I I

iii i i

¬°OPA b. Lamellar

Figure 6. Phase diagram of glyceryl mono-palmitate depicting molecular packing of cubic, lamellar, and gel mesophases. Source: Ref. 5.

Applications: Cake volume Starch complexing tion, associated as a three-dimensional cubic lattice (Fig. 6). It can absorb up to about 40% water. When dispersed in excess water, surrounded by a lamellar phase, Larsson (7) has referred to the phase as a "cubosome."

When the lamellar mesophase is further diluted with water at the proper temperature, a dispersion phase is formed. The dispersion phase is not merely a dilution of the neat or lamellar phase. The particles are spheres of bimolecular layers alternating with water, and the hollow center can be filled with an aqueous phase. The lamellar bilayer is not stable because the hydrophobic edges resist the aqueous media, and the lamellar bilayer rolls up to form a sphere, with the polar groups external, protecting the internal hydrophobic tails ("liposome"). The physical form of a surfactant drastically affects its functionality. In some cases, the surfactant is completely nonfunctional due to nondispersal on a molecular basis. In other cases, its functional value can be doubled simply by changing the physical form to provide full dispersion.

Although there had been some reference to the use of monodiglycerides in margarine as early as 1921 (4), the first significant commercial use of a surfactant in food was the inclusion of lecithin to improve the viscosity of molten chocolate in the early 1930s (8).

Lecithin was found to lower the viscosity of chocolate, reducing the level of expensive cocoa butter normally required to do so. Lecithin was found to form a monolayer over the hydrophilic constituents of chocolate (sugar, milk, etc) with the lipophilic hydrocarbon tail extended into the molten cocoa fat, facilitating the flow of the vehicle by reducing friction (9).

The incorporation of sorbitan monostearate in chocolate inhibited the formation of bloom (10). Bloom is the result of migration to the surface of chocolate of unstable polymorphs of cocoa butter. As the polymorphs of fat migrate, they leave behind the cocoa fibers, which impart color and ultimately resolidify on the surface of chocolate as light-colored blotches. Sorbitan monostearate forms a monolayer on chocolate nonfat solids, impeding the capillary migration of the unstable cocoa fat polymorph to the surface, thus, inhibiting bloom (11).

About 1933, the so-called superglycerinated shortenings were introduced in the United States for use in the production of cakes (12,13). These contained a significant amount of monoglycerides, for example, 3%. These were included either by production in situ by alcoholisis of the fat with excess glycerine during refining or by direct addition of a monodiglyceride.

Shortenings with added surface-active monoglycerides were found to impart greater structural stability to the cakes, allowing for the inclusion of higher ratios of sugar to flour. Cakes with improved texture, volume, and symmetry, as well as keeping quality, resulted from the use of such shortening.

In 1968 the following definition for cake was proposed (14): Cake is a protein foam stabilized with gelatinized wheat starch and containing fat, emulsifiers, mineral salts, and flour and aerated principally by gases evolved by chemical reaction in situ.

It is well known that oil is an antifoam that tends to destroy any foam structure, including cake, by weakening the protein film. Plastic shortenings are composed principally of an oil fraction, usually 70 to 75%, blended with solid fats for solidity at room temperature. One would expect the shortening to act as an antifoam, and indeed it does, unless properly encapsulated within an emulsifier. The a-tending surfactants such as propylene glycol monostearate and glyceryl lactopalmatate are exceptionally effective emulsifiers, especially when liquid oil is used as the shortening, because of their ability to form a crystalline membrane around the oil globules, preventing the oil from migrating into the protein lamellae (15). Monoglycerides are also very effective encapsulating agents.

The dispersion of the emulsifier in the batter is most important from the standpoint of obtaining functional improvement. Dispersion by inclusion within the shortening is adequate if the shortening is fully dispersed. Complete dispersion of a plastic fat in an aqueous medium may require extensive art and still might not produce optimum results.

Wren (16) has described six possible physical states for the inclusion of surface active lipids (Table 5). Inclusion of the surfactant in shortening or margarine would represent the anhydrous state. However, it is often necessary to include the emulsifier in the aqueous phase. As an example, the traditional sponge cake formula contains no shortening. To obtain the benefits of an emulsifier, it must be added as an aqueous dispersion. Krog (17) compared the effectiveness of six different mesophases of a distilled glyceryl monostearate in the aqueous state (Table 6). The best results were obtained with the dispersion phase, followed closely by the crystalline gel. Poorest results were obtained with the coagel of fi crystals in water. There is no doubt that the dispersion phase was more effective because it was more intimately dispersed in batter. The viscous isotropic cubic phase, poorly dispersible, provided very poor results as is but provided vastly improved results when dispersed in water.

Hydrophilic surfactants (eg, polysorbate 60) are added to cake batters to further reduce surface tension, improving the dispersibility of the lipid phase (eg, shortening) and

Table 5. Possible Physical States of a Surface-Active Lipid

Anhydrous

Polymorphic crystal

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