Razones Administrativas

Introduction

The encapsulation of flavors was first reported in the 1930s when it was observed that a volatile substance, isopropanol, was retained by a spray-dried particle (Thies, 1999). This observation catalyzed the development of spray dry flavor encapsulation, a technology responsible today for the daily production of tons of encapsulated flavor products globally.

Reineccius (2004) and others (Brenner, 1983; Re, 1998; Liu et al., 2001) have discussed spray dry encapsulation technology in some detail.

Although spray drying is currently the dominant flavor encapsulation technique, a num-ber of alternate encapsulation technologies exist and offer a potential means of producing unique flavor-loaded microcapsules. Complex coacervation encapsulation procedures fall into this category. Accordingly, this contribution is a discussion of various aspects of com-plex coacervation encapsulation technology and the encapsulation of flavors for food prod-ucts. Only complex coacervation processes based on food-grade polymers are considered here. Although fragrances are not considered in this contribution, much of the discussion is also applicable to the encapsulation of fragrances as well as other complex core materials.

Flavor Encapsulation

The preparation of flavor-loaded microcapsules is a complex task. It is much more compli-cated than it appears at first glance, because flavor microcapsules must meet a series of requirements. One requirement is the production of microcapsules that retain the desired properties of the flavor encapsulated. Each flavor is a unique and complex mixture of many compounds. These compounds have a broad range of structures with vapor pressures, sol-vent solubility, and stability that differ significantly. Any useful encapsulation technology must be able to accommodate this variability. Ideally, the chemical composition of the fla-vor is unchanged by the encapsulation process, and the encapsulated flafla-vor is identical in all respects to the unencapsulated flavor. In reality, this is generally not the case. Encapsula-tion processes typically change the chemical composiEncapsula-tion of a flavor in some way. Loss of more volatile or more water-soluble components during an encapsulation process is common.

Such losses can have a significant effect on the desired olfactory properties of the flavor.

Flavor-loaded microcapsules must contain enough active agents to cause the desired effect. The amount of flavor required varies with the nature of the flavor and intended food product. Although flavor impact can be altered by varying the flavor loading of a capsule of fixed size as well as by varying capsule size, the degree of variation may be limited by the nature of the food product. For example, capsule size variations may be limited by the need

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to retain structural integrity during processing while having the ability to be ruptured by chewing. Variations in amount of flavor carried by a capsule of fixed size may be limited by the capsule formation process as well as the retention or barrier properties of the cap-sule shell.

Stability of flavor-loaded capsules during processing and storage is an issue that must be addressed. Such capsules must have acceptable stability from the time of formation to con-sumption of the food product. Stability during processing of capsules as they are incorpo-rated into a food product can be a problem if this involves high shear or a combination of high temperature and shear. Shelf life stability after incorporation and storage in a food product is important as is the ability of capsules to release their contents during food prepa-ration or consumption. This series of stability requirements is imposing and often limits actual capsule performance. Factors that affect capsule stability include oxygen, moisture, heat, and light. In principle, the shell of a capsule should protect an encapsulated flavor from these agents, but deficiencies in either the capsule shell or the material(s) from which the shell is prepared may cause a shell to provide inadequate protection. Shell materials typically used to form food-grade flavor capsules may experience major property changes during food-processing steps that involve heat and moisture. Of course, flavor-loaded microcapsules must meet specifications imposed by governmental regulatory agencies responsible for food safety. This requirement puts a restriction on the shell materials that can be used. It also limits the nature and amount of processing agents used in a capsule-formation process.

In summary, the complex series of specifications associated with the formation of flavor-loaded microcapsules makes their preparation an interesting field of study. Much can be done in order to produce capsules that more closely approach the degree of perfection desired. Studies by various workers of the diffusion barrier properties of candidate shell materials merit review, because they provide much insight into the properties of such rials and help one develop a realistic appreciation for the limitations of specific shell mate-rials and capsule formation processes (Menting and Hoogstad, 1967; Kerkof and Thijssen, 1974; Thijssen, 1975; Rulkans and Thijssen, 1978; Goubet et al., 1998). Although most of these involve spray drying and freeze drying studies, the results obtained are applicable to capsules formed by any encapsulation process.

