Protein: A macromolecule composed of one or more polypeptide chains, each with a characteristic sequence of amino acids linked by peptide bonds.

This basic definition does not do proteins justice. Those in the food industry know that proteins do much more. Not only are they essential and often beneficial for health, they are also integral ingredients in food formulations. From texture enhancement and gelation to water binding and foaming, proteins add function and form to numerous foods. And research continues to expand on the benefits of proteins. This article looks at the functional aspects of soy, egg and whey protein.

Textural Appeal

Texture and mouthfeel are important characteristics of any food product. Isolated soy proteins, soy protein concentrate and soy flours provide unique textural characteristics, while boosting protein. "Textured soy protein concentrates can provide meat-like texture, mouthfeel and visual appearance to meat and meat analog systems," says Russ Egbert, director, protein research, Archer Daniels Midland Co., Decatur, Ill. Textured soy flours do the same. If the appropriate soy protein is selected, it can enhance texture in frankfurters, vegetarian analogs and yogurt.

Textured or structured soy proteins are made from soy flour, soy protein concentrate or isolated soy protein. Textured vegetable protein is manufactured through thermoplastic extrusion of soy flour under moist heat and high pressure, explains Egbert. There are many sizes, shapes, colors and flavors and they often come in bacon-colored and bacon-flavored varieties.

Unique textured protein products can be produced using combinations of soy protein or other powdered protein ingredients (i.e., wheat gluten), in combination with various carbohydrate sources, adds Egbert. Formulators use the products that contain wheat gluten more widely in vegetarian applications to simulate ground meats or meat chunks and strips. The textured products manufactured by thermoplastic extrusion technology are available in dry form, he says. Users can hydrate these products in water or flavored solutions prior to usage in processed meat products and in vegetarian analogs.

Gelation

Peptides, Appetite Control and Anti-inflammatories

Food scientists tend to regard proteins for their nutritional or food texture-providing value. However, interest is growing in the physiological or pharmacological activity of small "units" of protein, the peptides and amino acids produced during digestion or food processing.

An example of a peptide and its physiological activities is glutathione, which is made from three amino acids and is involved in cellular redox balance. Other examples include a thyrotropin-releasing hormone (a tripeptide) that acts as a neurotransmitter and beta-endorphine with opioid activity.

Normally, intact peptides and proteins absorbed from the gut into the bloodstream are removed by the body's immune system. However, diet-derived peptides from enzyme-treated casein and gluten have been shown to possess opioid activity1 and the ability to modulate the immune response.

Additionally, milk-derived peptides--shown in rodent studies to reduce hypertension when ingested--are now available.

Hypertensive peptides are a big story, says Eric Bastian, director of R&D, Glanbia. However, although the animal data is compelling, the human clinical research is still missing.

Commercially available glycomacropeptides (GMPs), a partially glycosylated fragment of the milk protein kappa-casein, are also gaining interest. It's been conclusively shown that if the protein portion of certain GMPs are released into the bloodstream, blood coagulation is inhibited, says Bastian. This would benefit those with thrombosis.

Additionally, some GMPs bound to cell receptors in the intestine illicit an immune response by prompting the release of interleukins. Interleukins are involved in the control of allergies, of autoimmune diseases and also function as anti-inflammatories. GMPs also mediate CCK (cholecystokinin) release into the bloodstream, which is involved in appetite suppression. Theoretically, GMP consumption could help prevent and treat these conditions.

Researcher Michael Kuhar, Ph.D., Yerkes Regional Primate Research Center, Emory University, is going one step further to investigate the ability of peptides (produced in rodents injected with drugs) to control drug addiction, as well as appetite. "Usually, ingested peptides don't enter the brain and act as chemical messengers that regulate appetite. We need medications to do that," says Kuhar.

The integration of foods and pharmaceuticals will continue to be an interesting area to watch.

1 Moreley, J.E., et al. 1983. Gastroenterology 84, 1517-1523.

Unique textures also are the result of a protein's ability to form gels and to affect product viscosity. Gelation involves an unfolding or dissociation of protein molecules--followed by aggregation--resulting in gel formation.

For example, the denaturation and coagulation of egg proteins is responsible for the thermal setting required for products like cake batters, custards, and puddings. When egg white is heated, a thermo-irreversible gel coagulum forms, resulting in desired textures.

Under appropriate heating conditions, whey proteins form irreversible gels by restructuring into extended three-dimensional networks. Gelation entraps water within the capillaries of the gel matrix, providing additional water holding capacity. Water binding is especially important in viscous food products, such as beverages, soups, sausages and custards. A strong gel network helps hold water and prevent moisture loss.

Traditionally, foods containing whey protein ingredients had to be heated above 65°C before the proteins would form gels or thicken solutions, limiting their use in many types of food products. Current research is focusing on creating cold-set gels: whey protein ingredients that are capable of thickening solutions or creating gels at ambient--and even refrigerated--temperatures.

Viscosity

Viscosity development is closely related to gelation properties and protein-protein interactions. Soy proteins can enhance viscosity. Isolated soy proteins and soy protein concentrates are usually the proteins used for viscosity modification, states Egbert. Isolated soy proteins provide a wide range of viscosity profiles. The modification of isolated soy proteins produces very low to very high viscosity products.

