Functions of Proteins in Food
The proteins in foods allow certain reactions to occur during preparation: hydration, denaturation/coagulation, enzymatic reactions, buffering, and browning.
The ability of proteins to dissolve in and attract water, a process called hydration, allows them to play several important roles in foods. One of these is the capability to form a gel, an intricate network of protein strands trapping water that results in a firm structure. Proteins from milk, meat, egg, and soy have all been used in a variety of gels. The gelling ability of proteins allows them to be used as binders, stabilizers, and thickeners in a variety of foods such as preserves, confectioneries (gums, marshmallows), and desserts (ice cream, puddings, custards, pie fillings, mousses, and plain-flavored gelatins).
Another example of protein’s hydrating ability in food preparation is in bread-making. Water or milk is combined with yeast and the two major proteins of wheat—gliadin and glutenin—through the process of kneading to yield the protein gluten, whose elastic qualities allow it to stretch with the carbon dioxide gas produced by the yeast during fermentation. This is how bread rises, and without protein’s ability to hydrate, rising would not take place.
Large protein molecules are sensitive to their surroundings. When subjected to heat, pH extremes, alcohol, and physical or chemical disturbances, proteins undergo denaturation. Denaturation can result in coagulation, which is described as a curdling or congealing of the proteins. Both denaturation and coagulation are irreversible in most proteins. Examples include the hardening of egg whites with heating, the formation of yogurt as bacteria convert lactose to lactic acid and lower the pH, and the stiffening of egg whites when they are whipped (4). Adding compounds like sugar to an unbeaten egg white stabilizes the denatured protein; therefore, sugar is often added near the end of whipping, just before the egg whites have reached their optimum consistency. Salt speeds the coagulation of proteins by weakening the bonds of protein structure, which is why it is frequently used by cheese makers to help produce a firm curd.
The chemical bond between two amino acids.
The irreversible process in which the structure of a protein is disrupted, resulting in partial or complete loss of function.
The clotting or precipitation of protein in a liquid into a semisolid compound.
Most enzymes are grouped into one of six different classes according to the type of reaction they catalyze. Hydrotases catalyze hydrolysis reactions. Examples include lipases, which hydrolyze lipids; proteases, which hydrolyze protein; and amylases, which hydrolyze starch. Hydrolases are the most common enzymes used by the food industry. These hydrolytic enzymes catalyze hydrolysis by breaking, or cleaving, a chemical bond within a molecule by adding a molecule of water. The two hydrogens and the oxygen of the water molecule become part of the two new molecules formed. Another type of enzyme, oxidoreductase, catalyzes oxidation-reduction reactions such as dehyrogenases, which act by removing hydrogen, and oxidases, which add oxygen. Lyases assist in breaking away a smaller molecule, such as water, from a larger substrate. Transferases, as their name implies, transfer a group from one substrate to another; tigases catalyze the bonding of two molecules; and isomerases transfer groups within molecules to yield isomeric forms.