Proteins are about a lot more than the latest headline-grabbing diet fad or nutritional wonder. Protein may be the most valuable arrow in the product developer’s quiver. But it’s packed with paradox. It can be a handy tool and a finicky reactant, a potential allergen and a vital macronutrient, a formulation’s salvation and its Achilles’ heel.
“Protein is so very functional in so many different ways,” says Starla Paulsen, applications department manager, Glanbia Nutritionals Inc., Monroe, WI, that to turn those functions to your favor, “you really have to understand what you’re working with. You have to have some sort of matrix to build structure in food. And one of the major ways you can get that is with protein.” It’s the gluten network in bread dough. It forms gelatin gels, cheese curds and yogurt. It stabilizes salad dressings and heat-set foams like meringue. And, it’s the very muscle tissue that we eat as meat.
Going against nature
All of protein’s functional properties—solubility, water-binding, viscosity, foaming, gelation, emulsification, film-forming—depend on its physical conformation.
Protein structure starts with 20 commonly occurring amino acids linked in peptide bonds to form a linear sequence. When we alter a protein’s structure from its native conformation, we denature it. Extremes of pH, temperature, shear force, salts or solvents disrupt the protein in ways that can exert powerful effects on its functionality.
For example, “if you denature a protein, it might not have any ability to form a network to hold air or water,” Paulsen explains. That could be disastrous in a formulation, or it could produce precisely the results you desire, as would be the case, she says, “if you want a protein that’s very inert or doesn’t absorb a lot of water—that just builds bulk and not structure.”
Such structural tweaking can enhance water binding, as happens with β-lactoglobulin, a relatively heat-sensitive whey protein frac-tion. When heated, “the bonds creating the tertiary structure of the protein globules are broken, unfolding of the protein molecules oc-curs, and new protein-protein interactions occur,” says Kimberlee J. Burrington, dairy ingredient applications coordinator, Wisconsin Center for Dairy Research, University of Wisconsin-Madison. “The unfolding of the product exposes more water-binding sites, so the ingredient will have enhanced water-binding ability.” This makes it useful in processed meats, baked goods, sauces and dressings.
A functional solution
Perhaps denaturation’s most important effect on protein function involves its ability to turn a previously soluble protein insoluble. Because solubility is a precondition for many protein functions—water-binding, foaming, emulsification and gelation among them—knocking a protein out of solution may knock it out of function.
Proteins lose solubility near their isoelectric point (pI), the pH at which a protein has no net charge. With no positively charged amino acids to repel each other—or negatively charged ones to do the same—the proteins aggregate in dense, bulky clusters that precipitate out of solution and drift to the bottom of the beaker or bottle.
The implication is that, if solubility is a priority, product designers best work with protein whose pI is not the same as that of the ma-trix. This is why manufacturers formulating with gelatin gravitate toward type-A, or acid-pretreated, gelatin as opposed to base-pretreated type-B. The pI of type-A hovers around pH 9; type-B’s is closer to 5. Thus, type-A gelatin “gives manufacturers a lot more flexibility,” says Jeremey Kaufmann, senior sales manager, edible and specialty gelatins, Gelita USA, Sioux City, IA. “All edible prod-ucts are going to be at a pH significantly less than 9, so type-A gives you a wide range where you’re not near that pI. If you’re working with gelatin at a pH near its pI, you get strange interactions going on, like influences on clarity, on viscosity, on gelling power.”
As an added bonus, notes Mindi McKibbin, associate chemist, Gelita USA, type-A gelatins foam better (why they’re ubiquitous in marshmallows) and are less prone to syneresis (why they often stabilize yogurt).
For beverage manufacturers, the main pI challenge has been finding a protein compatible with high-acid formulations. This, Paulsen notes, is an area where whey proteins shine. With pI values ranging from about 4.2 to 4.5 for the α-lactalbumin fraction, and from 5.3 to about 5.5 for β-lactoglobulin, “whey proteins happen to be one of the very unique proteins in that, as you push them down the pH scale, they become more positively charged,” she says. “Then the protein repels itself, so it cannot aggregate and it stays soluble in water. This makes it one of the only proteins that you can use at those really low pHs that you find in things like sports beverages.”
But, a whey protein may have more trouble in a neutral meal-replacement shake, or a yogurt smoothie whose intermediate pH nearly matches whey’s pI. “If you’re trying to work in that area, you deal with a lot of aggregation,” Paulsen says. “The proteins are pretty soluble as long as you don’t add any heat treatment, but once you add heat, because there is no charge on the proteins, all they do is ag-gregate and fall out of solution.”
This can actually be a boon in products where you deliberately seek texture, Paulsen notes. “When you start to get those aggregates, it causes viscosity in the beverage. If you figure out how to harness that correctly with the right additives and processing”—smoothing out the aggregates with homogenization, for example—“you can utilize an ingredient you already have in your formula to give you the vis-cosity and mouthfeel that you want,” she says.
Another dairy protein choice for beverages is casein. According to Jeffrey Kegel, business development manager, Milk Specialties Global, Eden Prarie, MN, what sets casein apart is that, unlike most other proteins, it doesn't exist in a crystalline structure. “It is this that gives them unique properties in various food applications, as they are very resilient and easily manipulated at the same time,” he says. Heat tolerance is one casein plus, with the proteins remaining soluble even at retort temperatures. “This allows them to work great in UHT or RTD beverages.”
Casein's flexible, open structure also lends it to use in viscosity control. For example, while sodium caseinates increase viscosity in products that need some texture, addition of calcium to the solution completely changes the profile. “The changes include the calcium causing aggregation of the casein structure, thus lowering the viscosity,” Kegel says, “as well as making the product a milky white color” because the aggregated protein better reflects light. “Depending on the application,” he says, “a sodium- or calcium-based casein-ate may be preferred.”
Starting to gel
For yogurt, Paulsen says, “you want to induce protein-to-protein bonding, because you want the protein to form that really strong network—that gel—to hold the water.” Gelation is an important precursor to a number of protein functions, from water absorption, thickening and adhesion to emulsifying, foaming and stabilization. Without protein gelation, we wouldn’t have coagulated egg white, gelatin desserts, surimi, textured vegetable proteins, bread dough or yogurt.