CARBOHYDRATES AS ENCAPSULATING MATRICES

16.3 CARBOHYDRATES AS ENCAPSULATING MATRICES

16.3.1 F UNCTIONALITY OF C ARBOHYDRATES IN T RIGGER E VENTS

As compared with proteins and lipids, carbohydrates have been the most common choice of shell materials in food ingredient encapsulations, especially in flavor

applications. 11 Some common examples from the three classes of natural materials are listed in Table 16.2. In practice, the best performance has most often been obtained by using special additives together with a single-carbohydrate material, or by applying many carbohydrates in combination with each other or with, e.g., proteins. Furthermore, several encapsulation processes can be applied to achieve better performance. Carbohydrates are, however, hydrophilic molecules, which means that they always interact with water. Changes in water activity can cause stickiness and caking of powders, and may also induce changes in the number of phases initially present in the system, which naturally can change the original functionality. The presence of water may also mean swelling, which might allow transport of water-soluble components across the matrix, or water can dissolve the shell matrix. In applications in which water interactions need to be avoided, instead of carbohydrates, lipid coatings could solve the problems.

Typically, glassy, highly viscous carbohydrates with low water contents form matrices, which have been used to prevent evaporation and minimize chemical reactions. 13,14 Amorphous glassy matrices are generally believed to be stable because of the low mobility of the matrix, which slows down, e.g., oxygen and flavor diffusion. In real coating-based encapsulations, film formation ability should

be the inherent property of the shell material. In this case, the question is whether the membrane behaves according to the target functionality. In matrix systems, the shell material forms a continuous region in which ingredients are distributed, which is a much more complicated control system than a single-core particle. Various trigger events that may be exploited when developing targeted delivery systems for processes or special delivery vehicles for improving the biological performance are described in Table 16.3. In addition to its chemical and physical

TABLE 16.2 Approved Food-Grade Shell Materials for Microencapsulation

according to Thies 12

Polysaccharides

Fats and Waxes

Proteins

Gum arabic

Gelatins Modified starches

Hydrogenated vegetable oil

Bee wax

Whey proteins Hydrolyzed starches a Soy proteins

Alginates Sodium caseinate Pectins Carrageenan

a Maltodextrins.

Functional Food Carbohydrates

properties and interactions between the shell material and the encapsulant, the final behavior of the shell material depends also on the technology applied to

produce the encapsulated product. The following discussion focuses on some selected carbohydrates that are already used in many food applications, or have been suggested to be exploited, or are potentials but so far not reported to be applied in the food area.

16.3.2 S TARCH

Starches have not been the usual choice of the encapsulation matrix for food ingre- dients in the most often applied spray-drying technology, because starches do not

dissolve in cold water. 15 Instead of starch polymers, their water-soluble hydrolysis products, maltodextrins and slightly derivatized starch hydrolysates, have widely been used as encapsulating matrices in the food industry (see 16.3.5 and 16.3.6 in this chapter).

Starches are unique among polysaccharides because they occur in nature as discrete particles, called granules. 16 For dissolvation of granular starches under aqueous environments and at normal pH values, temperatures above 40˚C are needed. Because amylases do not generally hydrolyze solids, starch granules are not easily digested in the human digestive tract. This, however, depends on starch origin. Native legume starches are more digestible than native potato or high-amylose cornstarch, but less digestible than native cereal or cassava starch. Native cornstarch granules have been shown to be hydrolyzed up to 75%, and legume starches 25 to 35% by porcine α-amylase. 17

Not only starch granules differ in their digestibility, but also dissolved starch polymers after gelation vary in their accessibility to amylases. The linear starch polymer — amylose — typically forms networks that are much more resistant to digestive tract conditions than amylopectin, which is the main starch polymer in all starch granules and the only polymer in so-called waxy varieties. The resistant starch

TABLE 16.3 Possible Trigger Events That Can Release the Active Substance from the Microcapsule

