Proteins are made up of a “team of amino acids”. Therefore, the capabilities of the “team” will depend upon the individual capabilities of its players, that is, its amino acids. For example, some amino acids are polar or charged, allowing them to dissolve in water, while others are non-polar, preventing them from dissolving in water. Water-soluble proteins are referred to as hydrophilic while those that are not soluble, are called hydrophobic. The functional properties of proteins are affected by their structure. Examples of functional properties include water absorption and retention, solubility, color, gelation, viscosity and texture, emulsification, foam formation, flavor-binding properties, curdling, and enzymatic browning. I will address each of these in this lesson.
Water Absorption and Retention
Proteins that are made up of mostly hydrophilic amino acids will tend to absorb and retain more water. For example, bakery products containing high-protein ingredients such as soy and other pulses will be more moist and heavier due to greater water retention. This is an important functional property for bakers because more water retention means greater product yield and higher profits.
Proteins that are made up of mostly hydrophilic amino acids will be more soluble. This is particularly important when you are making beverages. For example, soybean protein and pea proteins are found to be highly soluble in water. This makes them ideal for use in beverages and soups.
Proteins react with reducing sugars to form flavors and color compounds in a process called Maillard reaction. The dark colors that you see on the surface of bread, and the grill marks on steak is due to Maillard reaction. For this reason, bread containing milk or soy flour will have a darker color.
Some proteins have the ability to form a gel. A prime example of this type of protein is gelatin. Gelatin is made from collagen which is a rope-like protein polymer from the bones and tissue (skin, tendons, and ligaments) of animals (usually pigs and cows). When gelatin is heated, it dissolves and is dispersed in solution. But, as it cools, the rope-like strands bond together and trap water between them in the process. This results in the formation of a gel. An example of this would be products made using the Jello-O brand of products from Kraft.
Viscosity and Texture
Proteins can make foods not only more viscous (thicker) as we saw in the formation of gels, but also elastic. We call this property, visco-elasticity. The best example of this is seen in gluten proteins. As water is added to wheat and the mixture is molded, dough is formed. We are able to stretch this dough like an elastic which recoils when it is released. This is an important characteristic that gives bread its texture. As the bread dough rises during fermentation, the strong visco-elastic property of the gluten proteins prevents the bread from collapsing. Higher protein content in bread and bakery products will produce a firmer texture. Cakes are generally made using wheat with a low protein content (7-9%) to give a soft texture, whereas bread flours have high protein (14-16%) for firmness.
Emulsifiers are substances that are able to prevent the separation of oil and water in food. They are able to do this because part of their structure allows them to interact with water and another part with oil. Therefore they can grab onto both water and oil to form a bridge between them. Many proteins have this property because they contain both hydrophilic and hydrophobic amino acids. For example, milk in ice cream contributes to an emulsification effect by helping to prevent the separation of fat and water.
A food foam is formed when air bubbles are dispersed in water. Examples of food foams are whipped cream, ice cream, marshmallows, and beaten egg whites. Proteins stabilize foams by forming a protective coat around the air bubbles in the foam, which prevents the bubbles from collapsing. They are able to form this coat because of their hydrophilic/hydrophobic nature. The hydrophilic part of the protein will bind to water and the hydrophobic part will bind with the air, creating a stable bridge.
Proteins are generally odorless compounds on their own, but they can bind flavor compounds and therefore impart new flavor to foods. I have observed this in my study looking at the flavor profile of bean flour fractions. What I have found is that flour fractions with more protein, have a more diverse range of flavor compounds, and these are present at very high concentrations. The ability of proteins to bind favors can have a negative impact on the flavor of end-products if off-flavors are trapped. On the positive side, manufacturers can use this property to trap and retain certain flavor ingredients in food.
Proteins can coagulate with the addition of acids. For example, in my research, I am able to separate bean protein from other components of bean flour by adding acid to the flour slurry At a pH of 4.6, proteins in the slurry curdle and fall out of solution. The point at which proteins precipitate or fall out of solution, is called the isoelectric point. It is a point where the charge on the protein changes to neutral. At a neutral charge, it is no longer capable of dissolving in water. The same principle is followed in the production of yogurt and cheese. For example, lactic acid bacteria produce lactic acid, in cheese making resulting in curdling of milk and the production of milk curds which can then be fermented into cheese.
I talked about Maillard browning earlier. Maillard reaction is called non-enzymatic browning since it is not dependent on the work of enzyme. Remember that with Maillard browning you need only proteins and reducing sugars. Another type of browning is called enzymatic browning. That is when browning is caused by enzymes. Enzymes are proteins that speed up the rate of chemical reactions in living systems. One type of reaction that they speed up is the browning reaction. This reaction is caused by the action of the enzyme polyphenol oxidase on phenol compounds in foods, in the presence of oxygen. The result is a brown compound called melanin. This reaction is evidenced in apples and potatoes after they are cut and left exposed to oxygen.
This concludes a general overview of the most important functional proteins of proteins. You will be able to use these basic principles to solve food science problems and create new food product ideas.
Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.