 percent sodium s u l f a t e solution or ethanol i s added at 50 to 60 percent by f i n a l totalvolurae, i n order to induce the coacervation. This system i s cooled to 5 C; then, i t i s necessary to insolubilize the coacervate capsules suspended i n the equilibrium l i q u i d by the addition of a hardening agent such as gluteraldehyde and adjusting the pH. The resulting microcapsules are washed, dryed and c o l l e c t e d .
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Complex Coacervation Complex coacervation ( 3 ) can be induced i n systems having two dispersed hydrophilic c o l l o i d s of opposite e l e c t r i c charges. Neutralization of the o v e r a l l positive charges on one of the c o l l o i d s by the negative charge on the other i s used to bring about separation of the polymer-rich complex coacervate phase. The gelatin-gum arabic (gum acacia) system i s the most studied complex coacervation system. Complex coacervation i s possible only at pH values below the i s o e l e c t r i c point of g e l a t i n . It i s at these pH values that g e l a t i n becomes p o s i t i v e l y charged, but gum arabic continues to be negatively charged. A t y p i c a l complex coacervation process using g e l a t i n and gum arabic c o l l o i d s i s as follows: The core material i s emulsified or suspended either i n the g e l a t i n or gum arabic solution. The aqueous solution of both the g e l a t i n and gum arabic should each be below 3 percent by weight. Then, the g e l a t i n or the gum arabic solution (whichever was not previously used to suspend the core material) i s added into the system. The temperature of the system must be higher than the gel point of an aqueous g e l a t i n solution (greater than 35 C). The pH i s adjusted to 3.8-4.3 and continous mixing i s maintained throughout the whole process. The system i s cooled to 5 0 C and the gelled coacervate capsule walls are i n s o l u b i l i z e d by either adding glutaraldehyde or another hardening agent and adjusting the pH. The microcapsules are washed, dryed and collected. Aqueous Phase Separation The term aqueous phase separation i s 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 i s the o i l and i t should be immisible i n the continuous phase, namely water. A commercial example of aqueous phase separation would be the microencapsulation of an o i l y flavor such as sour cream with a g e l a t i n wall. These microcapsules would then be dispersed i n 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 f i r s t swell and then rupture.
In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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O r g a n i c Phase S e p a r a t i o n The term o r g a n i c phase s e p a r a t i o n (4) i s sometimes more s i m p l y referred to as " w a t e r - i n - o i l " m i c r o e n c a p s u l a t i o n . I n t h i s case the p o l a r c o r e i s d i s p e r s e d i n t o an o i l y or n o n - p o l a r continuous medium. The w a l l m a t e r i a l i s then d i s s o l v e d i n t h i s continuous medium. A simple technique for encapsulation consists of d i s s o l v i n g e t h y l c e l l u l o s e i n c y c l o h e x a n e a t a temperature of 50° C with continuing mixing. Only one phase i s p r e s e n t . The cyclohexane i s the o i l y , c o n t i n u o u s phase and the e t h y l c e l l u l o s e will l a t e r form the c o a c e r v a t i v e wall. The temperature is elevated to 70°C over a p e r i o d of 20 to 30 m i n u t e s . The c o r e m a t e r i a l i s added and the temperature r a i s e d to 8 0 C over a p e r i o d of time and i s h e l d a t t h a t temperature f o r one h o u r . The system is allowed to c o o l r a p i d l y to 2 0 - 4 0 ° C. Upon c o o l i n g , the ethylcellulose w i l l g r a d u a l l y emerge as a s e p a r a t e d coacervate phase which w i l l then g r a d u a l l y s o l i d i f y by the time 20° C i s reached ( u n l i k e hot c y c l o h e x a n e , the c o l d m a t e r i a l i s a n o n solvent). The c a p s u l e s a r e washed, f i l t e r e d and a i r d r y e d . It s h o u l d be noted t h a t e t h y l c e l l u l o s e i s g e n e r a l l y approved f o r use i n the p h a r m a c e u t i c a l i n d u s t r y . However, f o r i t s use i n the food i n d u s t r y the "Code of F e d e r a l R e g u l a t i o n s " s h o u l d be consulted under the categories of both microencapsulation and ethylcellulose.