Trigger Event Release Mechanism

Mechanical treatment

Fragmentation and release

Solvent Dissolvation of shell matrix and release, or swelling and diffusion

across

Solvent + enzymes/microbes Release only in the presence of specific enzymes or microorganisms

due to degradation/dissolvation

Solvent + pH E.g., stable at neutral pH, release at acid/alkaline pH, similar with

solvent + temperature

Solvent + temperature Release in solution at elevated temperature due to dissolvation or

swelling

Temperature Release from solids when heating or cooling due to melting

Potential Use of Carbohydrates as Stabilizers and Delivery Vehicles

structures — granules, gels, and films — are fermented by the colonic microflora as part of dietary fiber. 18–20 On the other hand, films and gels prepared of amylopectin are mechanically weak and easily hydrolyzed by either acids or amylases. 21–23

Investigations on the possible utilization of amylose for encapsulation of organic molecules have been conducted. 24,25 This behavior is based on the tendency of amylose to form helical conformations with an inner surface consisting pre- dominantly of hydrogen atoms, making it hydrophobic, a structure that is closely related with cyclodextrins. About 6% unsaturated fatty acids were bound within amylose helix in both potato starch and high-amylose cornstarch and were very stable to oxygen. 25

Native starch matrices have been developed to function as encapsulation carriers, e.g., for slow release. 26,27 Both methods were based on a similar process, which was total dissolvation of starch granules in water, followed by addition of the encapsulant. The final particles were produced by drying the rapidly cooled gel. Virtually any material may be encapsulated by the described process, and the release of the encapsulant could be controlled by the choice of the starch, high-amylose cornstarch giving prolonged release. The insolubility of amylose in gastric juices and its degrad- ability by colonic bacteria were for the first time technologically exploited in a novel way when colon-specific drug delivery formulation — a macrocapsule — was developed about 10 years ago. 28,29 In another study based on starch polymers’ ability to retrogradate from solutions, it was suggested that this phenomenon can be utilized when developing microencapsulation technology for lipophilic drug particles. 30

Because of the exceptional granular nature of native starches, it is no wonder that specially treated starch granules have been suggested to be potential carriers for many active substances that could be utilized in the areas of food, cosmetics, agriculture, and medicine. Partially hydrolyzed and cross-linked starch granules

were suggested to be suitable carriers for functional substances. 31 Hydrolysis was performed with the aid of amylases, and cornstarch was suggested to be the preferable starch used in the application. To improve the absorption capacity, the surface of the granules could be treated with proper agents or, in the case of absorbing lipophilic substances, chemically modified. Another investigation focused on various amylase-treated starch granules without any cross-link forma-

tion. 32 The produced granules were suggested to have broad application potential in food formulations due to low viscosity, and they also were claimed to be able to act as carriers for hydrophobic components.

In addition to exploiting partially hydrolyzed starch granules, starch granule aggregates were discovered to function as, e.g., a food ingredient carrier. 33,34 Water dispersion of starch granules, from small starch granules such as those of rice starch or the size-classified small starch granule fraction of, e.g., wheat starch, was partic- ularly preferred to form spherical aggregates when dehydrated in a spray dryer in the presence of a proper binder. The active substance could be introduced into the porous aggregate either as a component of the spray-dried dispersion or as solution in an inert low-boiling solvent, which can be removed by evaporation following loading of the aggregate matrix. The aggregates filled with active components could further be coated with bio- or synthetic polymers to improve the performance of the overall product. Additionally, the surface of the granules could be pretreated before

Functional Food Carbohydrates

aggregate formation by proper agents to improve absorption of the encapsulant. Specially treated potato starch granules were observed to have the capacity to be filled with fragrant compounds up to 30%. 35

A special technique for microencapsulation of living microorganisms in starch was developed using properties of both starch granules and starch polymers. 36 In this process, partially hydrolyzed granules offered the carrier matrix for the microbes, and after a coating process based on water-dissolved high-amylose cornstarch poly- mers, the final powdery microcapsules were produced. The presence of high-amylose cornstarch was also recently observed to increase survival of certain health-promot- ing bacteria at low pH, and during passage through the intestinal tract of mice. 37 Adhesion of the bacteria on the starch granule surface was considered to be a possible mechanism for increased bacterial survival.

