Amino acids, peptides, and proteins are important constituents of food. They supply the required building blocks for protein biosynthesis. In addition, they directly contribute to the flavor of food and are precursors for aroma compounds and colors formed during thermal or enzymatic reactions in production, processing, and storage of food. Other food constituents, e.g. carbohydrate, take part in such reactions. Proteins also contribute significantly to the physical properties of food through their ability to build or stabilize gels, foams, dough, emulsions, and fibrillar structures.
There are a number of ways of classifying amino acids. Since their side chains are the deciding factors for intra- and intermolecular interactions in proteins and, hence, for protein properties, amino acids can be classified as:
- Amino acids with non-polar, uncharged side chains.
- Amino acids with uncharged, polar side chains.
- Amino acids with charged side chains.
Based on their nutritional/physiological roles, amino acids can be differentiated as:
- Essential amino acids
- Nonessential amino acids
The solubility of amino acids in water is high variable. Besides the extremely soluble proline, hydroxyproline, glycine, and alanine are also quite soluble.
Amino acids show the usual reactions of both carboxylic acids and amines. Reactions specificity is due to the presence of both carboxyl and amino groups and, occasionally, of other functional groups.
Free amino acids can contribute to the flavor of protein-rich foods in which hydrolytic processes occur (e.g. meat, fish, or cheese). Taste quality is influenced by the molecular configuration.
Peptides are formed by binding amino acids together through an amide linkage. On the other hand, peptide hydrolysis results in free amino acids. By convention, the amino acid residue with the free amino group is always placed on the left. The amino acids of the chain ends are denoted as N-terminal and C-terminal amino acid residues. The peptide linkage direction in cyclic peptides in indicated by an arrow.
While the taste quality of amino acids does depend on configuration, peptides, except for the sweet dipeptide esters of aspartic acid, are neutral or bitter in taste, with no relationship to configuration. As with amino acids, the taste intensity is influenced by the hydrophobicity of the side chains. The taste intensity does not appear to be dependent on amino acid sequence.
Peptides are widespread in nature. They are often involved in specific biological activities (peptide hormones, peptide toxins, peptide antibiotics).
The structure of a protein is dependent on the amino acid sequence (the primary structure), which determines the molecular conformation (secondary and tertiary structures). Proteins sometimes occur as molecular aggregates, which are arranged in an orderly geometric fashion (quaternary structure).
The term denaturation denotes a reversible or irreversible change of native conformation (tertiary structure) without cleavage of covalent bonds (except for disulfide bridges). Denaturation is possible with any treatment that cleaves hydrogen bridges, ionic or hydrophobic bonds. This can be accomplished by: changing the temperature, adjusting the pH, increasing the interface area, or adding organic solvents, salts, urea, guanidine hydrochloride or detergents.
From a food processing point of view, the aims of modification of proteins are:
1. Blocking the reactions involved n deterioration of food.
2. Improving some physical properties of proteins.
3. Improving the nutritional value (increasing the extent of digestibility, inactivation of toxic or other undesirable constituents, introducing essential ingredients such as some amino acids).
The following protein sources are suitable for the production of texturized products: soya; casein; wheat gluten; oilseed meals such as from cotton-seed, groundnut, sesame, sunflower, safflower, corn protein; yeast; whey; blood plasma; or packing plant offal such as lungs or stomach tissue.