Food Proteins: Interactions and Functionality

Abstract

Proteins are the most abundant macromolecules found in living cells and approximate half of the cell's dry weight. They are required in the food of humans, fish, and most higher animals. Historically, food proteins have been selected for their nutritional value and can be obtained from a wide variety of naturally occurring sources. Proteins undergo a wide range of structural and conformational changes through a variety of complex interactions during processing and storage. Such changes can affect the principal purpose of dietary proteins, which is to supply nitrogen and amino acids for the synthesis of proteins in the body. It is through an understanding of these interactions and their effects on functionality that food proteins have played a major role in the food supply. The macrostructure of a protein is primarily determined by its amino acid sequence. Amino acids are essential in basic nutrition, growth, and maintenance. Nine of the 20 identified amino acids, namely, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine, are considered essential. They are so classified because they cannot be synthesized by humans and must therefore be supplied in the diet. When essential amino acids are not supplied in sufficient quantity, they restrict the adequate utilization of other amino acids and are thus termed limiting amino acids. The processing of food proteins is designed to reduce microbial and enzymatic spoilage, inactivate antinutritional substances, improve the availability of perishable foods, and enhance the sensory quality of the food. Chemical changes that occur during food processing can have a significant impact on the nutritional value, the sensory properties, as well as the general functionality of the food. Heat treatment of proteins under acidic conditions can result in denaturation or even total unfolding of proteins and inactivation of enzymes and antinutritional substances. More severe heating can cause proteins and/or enzymes to cross-link with one another. Process-induced chemical interaction can also occur between proteins and carbohydrates, which leads to nonoxidative, nonenzymatic browning (the Maillard reaction). Lipids, especially when unsaturated, are susceptible to oxidation. They can form lipid peroxides, which interact with proteins to yield lipid–protein complexes, thus decreasing the nutritional value of the food. In addition to the above-mentioned purposes of food protein processing, proteins play an important role in improving the functionality of food ingredients. For instance, heat denaturation of whey proteins before spray-drying improves their water-holding properties. Alkali treatment of soy proteins is used to improve their solubility and textural properties, and to obtain desirable rheological properties. Viscosity and solubility measurements are commonly used to obtain information about the functional behavior and physicochemical nature of proteins.