A recently reported study demonstrated formation of small crystalline aggregates from, e.g., cornstarch water solution when the solution was slowly cooled after preparation in a jet cooker. 38 Crystalline aggregates were observed to be composed of amylose and suggested to be a result of crystallization of helical inclusion com- plexes formed from amylose and the native lipids present in cereal starch granules. The cavity of the amylose helices is chemically very similar to the hydrophobic cavity of β-cyclodextrin. Because it is known that β-cyclodextrin stabilizes various flavors (see section 16.3.6), jet cooking may offer a new process to exploit the binding ability of amylose.

Thus, starches offer porous carrier matrices, water-soluble and -insoluble film materials, and a special binding ability of the amylose helical structure, which all

might be exploited when developing delivery vehicles for bioactive components. These materials may further be combined with proteins or polyelectrolytes to improve the overall performance.

16.3.3 P ECTINS

Pectins are anionic polyelectrolytes and resemble alginates. The key feature of all pectin molecules is a linear chain of (1-4)-linked α-D-galactopyranosyluronic acid

units. 16 Thus, pectins are polygalactouronic acids, and the chain molecules are negatively charged at neutral pH. The pK values obtained have been in the range of

3.0 to 3.3. 39 Commercial pectins are mainly prepared from citrus peels and apple pomace. The degree of esterification (DE) varies among pectins and controls gelation and film formation properties. Additionally, sugars have a large influence on pectin gelation because sugar competes with hydration water. Dry pectin coatings devel- oped for drug formulations have been investigated. 40,41 High methyl ester pectins (methoxylation degree > 50%) are less water soluble and have been observed to have more potential in delivery formulations than pectins with lower degrees of

esterification. 42 The gelation ability of high methyl esters makes it possible to reduce the penetration of water into the dosage form, and hence the dissolution of an active

component incorporated into it. Gels prepared of high methyl ester pectins are also heat stable, which could be a beneficial property when exploiting encapsulated

ingredients in processes.

Potential Use of Carbohydrates as Stabilizers and Delivery Vehicles

The presence of calcium salts or other multivalent cations makes it possible for low methyl esters (methoxylation degree < 50%) to also form rigid gels. 43 The gelation is believed to occur in a manner similar to that of alginates, with formation of strong interchain binding resulting in the conformation known as the egg-box model. A very useful property of these gels is their stability in solutions with low pH and swellability under slightly alkaline conditions. The functionality of these matrices, however, depends much on the number of methyl ester groups and calcium ion content.

An important technological property related to development of delivery vehicles is that all pectins are resistant to digestion, but are fermented by the colonic micro- flora. 42 That is why development of pectin-based formulations aiming at colonic drug delivery has recently been reported. Studies on drug release have demonstrated that pectin salts (calcium or other metals) with different solubilities are able to form matrices that offer control of colon drug release.

pH-dependent swelling of pectin gels may be exploited in applications in which the target encapsulant is able to diffuse across the swollen matrix. Gels prepared of high methyl ester pectins may also offer delayed release properties for certain ingredients. The rate of release can be suggested to depend greatly on the molar

mass of the encapsulant, as has been observed for the alginate gels. 44 This gives further possibilities to control the release characteristics. Furthermore, the observed colon drug delivery behavior — degradation of the gel and release of the encapsulant in the presence of bacteria — could perhaps be used in certain special food ingredient applications. Additionally, due to their anionic character, pectins could well be mixed with other biopolymers to achieve a wider variety of functionalities.