Complex Coacervation

Before discussing complex coacervation encapsulation processes, it is appropriate to con-sider the nature of complex coacervate systems and some of their characteristic features.

Complex coacervation is the liquid/liquid phase separation that occurs when solutions of two or more oppositely charged polyelectrolytes are mixed under suitable conditions.

Two liquid phases are formed: the coacervate phase and the supernatant or equilibrium liquid phase. The coacervate phase is a relatively concentrated polymer solution that par-ticipates in a complex coacervation encapsulation system. It is in this phase that capsule shell forms. The supernatant phase is a dilute polymer solution and serves as the continu-ous phase in which capsule formation occurs. Dilution favors complex coacervation and is a property that distinguishes complex coacervation from other polymer phase-separation phenomena.

Complex coacervation is affected by many variables. Bungenberg de Jong’s experimental studies in the 1930s and 1940s provide much useful background data about the phenomenon 150 Chapter 7

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(Bungenberg, 1949). Since then, a variety of workers have considered various aspects of coacervation including theoretical analyses based on polymer solution thermodynamics (Burgess, 1990; Veis, 1970; Schmitt et al., 1998). Although this information provides a guide for developing complex coacervation microencapsulation procedures, it is important to rec-ognize that conditions that optimize the degree of coacervation in a specific system may not be conditions under which useful microcapsules can be formed. For example, these condi-tions may produce a complex coacervate that is too viscous to yield acceptable capsules.

Selected Properties of Complex Coacervates

Many complex coacervation systems suitable for the production of microcapsules exist. In virtually all cases, gelatin is the polycation used. A wide range of polyanions is used. Each system operates under a unique set of conditions and has a unique set of properties once formed. This reflects differences in nature and frequency of ionic groups distributed along the chains of the polymers involved in a specific complex coacervation procedure. Differ-ences in polymer chain structure and molecular weight (MW) are other factors that influ-ence coacervation.

One of the polyelectrolytes used in a typical complex coacervation encapsulation proce-dure is a natural polymer with a complex molecular structure. For example, gelatin polymer molecules are made up of a number of different amino acids with different pendent groups.

Because anionic and cationic pendent groups are distributed along the polymer chain, gela-tin is a polyampholyte. The cationic groups are primary amino groups, while the anionic groups are carboxyl groups. The degree of ionization of these ionic groups varies with pH, so the net charge carried by a gelatin molecule varies with pH.

Gelatins formed by acid hydrolysis of collagen are classified as Type A or acid precursor gelatins. Alkaline hydrolysis yields Type B or alkaline precursor gelatins. The isoelectric point (pI) of Type A gelatins is typically 8–9, while the typical pI of Type B gelatins is 4–5.

The reduced pI value of Type B gelatins is caused by hydrolysis of pendant amide groups under alkaline conditions. The number of primary amino groups distributed along a gelatin chain is essentially independent of the hydrolysis procedure. Although both types of gelatins produce complex coacervates suitable for microcapsule formation, Type A gelatins historically have been used most. Significantly, for gelatin to carry a net cationic charge, it must be at a pH lower than its pI.

Although gelatin is the polycation involved in the formation of complex coacervates used in microencapsulation processes, many different polyanions are used. They differ greatly in anion group distribution along a polymer chain as well as the nature of this group. This is particularly true of natural polymers that carry an anionic group. Gum arabic (GA) and alginate, two polysaccharides, derive their anionic character from carboxyl groups distributed along their polymer chains. In GA, such groups are located on short-chain branches hanging off the primary polymer short-chain. Approximately 20% of the sugar units in GA contain a carboxyl group. In contrast, alginate molecules are linear polymer chains and every sugar in the chain has a carboxyl group. The degree of ionization of carboxyl groups is a strong function of pH and steadily decreases as pH decreases. The anionic character of sodium polyphosphate, an inorganic material, is due to the phosphate group. Carrageenan, a polysaccharide with a linear chain, has sulfate groups distributed along its chain. The average number of sulfate groups per sugar unit varies with the type of carrageenan.