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A n o t h e r e t h y l c e l l u l o s e w a l l e n c a p s u l a t i o n system i n v o l v e s the a d d i t i o n of p o l y i s o b u t y l e n e to the ethylcellulose/cyclohexane. The procedure begins w i t h the a d d i t i o n of ethylcellulose to a m i x t u r e of c y c l o h e x a n e and p o l y i s o b u t y l e n e a t room temperature. P o l y i s o b u t y l e n e , w h i c h i s more s o l u b l e i n c y c l o h e x a n e than i s ethylcellulose, i s more e f f e c t i v e i n c a u s i n g the l a t t e r to emerge as a s e p a r a t e l i q u i d c o a c e r v a t e than would be the case w i t h merely c o o l i n g the c y c l o h e x a n e . The s o l u t i o n i s then heated to 80°C and the c o r e m a t e r i a l added. The system i s c o o l e d to 40°C i n 60 minutes and c o o l e d q u i c k l y to 2 0 - 2 5 C. Microcapsules are filtered, washed and d r y e d . A m o d i f i c a t i o n of this system involves the f o l l o w i n g p r o c e d u r e : The p o l y i s o b u t y l e n e is dissolved i n cyclohexane a t a temperature of 70° C and w i t h continuous stirring. After cooling the system to 4 0 C, e t h y l c e l l u l o s e i s added and d i s s o l v e d . The c o r e i s d i s p e r s e d in t h i s s o l u t i o n and the system a g a i n heated to 7 8 C and h e l d f o r 10 minutes a t t h i s t e m p e r a t u r e . Then i t i s s l o w l y c o o l e d to room temperature f o l l o w e d by c o o l i n g to 10°C over two h o u r s . Besides p o l y i s o b u t y l e n e , o t h e r c o a c e r v a t i o n i n d u c i n g agents have been used such as p o l y e t h y l e n e and b u t y l r u b b e r . e
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D e s i g n i n g the P r o c e s s — P r o c e s s and M a t e r i a l S e l e c t i o n C o a c e r v a t i o n i s a v e r y c o m p l i c a t e d p h y s i c a l phenomenon. A n d , many factors affect the p r o p e r t i e s of the r e s u l t i n g microcapsules. C o a c e r v a t i o n and phase s e p a r a t i o n from o r g a n i c and aqueous media i n v o l v e many p r o p e r t i e s , m a t e r i a l s and p r o c e s s e s such a s : phase inducing agents, stirring rates, c o r e to w a l l r a t i o s , polymer characteristics, core c h a r a c t e r i s t i c s ( w e t t a b i l i t y , s o l u b i l i t y ) , c o o l i n g r a t e s and r a t e s of a d d i t i o n .
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The b a s i c p r o d u c t i o n of m i c r o c a p s u l e s a c t u a l l y involves three distinct steps as d i s c u s s above. The h a r d e n i n g of the microcapsules sufficient f o r i s o l a t i o n of them i n t o a d r y freef l o w i n g powder remains as a p e r s i s t e n t problem. One of the e a r l i e s t attempts i s the g e l a t i n - t a n n i n r e a c t i o n ( 5 ) . Tannic a c i d i s r e c o g n i z e d as a means f o r " h a r d e n i n g " g e l a t i n w a l l e d capsules. I t s h o u l d be noted t h a t a p a r t i c u l a r e n c a p s u l a t i o n s y s t e m , such as would be d e s c r i b e d i n any one of the p a t e n t s i n the l i t e r a t u r e , i s not n e c e s s a r i l y effective i n encapsulating a given flavor. Consequently i t i s o f t e n n e c e s s a r y t o d e v e l o p o r modify an e n c a p s u l a t i o n system f o r each f l a v o r . This i s p a r t i c u l a r l y true when one l o o k s a t the m u l t i t u d e o f r e q u i r e m e n t s r e l a t e d to s t o r a g e and r e l e a s e of the m i c r o c a p s u l e c o r e . In developing or m o d i f y i n g an e n c a p s u l a t i o n s y s t e m , i t is very h e l p f u l to l o o k a t the a v a i l a b l e w a l l m a t e r i a l s t h a t one may have f o r use. A r e a s o n a b l y comprehensive l i s t i s a v a i l a b l e ( 6 ) . This list s h o u l d be used i n c o n j u n c t i o n w i t h m a t e r i a l s a l s o l i s t e d i n the "Code of F e d e r a l R e g u l a t i o n s " , T i t l e 21, p a r t i c u l a r l y parts 1 to 199 ( 7 ) . The f o l l o w i n g m a t e r i a l s s h o u l d be u s e f u l in d e v e l o p i n g a m i c r o e n c a p s u l a t i o n system u s i n g c o a c e r v a t i o n :