16.3.4 O THER P OLYSACCHARIDES

Gum arabic has been the standard of excellence as an encapsulating matrix for food ingredients, especially in applications based on spray-drying technology. 4 It is a very good emulsifier, bland in flavor, and provides good retention of volatiles during the drying process. Gum arabic is a complicated polymer and is composed not only of polysaccharides, but also of protein units, which probably explains its emulsifying

potential. 16 The overall protein content is about 2%, but specific fractions may contain as much as 25% protein. Due to its high retention property, gum arabic has been much used for flavor carriers, especially for citrus and other flavor oils. An important characteristic is flavor load, which is closely related to retention because higher loadings generally tend to produce poor retention. The load ratio 4:1 (car- rier:flavor) has most commonly been used in practice. Thus, gum arabic is well suited for the change of the physical form of an active substance, which normally is a change from a liquid to a solid to improve the processing. A beneficial property, as compared, for example, to protein carriers, is the stability of the emulsion formed by gum arabic under acidic pHs. Another important characteristic is its compatibility with a high concentration of sugars. Additionally, gum arabic is anionic and may

react with other polymers. 44 Whether improvement of oxidation stability is achieved by binding food ingredients within gum arabic matrices cannot be answered based on reported literature, 4 in spite of the long history in using gum arabic as an

Functional Food Carbohydrates

encapsulating matrix. Gum arabic may or may not offer protection against oxidative deterioration, depending upon the gum. Some species are claimed to offer outstand- ing protection, while others offer little or no protection to the active substance.

Alginate is a linear polysaccharide and is composed of two monomeric units: β-D-mannuronic and α-L-guluronic acids. 45 Alginates prepared from various sources may differ in either molecular weight or proportion of monomers. The content of guluronic acid ranges from 10 to 80%, but the length of the guluronic sequence is the more important functional property, since it strongly influences gel formation. The viscosity of alginate solutions is mainly determined by the size of the molecule. The presence of calcium (or other divalent cations) increases drastically the viscosity due to gel formation, which is believed to occur via bridge formation between calcium and carboxyl, as well as hydroxyl groups of the parallel guluronic chains, resulting in an egg-box arrangement. The gel strength depends on the content of the guluronic units in the alginate polymer and the calcium concentration. Commercial alginate is usually sodium salt of the alginic acid and prepared from brown seaweeds.

The major development of encapsulation processes based on alginates has been with the aim to improve cell viability, but also vitamins, enzymes, and other active components have been of interest. 45 Many technologies exist to apply alginate in encapsulated form, but beads formed by external gelation, i.e., by dropping the alginate-encapsulant solution into a calcium chloride solution, is the most common technique. To achieve protection of the encapsulant against oxygen, denaturation, or light, alginate beads must be dried. Unfortunately, little information is available concerning either direct production of dry beads or drying of wet beads. In addition, the rehydration process of beads has not been dealt with in the literature. However, alginates are interesting, safe, and functional biopolymers offering matrices for food ingredient delivery vehicles, in which delivery may be controlled, e.g., by degree of swelling. Alginates could also be combined with other biopolymers to achieve better performance.

Water-soluble polysaccharides known as β-glucans are linear glucose chain polymers composed of 1 →4 and 1→3 linkages. 16 Oat and barley brans are the source of commercial β-glucans. β-Glucan-rich fractions from oats were discovered to be able to function as a pH-depending encapsulation matrix for bioactive components

such as living bacteria and enzymes. 46 The hydration of the matrix occurred under slightly alkaline conditions, where the encapsulant could also be added. No release under acidic conditions took place because no hydration occurred. Thus, β-glucans have similar functional properties as pectins in that their swelling depends on pH, offering pH-dependent release of the encapsulant from the matrices. It has also been reported that water-soluble polysaccharides from linseed (mucilage) could function as delivery systems for certain cosmetic, therapeutic, or nutritional substances. 47

16.3.5 M ALTODEXTRINS ,S YRUPS , AND S UGARS

Starch hydrolysis products with a dextrose equivalent (DE) below 20 are called maltodextrins. Maltodextrins have traditionally been used as encapsulation matrices, especially in spray drying. They are inexpensive, bland in flavor, very easily soluble in water (up to 75%), and exhibit low viscosity in solutions. 4,11 Generally in spray