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Because the polymers used to form complex coacervates differ significantly in composi-tion and structure, properties of complex coacervates formed by different polymers differ significantly. In order to illustrate this point, Table 7.1 contains degree of coacervation and enrichment data at 50°C for three gelatin-based complex coacervation systems used to pre-pare microcapsules: gelatin/gum arabic (GGA), gelatin/polyphosphate (GP), and gelatin/

sodium alginate (GAlg) (Commandur et al., 1989). Degree of coacervation (ρ) is defined as the fraction of total polymer in the system that is in the coacervate (Veis and Aryani, 1960).

Enrichment (ε) is defined as the ratio of polymer concentration in the coacervate phase to that in the supernatant phase (Veis and Aryani, 1960). GGA and GP coacervates were formed by interacting 285 bloom Type A gelatin with GA and sodium hexametaphosphate, respectively. GAlg coacervates were formed by interacting 231 bloom Type A gelatin with a hydrolyzed alginate.

Each coacervation system was studied at three initial solids concentrations and three pH values in order to illustrate how changes in these variables affect coacervate formation.

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Coacervate system Initial solids w/v (%) pH Degree of coacervation Enrichment

GGA 3.96 4 0.86 22

3.3 4 0.88 34.8

2.83 4 0.89 47.7

3.96 4.2 0.78 11.3

3.3 4.2 0.81 17

2.83 4.2 0.81 21.2

3.96 4.4 0.81 12.1

3.3 4.4 0.8 16.6

2.83 4.4 0.83 24.2

GAlg 2.11 4 0.86 66.7

1.81 4 0.83 68

1.59 4 0.82 71.7

2.11 4.2 0.66 25.3

1.81 4.2 0.73 42.5

1.59 4.2 0.7 54.7

2.11 4.4 0.78 35.7

1.81 4.4 0.81 53

1.59 4.4 0.79 54.7

GP 5 4 0.74 11.5

4.54 4 0.78 15.5

4.17 4 0.78 20.9

5 4.2 0.72 9.4

4.54 4.2 0.71 11.3

4.17 4.2 0.74 14.5

5 4.4 0.61 4.8

4.54 4.4 0.57 5.7

4.17 4.4 0.65 10.2

Table 7.1. Tabulation of degree of coacervation (ρ) and enrichment data (ε) at 50ºC for several coacervation systems

Indeed, at sufficiently high concentration and pH values, complex coacervation does not occur. Initial solids content is defined as the total polymer solids in the system at the time when complex coacervation occurred. The data in Table 7.1 were obtained by using initial solids of 1.6–5 w/w% and pH values of 4.0–4.4 pH. Although these initial solids and pH values are typical for many coacervation systems used to make microcapsules, situations exist where suitable coacervates will form when values of one or both parameters fall out-side these ranges.

The gelatin/polyanion ratio used for forming gelatin-based complex coacervates varies with ionic equivalent weight of the polyanion(s) used. The GGA complex coacervate data in Table 7.1 were formed by using a 1:1 w/w ratio of gelatin and GA, because both poly-mers have an ionic equivalent weight of roughly 1000. Alginates have an ionic equivalent weight of approximately 180, so the gelatin/alginate the ratio used was 3.7:1 w/w. The w/w gelatin/polyphosphate ratio was 9:1. It is high, because polyphosphates have a low ionic equivalent weight.

In all cases, the polyanion ratios reported here are those typically used by the author to produce complex coacervates suitable for microcapsule formation. Complex coacervates will form when a coacervation system contains excess gelatin or polyanion, but it is com-mon practice to use gelatin/polyanion ratios that approach ionic equivalency. Although the data in Table 7.1 are for coacervation systems based on one polyanion, mixtures of several chemically different polyanions can be used to produce gelatin-based coacervates suitable for microcapsule formation. This enables one to develop a broad range of complex coacer-vate systems suitable for microcapsule formation.

The data in Table 7.1 (Commandur et al., 1989) show that ρ and ε at 50°C are affected by the nature of the polyanion involved in coacervate formation, system pH, and initial solids content of the system. For all coacervation systems examined, values of ε at constant pH increase as the initial solids content decreases. This reflects the increase in intensity of coacervation upon solution dilution, a characteristic feature of complex coacervation. In contrast, other polymer phase-separation phenomena such as polymer/polymer incompati-bility and salting out (simple) coacervation are favored by increasing the concentration of the molecules involved.

