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Solubilizers

Table 4. Estimation of HLB by Water Solubility

Application in water HLB range

Not dispersible 1-4

Poor dispersibility 3-8

Milky dispersion after vigorous agitation 6-8

Stable milky dispersion 8-10

Translucent to clear dispersion 10-13

Clear 13 +

low HLB values will be more easily dispersed in the lipid phase. However, there are important other factors that significantly affect surfactant dispersibility in any phase; most surfactants, especially those most effective when used in food processing, are lipophilic and solids at room temperature. As such, they disperse in aqueous media with difficulty. When first introduced as cake emulsifiers, the lipophilic monoglycerides were dispersed in the shortening, which was then added to the batter or cake mix. Monoglycerides and other lipophilic surfactants such as the sorbitan esters, when used in dairy products, were melted into the mix during pasteurization and dispersed to the interface of the fat and water on a molecular basis with the aid of hydrophilic surfactants naturally occurring in the cream phase. When lipophilic or high-melting surfactants such as monoglycerides were required for inclusion in a bread dough at room temperature, it was found necessary to "hydrate" them into a paste emulsion form with water to disperse them in dough or include less effective unsaturated fluid monoesters to form a soft, dispersible plastic.

The first monoglycerides used for the production of hydrates were fully saturated mono/diglycerides. They formed stable emulsions in paste form with about 75% water. Small quantities (ca 0.5%) of hydrophilic coemulsifiers such as sodium stearate or lecithin were added to improve emulsion stability.

When monoglycerides were later distilled to provide a higher monoester content of about 90%, it was found that such products formed gel-like lumps with water, making dispersion in the aqueous phase impossible (4). It was found that distilled monoglycerides and some other surfactants formed mesophases with water and that temperature and water content had to be carefully controlled to obtain adequate dispersion. Some mesophases are dispersible in the aqueous phase, others are more compatible with the lipid phase.

Surface active agents, when dispersed in an aqueous phase, can exist in essentially six mesomorphic or liquid crystalline phases. These phases are anhydrous crystals, fluid isotropic, neat lamellar, gel, dispersion, hexagonal and cubic or viscous isotropic (Fig. 6). When a liquid crystal results from heating the anhydrous crystal, it is known as a thermotropic mesomorph; when resulting from a crystallization from solvent such as water, it is known as a lyotropic mesomorph.

When existing as a pure crystal, the polar heads of the surfactant adjoin one another head to head, and the rigid hydrocarbon tails also adjoin. When heat is applied in the presence of water, the hydrocarbon tails liquify because the weak energy of the van der Waals forces between them is overcome. However, the hydrogen bonding between the polar heads remains, resulting in a semiliquid crystalline structure. The contraction of the disordered hydrocarbon chains opens a gap between the polar heads, allowing water to enter. A lamellar mesophase is formed. When this phase is cooled below the temperature where the hydrocarbon chains resolidify (known as the Krafft Point), a gel phase is formed with the water remaining between the polar groups. The gel is metastable; the water is eventually expelled, and a coagel of fine p crystals dispersed in water is formed.

When some surfactant crystals, such as monoglycer-ides, are heated to relatively high temperature with little water, the water is dispersed in the fluid lipid as micellar aggregates, cylinders at low water content, discs at higher. This phase is known as fluid isotropic.

When some surfactants are heated to higher temperature in the presence of water, they may form hexagonal cylindrical aggregates. Two types may be formed. The hexagonal I phase consists of cylinders with the polar heads on the outside and the hydrocarbon chains oriented inward as a core. Such phases are infinitely dilutable in aqueous media. The hexagonal II cylinder is the reverse of the first, with the polar moiety in the interior, surrounding a core of water, with the hydrocarbon chains oriented to the exterior. This phase is found only at low water content, usually less than 30%. At higher water content, this phase will separate from the aqueous media as a viscous mesophase.

Another lipophilic phase, somewhat associated with the hexagonal II phase, is the liquid crystalline cubic phase, frequently encountered with unsaturated monoglycerides. It is viscous and isotropic. Larsson (6) has described a cubic phase of monoglyceride as a polygon aggregate of water cylinders enclosed in a matrix of hexagonal II configura-

Applications: Dental paste Flavor encapsulation o o <D

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