Potential Use of Carbohydrates as Stabilizers and Delivery Vehicles

drying, maltodextrins have been used in combination with emulsifiers such as gum arabic for the preparation of stable emulsions of hydrophobic ingredients prior to

the drying step to achieve best possible performance. Furthermore, it has been observed that the use of high-DE materials — syrups and sugars — together with maltodextrins often results in more stable formulations. There are also studies that have shown that higher-DE matrices give better protection against oil oxidation than

lower-DE matrices. 4 This is not in agreement with the general thinking that glass transition temperature controls the stability; a possible explanation is the presence of trace minerals in the carrier matrix, or that the carrier may act as an antioxidant. Matrix porosity has also been suggested to affect oxidation. In addition, mixtures of sucrose and maltodextrins have been used in spray drying. They have also often been used in extrusion encapsulations. 48

The major shortcomings of starch hydrolysates are a total lack of emulsifying capacity and low retention of volatiles. Caking of the microcapsule powders during storage is an additional problem associated with the higher-DE products. The lack of emulsification is not any problem if a water-soluble substance is the target encapsulant, or if a secondary emulsifier can be used in processing. As mentioned above, the efficiency to inhibit, e.g., oxidation cannot be clearly answered. Malto- dextrins and sugars have been combined with gelatin in many commercially available bioactive substance preparations, especially for encapsulation of unsaturated lipids that are easily oxidized. These matrix formulations also contain various additives, such as antioxidants, for improvement of the performance of the powders.

Less commonly practiced encapsulation techniques include co-crystallization of flavors within sugars and adsorption of flavors into microporous carbohydrates such

as sugars, of which the potential of the latter has only recently been recognized. 49 Although crystalline sucrose is a poor carrier for flavors, the co-crystallization

process has been claimed to improve the stability. The binding of volatile flavors on highly porous carbohydrates is based on physical adsorption, which means reversible

condensations of flavor gases onto the surface of solid carbohydrates due to weak attractive forces. Especially high porous sugar matrices using special drying tech-

nologies have been developed to function as adsorption carriers.

16.3.6 S TARCH D ERIVATIVES

Starch derivatives that are made more hydrophobic by replacing hydroxyl groups with more lipophilic groups were developed to function alone as microencapsulation matrices for lipophilic flavors. To perform the spray-drying process with enough solids, starch carrier can also be depolymerized. Starch octenyl succinate is one such

derivative and, in fact, the only one allowed for emulsifying foods in Europe. 50 Starch octenyl succinates have excellent emulsifying and flavor retention properties, but

unfortunately, they do not prevent much oxidation. 4,51,52 The protection efficiency against oxidation can be improved by combining glucose or glucose syrups with the

starch derivative. 50 Starch derivatives have been reported to be used as carrier matri- ces alone or in combination with other carrier carbohydrates or proteins in producing many commercial powdered ingredients, such as fish oils, vitamins, and amino acids. Usually, the commercial products are recommended to be stored under dry and dark

Functional Food Carbohydrates

conditions and at temperatures below 15˚C. The shelf-life reported varies from 18 to 24 months. The encapsulant load is in the range of 10 to 40%. After the package

is opened, the product should be used within 1 month. Cyclodextrins are a special group of carbohydrates that are produced enzymat- ically from starches and that are cyclic molecules made of glucose units. 48 These molecules have an inner hydrophobic cavity in which several hydrophobic substances can be solubilized. β-Cyclodextrin is approved to be used in food formulations. Garlic and onion oils can be complexed as odorless components by cyclodextrin, and stabilization of fat-soluble vitamins can also be performed with the aid of

cyclodextrin. 1 Generally, cyclodextrin complexes can protect ingredients from oxi- dation, light-induced reactions, thermal decomposition, and evaporation losses. 48 Crystalline cyclodextrin complexes are stable and greatly improve processing per- formance, handling, and storage of food ingredients. The odorless complexes are claimed to be stable up to temperatures of 200˚C. In mouth conditions, however, the dissociation of the complexes occurs. In contrast to starch hydrolysis products, cyclodextrin powders are nonhygroscopic and very heat stable, but the load is only in the range of 6 to 15%, which is one limitation of using β-cyclodextrin as a carrier. The other limitation is the size of the molecule, which has to fit exactly into the cavity.