Values of ρ for the GGA system fall between 0.8 and 0.9 over the range of initial solids and pH values examined. Thus, in these GGA systems, 80–90 w/v% of the polymers were concentrated in the coacervate phase. The GGA coacervate phases formed are 15–26 vol%

of total system volume and have a solids content of 10–14 w/v%. Solids content of the supernatant phase was 0.3–0.9 w/v%. Since values of ε found for the GGA systems range from 11 to 35, a high degree of polymer partitioning was achieved. At constant initial solids, values of ε decrease as pH increases.

GP coacervate phases occupy 13–21 vol% of total system volume and have a solids con-tent of 13–21 w/v%. GP ρ values of 0.6–0.8 and ε values of 5–21 are lower than the range of ρ and ε values found for GGA and GAlg systems. Thus, the GP coacervate system achieves a lower degree of polymer partitioning. The solids content of GP supernatant phases range from 1 to 2.3 w/v%, considerably higher than that observed with GGA and GAlg systems. Because gelatin concentrations of approximately 2 w/v% approach the con-centration at which gelatin solutions gel, operating conditions of an encapsulation process based on GP must be adjusted to keep the supernatant solids concentration below 2 w/v%.

The solids content of a GAlg coacervate phase at 50°C varies from 15 to 22 w/v% while the solids content of a GAlg supernatant phase varies from 0.3 to 0.9 w/v%. The GAlg

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coacervate is 4–7 vol% of total system volume, considerably lower than the values observed with GGA or GP coacervates. Values of ρ for the GAlg systems are 0.7–0.9, a range similar to but broader than that observed with GGA coacervates. The 25–72 range of ε values for GAlg coacervate systems is higher than that observed with GGA or GP sys-tems. Thus, GAlg coacervate systems more effectively concentrate or partition into the coacervate phase the polymers involved in complex coacervation. At first glance, this is surprising, because the volume of the GAlg coacervate phase is much smaller than that of the GGA and GP coacervate phases, while the solids content of the GAlg coacervate phase is similar to that of the GGA and GP coacervate phases. Closer analysis leads to the recog-nition that the initial solids content of the GAlg system is measurably lower than that of the GGA and GP systems. Thus, the GAlg coacervate phase contains a higher percentage of polymers present in the GAlg system, even though it occupies a smaller volume fraction than the GGA and GP coacervate phases and has a solids content similar to these systems.

The ρ and ε data reported in Table 7.1 provide valuable insight into the nature of three complex coacervate systems, but they reveal nothing about coacervate rheology or the tem-perature at which the coacervates gel. Coacervate rheology is a primary variable that affects capsule shell formation and capsule aggregation. Accordingly, the viscosity of a number of coacervate phases was measured over a range of temperatures by capillary viscometry (Commandur et al., 1989). Table 7.2 summarizes results of these measure-ments. The data show that the sodium alginate and GA solutions used to form GGA and GP coacervates have a viscosity of 3–6 cS at 50°C. This viscosity increases as the temperature is reduced to 30°C, but the viscosity increase caused by cooling is not pronounced, because neither polymer alone gels on cooling. In contrast, viscosity of the gelatin solutions exam-ined steadily increases as the solution temperature is reduced from 50°C to 35°C. The viscosity of most such solutions becomes unstable at 32°C. That is, the recorded viscosity steadily increases toward infinity as the time at 32ºC increases. This viscosity increase is due to the onset of gelation.

Not shown in Table 7.2 are viscosity data for GGA, GP, and GAlg supernatant phases that exist in equilibrium with the GGA, GP, and GAlg coacervate phases for which viscos-ity data were obtained. Most supernatant phases have a viscosviscos-ity below 1 cS at tempera-tures ranging from 50°C to 35°C and provide no indication that they will gel on further cooling. Exceptions are two GP supernatant solutions isolated from pH 4.4 GP coacervate systems. The viscosity of both solutions remained below 1.5 cS as they were cooled to 35°C, but the upward slope of their temperature–viscosity plots suggests that both will ulti-mately gel.