"The functional properties of isolated soy proteins can vary dramatically. Functionality is determined, in a large part, on the specific processing parameters for any given isolated soy protein. Heat, homogenization and pH are three factors that greatly influence the functional characteristics of the finished isolated soy proteins," notes Egbert.

Product viscosity is important in a wide range of beverage applications. Enzyme modification creates isolated soy proteins that produce very low viscosity for production of high protein beverages and infant formula, he explains. Viscosity and gelation properties are critical in the manufacture of soy yogurt. In cream soups and high fat sauces, emulsification and viscosity are important to the stability and texture of the finished products. Processed meat applications require isolated soy proteins and soy protein concentrates with good emulsification and gelation properties.

Emulsification

Proteins act at oil/water interfaces to form and stabilize emulsions. Both protein and lipids contribute to the emulsifying properties of whole eggs and yolks. Eggs play a crucial role in emulsifying fat and liquids in all types of baked products.

Controlled denaturation of the protein can enhance emulsification properties. For example, treated whey protein isolates perform as emulsifiers in beverage formulations. Through the use of a simple processing step, beverage manufacturers can take a commercially available whey protein isolate and modify it for their personal specifications, according to information from Dairy Management Inc., Rosemont, Ill. Applications might include infant formulas, sodas, fruit-based drinks and sports and nutritional beverages.

Bio-processing with enzymes enables customized whey protein functions such as gel strength, emulsion stability and solubility. Researchers are exploring this area to benefit areas like edible films, nutritional drinks and lowfat meat products.

Whip Appeal

Foaming is the creation and stabilization of gas bubbles in a liquid. Essential for the formation of protein-based foams is a rapid diffusion of protein to the air-water interface to reduce surface tension, followed by partial unfolding of the protein. This results in the encapsulation of air bubbles and in the association of protein molecules leading to an intermolecular cohesive film with a certain degree of elasticity. Foaming of protein solutions can be desirable in some applications, such as aerated frozen desserts and meringues.

Egg white is an excellent food foaming agent. The unique foaming properties of egg white are the result of the interaction between the various constituent proteins. Furthermore, the thermal coagulation of egg white foams results in products that have stable textural properties. Egg products that contain whipping agents to enhance foam formation are also available.

Enhanced Proteins

High pressure treatment and enzyme modification expand upon the functionalities of proteins. High pressure affects protein conformation and can lead to protein denaturation, aggregation or gelation. High-pressure treatment of proteins may ultimately create new textures and tastes or additional protein ingredients that have a minimal effect on flavor and color.

Potential applications of high-pressure processed egg whites include gelation, with maintenance of natural flavor and nutritional value. Various process parameters, which include pressure, time, temperature, protein concentration, pH and presence of salts, influence the effects. Additional experimental research on protein model systems and real food products is required to understand the potential of high pressure in the restructuring of food proteins.

Hydrolysis has a variety of effects on the gelation properties of proteins. Limited hydrolysis--using various enzymes--can effectively control the gelling ability, as well as the strength, of the gel. An article in Trends in Food Science and Technology by Dinakar Panyam and Arun Kilara, at Pennsylvania State University, described tryptic hydrolysis of whey proteins. Tryptic hydrolysis between 2.3% and 6.7% degree of hydrolysis (DH) prevented gelation at pH 3.0 and pH 7.0, whereas hydrolysis to 2.3% DH with a Bacillus subtilis protease dramatically increased whey protein's gelling ability and gel strength at neutral pH. This suggests that it may be possible to use tryptic digests at a high concentration in acidic beverages without them resulting in gelation. Similarly, the B. subtilis protease digests may have application in neutral pH foods, such as surimi.

Limited proteolysis using pepsin at pH 4.0 improved the characteristics of egg albumin gels. For certain dessert applications, the ability to obtain deliberately weaker gels in a controlled manner may be advantageous.

Partial hydrolysis of soy protein can result in an increase in solubility. Panyam and Kilara noted that soy protein hydrolyzed with 'Alcalase' (bacterial protease from Bacillus licheniformis) results in a product that had excellent solubility over a wide pH range. Whey proteins have very good solubility over a wide pH range, but denaturation tends to decrease their solubility at a low pH--the result of increased aggregation. Hydrolysis of heat-denatured whey protein isolate, especially by trypsin, partially restored the loss of solubility. PF

SIDEBAR - Thaumatin: A Potentially Tasty Protein

Taste-modifying proteins (TMPs) function as natural sweeteners, or flavor enhancers. They can contribute other attributes, such as flavor enhancement, bitterness suppression and umami. TMPs function at extremely low concentrations, are effectively non-cariogenic and are acceptable for diabetic formulations.

Thaumatin shows the most potential of the TMPs. It is a component of the fruit of Thaumatococcus daniellii, which grows in forest areas of Western Africa. Thaumatin is about 3,000 times as sweet as sucrose. It is used in the U.S. in applications such as beverages and chewing gum.

Sweet proteins tend to have slow taste profiles. Sweetness onset may take several seconds to reach maximal level and the sweetness can linger in the mouth for a very long time. When using thaumatin, formulators must carefully consider their processing steps. Heating can cause taste-modifying proteins to denature and the sweetness is lost.

Thaumatin may find more widespread use once certain limitations are overcome. The expense of protein purification and the availability of fruit to process limit mass production. Recent advances in biotechnology will one day increase the availability of TMPs. In addition, molecular biology will allow scientists to manipulate the taste intensity, aftertaste and taste profile of taste-modifying proteins.