Figure 9. Effect of emulsifier on fat destabilization in the freezer. Source: Ref. 24.

sion, preventing leakage of the aqueous phase (weeping) and reducing spattering when the margarine is used for frying.

Surface-active agents with a high degree of unsaturation are usually preferred for margarine, especially the low-fat/high-aqueous types as they more tenaciously incorporate the high-level water into the oil phase.

Another category of simulated dairy products with a large market is milk replacers for young animals. Such emulsions are usually spray-dried in such a manner as to retain the emulsion potential intact, and the emulsion reforms instantly on rehydration.

The largest consumption of surfactants within the food industry is by the baking industry. Aside from the use of surfactants for cake-batter emulsification previously discussed, surfactants are used in yeast-raised products such as bread and rolls for the purpose of reducing the rate of crumb firming, the principal factor associated with staling, and to strengthen gluten (dough conditioning).

As long ago as 1852, the French chemist Boussingault (27) proved that bread did not stale because of moisture loss simply by hermetically sealing it in cans, where the crumb firmed as rapidly as that of unsealed loaves. It is now generally conceded that bread crumb firms primarily owing to the rétrogradation of the gelatinized wheat starch. The linear amylose fraction (Fig. 10) retrogrades almost immediately after the baked product cools, thereby supplying physical structure to the baked food. It is not further involved in an ongoing firming. The branched amy-lopectin fraction (Fig. 10), however, continues to retrograde slowly over a period of days, causing the crumb to become firmer with aging.

Certain surfactants can retard crumb firming because they are able to form complexes with the glucose polymers of starch. Although these surfactants will form complexes with both the straight-chain amylose and the branched-chain amylopectin, it is now understood that it is most important to complex with the amylopectin since it is responsible for the progressive firming of crumb. It has been shown that the starch fractions will form a helix around the hydrocarbon chain of surfactants possessing the proper steric configuration (Fig. 11). The inside of the helix is lipophilic owing to the CH groups, and the exterior is hy-drophilic owing to the presence of OH groups. Straight hydrocarbon chains such as stearic acid can accommodate the core of the helix with its diameter of 4.5 to 6.0 A. On the other hand, unsaturated fatty acids of the cis configuration cannot enter the helix, as they are not straight.

The complexing ability of a number of surfactants has been evaluated (28); some of these are given in Table 7. A correlation has been established between the ability of a surfactant to complex with amylose and the ability to also complex with amylopectin and retard crumb firming (29,30). The starch components are not as prone to retrograde, owing to interference with hydrogen bonding when the helical clathrates form. For complex formation to take place, the surfactant must possess not only the proper steric configuration but must be dispersible on a molecular basis in the aqueous phase of the dough or batter.

Surfactants such as monoglycerides are used because of their starch-complexing ability in other foods containing

0.1% Glycerol monooleate

0.1% Glycerol monooleate

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