Low-pH wheat protein isolates exhibit unique film-forming properties in low-water systems, where they act like a very relaxed glu-ten. By whipping the isolate and adding more water, their texture approaches that of egg-white foam. “Isolates are being used in cakes, cookies and other sweet goods, providing structure and aeration while maintaining the desired texture and tenderness,” Carson says. “This allows wheat proteins to be used when formulating for reduced sugar and fat, creating better-for-you snacks. Wheat proteins can also be used when replacing or reducing eggs in reformulating for cost savings.”
When used at levels as low as 2% in pizza dough, wheat protein isolates ease sheeting and prevent cracking. A 1:1 blend of wheat protein isolate and water acts as a low-sugar, protein-enriched adhesive in bars and on breakfast cereals. And 2% of wheat protein isolate improves rheology and flavor in whole-grain goods, restoring strength and boosting volume without compromising softness.
Oil and water
Another principle protein function involves emulsion formation and stability. “When a protein stabilized emulsion is formed,” ex-plains Kevin Segall, Ph.D., food scientist, Burcon NutraScience, Vancouver, British Columbia, “the oil phase is dispersed by disruption into small droplets, which become coated with proteins. The proteins orient at the oil-water interface, lowering the interfacial tension with their hydrophobic regions exposed to the oil phase and their hydrophilic regions associated with the water phase.” Thus oriented, the proteins keep the phases separate and the emulsion stable.
Factors affecting emulsifying capacity include protein solubility—the more, the better—and flexibility, which allows the protein to uncoil and adsorb at the oil-water surface more readily. Thus, globular proteins with relatively stable structures, like many whey and soy proteins, don’t make the best emulsifying agents unless manufacturers fiddle with their conformation first. “With soy,” Nguyen says, “what we have is a globular protein. Therefore, as is, it wouldn’t be expected to be effective at emulsification. But, when we apply pro-prietary technology to it in order to liberate the side groups from its globular structure, we now have very good emulsifying soy pro-teins.”
Burrington says whey “emulsification properties can be enhanced by controlled denaturation of the protein.” The pH and ionic strength of the aqueous phase also come into play, with the presence of salts affecting whey protein’s emulsion capacity through their influence on conformation and solubility.
And, notes Paulsen, the fat and phospholipids in WPCs aids emulsification because they make the proteins “very stable and very will-ing to participate at the oil-water interface for interaction.”
The balance of proteins, lipoproteins and phospholipids in egg yolk offers another example of the right functional combination for emulsification. Egg yolk itself is an emulsion, and mayonnaise, Hollandaise, baked goods, and dressings all make use of its emulsifying properties. Egg yolk emulsions are stable to high shear and low pH, and don’t react adversely to ions. Perhaps the most-important func-tional phospholipid in egg yolk is lecithin, but other yolk constituents, including low- and high-density lipoproteins and myelin figures, also aid emulsion stability by sequestering those oil droplets.
The more hydrophobic the protein, the better it concentrates at the oil-water interface to lower surface tension and stabilize emul-sions. Canola proteins, isolated from canola meal occur in two major fractions: high-molecular-weight globulins, and lower-weight al-bumins. One ingredient, comprised principally from isolated canola globulins, emulsifies readily, in part because of “the relatively high surface hydrophobicity of the component proteins,” Segall says, “and also perhaps due to stabilizing steric interactions between droplets, introduced by the dimensions of the absorbed protein.” Although the ingredients are just emerging, he predicts their use in a broad range of emulsified applications, including spoonable and pourable salad dressings, sauces, beverages, and processed meats.
Stable airheads
“When air is introduced into a protein solution,” says Segall, “proteins orient at the air-water interface with their hydrophobic regions exposed to the air phase and their hydrophilic regions associated with the water phase, coating the air bubbles and lowering the interfa-cial tension.”
Voluminous, stable foams—like meringues, angel food cakes, marshmallows, whipped cream, mousses, soufflés and the head on beer—also require high viscosity in the aqueous phase and a layer of adsorbed proteins strong enough to hold air, yet elastic enough to expand without breaking. The protein qualities that help form foams differ from those that help stabilize them. Foam formation calls upon soluble proteins that rapidly migrate and unfold at the air-water interface. This usually means flexible proteins with limited secon-dary or tertiary structure, and a mild heat treatment may actually aid the unfolding. Stabilization, on the other hand, calls for proteins that form viscous films, called lamellae, that surround the gas bubbles in a continuous, elastic membrane: globular proteins of high molecular weight, for example.