GGA coacervate viscosity at pH 4.4 and 50°C ranged from 23 to 58 cS, i.e., 2 to 5 times greater than the 11.3 cS viscosity of a 10% solution of 285 bloom Type A gelatin at 50ºC.

Viscosity of the GGA coacervates steadily increases as the system is cooled. They either gel or become unstable due to onset of gelation as the temperature falls below 35°C.

Decreasing the pH of GGA coacervate formation from 4.4 to 4.0 increases coacervate vis-cosity at all temperatures examined.

The viscosity of GP coacervates at 50°C is 47–373 cS, measurably higher than the vis-cosity of GGA coacervates. Reducing the pH of a GP coacervate from 4.4 to 4.0 causes a major increase in coacervate viscosity and raises the GP coacervate gelation temperature above 35°C.

The viscosity of all GAlg coacervates at 50°C is 20–60 times higher than that of most GGA and GP coacervate phases at 50°C. Although GAlg coacervate phases have a very 154 Chapter 7

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Table 7.2.Changes in viscosity of various coacervate systems at different temperatures (from Commandur etal., 1989, with permission) Initial solids SolidsViscosity (cS) System(w/v%)apH(w/v%)b50°C 45°C40°C37°C35°C32.5°C30°C GGA coacervate3.964.410.42326293546107Gelled 2.834.412.13942476296UnstableGelled GGA coacervate3.96413.23942487997143Gelled 2.83414.3586374UnstableGelled GP Coacervate54.412.3475698221Gelled 4.174.414.36774120679Gelled GP Coacervate4.554.219.82983421782Gelled 4.17420.93734022606Gelled GAlg Coacervate2.114.415.667980616363396Gelled 1.594.218.41245148024857404Gelled GAlg Coacervate2.114201391166528066220Gelled 1.59418.61382169832547658Gelled Gelatin solution Type A 1011.513.214.81720UnstableGelled (285 bloom)1531354052.166.6UnstableGelled Gelatin solution Type A 105.27.389.210.311.516.2Unstable (231 bloom)155.415.917.820.624.628.9UnstableGelled Sodium alginate solution25.533.43.744.24.8 Gum arabic solution104.24.14.54.95.45.56.3 aTotal system solids at the time of coacervation. bTotal solids of solution used for capillary viscosity measurement.

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high viscosity at temperatures ranging from 50°C to 40°C, they do not appear to gel until cooled to 35°C.

As the data in Table 7.2 show, complex coacervate viscosity is a strong function of the coacervation system, pH, and temperature. Characterizing the effect of temperature changes on rheology of a coacervate system is an important task, because all gelatin-based complex coacervation encapsulation protocols involve a cooling step that lowers the sys-tem sys-temperature below the gel sys-temperature of the coacervate. In such processes, complex coacervate formation always occurs above the coacervate melt temperature so that the coacervate formed is initially a liquid. It must have a viscosity that is sufficiently low to enable it to engulf dispersed core material droplets or particles, thereby coating them with a thin film of liquid coacervate. Once a coacervate is formed, the two-phase system is cooled below the gel temperature of the coacervate, thereby setting the gel structure of the coacer-vate. This transforms the thin liquid film that surrounds the small droplets of core material into a thin gel coating. Viscosity of a coacervate above its melt temperature and changes in its rheology as cooling occurs have a major impact on the success of a complex

As the data in Table 7.2 show, complex coacervate viscosity is a strong function of the coacervation system, pH, and temperature. Characterizing the effect of temperature changes on rheology of a coacervate system is an important task, because all gelatin-based complex coacervation encapsulation protocols involve a cooling step that lowers the sys-tem sys-temperature below the gel sys-temperature of the coacervate. In such processes, complex coacervate formation always occurs above the coacervate melt temperature so that the coacervate formed is initially a liquid. It must have a viscosity that is sufficiently low to enable it to engulf dispersed core material droplets or particles, thereby coating them with a thin film of liquid coacervate. Once a coacervate is formed, the two-phase system is cooled below the gel temperature of the coacervate, thereby setting the gel structure of the coacer-vate. This transforms the thin liquid film that surrounds the small droplets of core material into a thin gel coating. Viscosity of a coacervate above its melt temperature and changes in its rheology as cooling occurs have a major impact on the success of a complex