Proteins are polymers of amino acids linked by peptide bonds and are essential to the structure and function of all living cells. In food systems, proteins contribute not only to nutritional value but also to texture, flavor, and stability through their diverse functional properties.
Amino Acid Composition
Twenty standard amino acids comprise food proteins, of which nine are essential in the human diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The amino acid profile of a protein determines its nutritional quality. Limiting amino acids — those present in the lowest quantity relative to human requirements — are commonly lysine in cereal proteins and methionine in legume proteins.
Protein Classification
Proteins are classified by their shape as globular or fibrous. Globular proteins, such as whey protein and egg albumin, are soluble in aqueous solutions and possess compact, spherical structures. Fibrous proteins, such as collagen and gluten, are elongated and insoluble, providing structural integrity to foods. This classification influences processing behavior: globular proteins tend to form stable foams and emulsions, while fibrous proteins contribute to dough elasticity and meat texture.
Functional Properties
Protein functionality refers to any physicochemical property that affects food processing or quality. Solubility is a prerequisite for many other functional properties and is influenced by pH, ionic strength, and temperature. Emulsification capacity allows proteins to stabilize oil-water interfaces, as seen in mayonnaise and salad dressings. Foaming properties enable the incorporation and stabilization of air in products such as meringues and whipped toppings. Gelation, the formation of a three-dimensional network, is critical in yogurt, cheese, and processed meats.
Protein Quality Scores
The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) evaluates protein quality by comparing the amino acid profile of a test protein to a reference pattern, adjusted for true fecal digestibility. The newer Digestible Indispensable Amino Acid Score (DIAAS) improves upon PDCAAS by using ileal digestibility values and accounting for each indispensable amino acid individually. These scoring systems are essential for regulatory and nutritional labeling purposes. Protein quality is assessed through the protein digestibility-corrected amino acid score (PDCAAS) and the digestible indispensable amino acid score (DIAAS). Accurate determination requires reliable protein analysis methods, while functional properties such as foaming and emulsification are relevant to emulsions and foams. Protein reactions also contribute to enzymatic browning in fruits and vegetables.