A c a c i a (Gum A r a b i c ) B u t a d i e n e - S t y r e n e 75/25 Rubber B u t a d i e n e - S t y r e n e 50/50 Rubber B u t y l Rubber Carob Bean Gum Carrageenan C i t r i c Acid Dextrin Dimethylpolysiloxane Dimethyl S i l i c o n e Ethylcellulose Food S t a r c h , M o d i f i e d Guar Gum Hydroxpropyl C e l l u l o s e Hydroxypropyl M e t h y l c e l l u l o s e Isobutylene-Isoprene Copolymer L o c u s t Bean Gum Methylcellulose Methyl E t h y l C e l l u l o s e M i c r o c r y s t a l l i n e Wax P a r a f f i n , Synthetic P e t r o l e u m Wax P e t r o l e u m Wax, S y n t h e t i c Poloxaraer Polyethylene
Polyethylene Glycols Polyisobutylene P o l y v i n y l Acetate Polyvinylpolypyrrolidone Potassium A l g i n a t e Potassium C i t r a t e P o t a s s i u m Polymetaphosphate Potassium Tripolyphosphate Povidone PVP R e f i n e d P a r a f f i n Wax S h e l l a c , Bleached Sodium A l g i n a t e Sodium C a r b o x y m e t h y l c e l l u l o s e Sodium C i t r a t e Sodium F e r r o c y a n i d e Sodium P o l y p h o s p h a t e s , G l a s s y (Sodium hexaraetaphosphate) Sodium Trimetaphosphate Sodium T r i p o l y p h o s p h a t e S y n t h e t i c Wax ( E t h y l e n e Polymer) Tannic Acid Terpene R e s i n , N a t u r a l Tragacanth White S h e l l a c Xanthan Gum
In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
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Conclusions A number o f e n c a p s u l a t i o n systems such as s p r a y d r y i n g appear to be s u p e r i o r to c o a c e r v a t i o n because o f c o s t and a v a i l a b i l i t y of materials. There a r e t i m e s , however, when c o a c e r v a t i o n is absolutely necessary. This g e n e r a l l y occurs when r e s e r v o i r m i c r o c a p s u l e s o f a s m a l l s i z e , say 10 t o 70 m i c r o n s , a r e needed o r when the c o r e m a t e r i a l i s a p o l a r l i q u i d and c a p s u l e s can o n l y be made by o r g a n i c phase s e p a r a t i o n . W i t h o i l y c o r e s i t i s g e n e r a l l y b e s t to s t a r t w i t h a g e l a t i n system and modify i t accordingly. The g l u t e r a l d e h y d e h a r d e n i n g o f g e l a t i n s h o u l d be j u d i c i o u s l y used i n accordance w i t h the r e s t r i c t i o n s s t a t e d i n the "Code o f F e d e r a l Regulations". There y e t remains c o n s i d e r a b l e amounts of a r t to coacervative microencapsulation. Here a r t i s b e s t d e s c r i b e d as a phenomenon awaiting a s c i e n t i f i c explanation. I n c o a c e r v a t i o n the k i n d of a d d i t i o n and the r a t e of and o r d e r of a d d i t i o n a r e e x t r e m e l y critical. I n g e n e r a l , t h e s l o w e r the p r o c e s s the b e t t e r i t i s f o r coacervative encapsulation. I t i s the i n t u i t i v e f e e l that e n c a p s u l a t o r s p r a c t i c e t h a t i t f r e q u e n t l y termed " a r t . "
1. 2.
Green, B . K . ; Schleicher, L. U.S. Patent 2 800 457, 1957. 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, Α., Industrial and Engineering Chemistry (The Gelatin-Tannin Reaction), 15, p. 839-841, (1923). 6. Food Chemicals Codex; 3rd edition, National Academy Pres, Washington, DC 1981. 7. Title 21, Code of Federal Regulations, Superintendent of Documents, Government Printing Office, Washington, D.C. 20402. RECEIVED February 23, 1988
In Flavor Encapsulation; Risch, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.
