Proteins

Chemical Reactions in Processing

Because proteins contain many amino acids with reactive side-chain groups, it is expected that a variety of reactions may occur during food processing. One of the most extensively studied reactions is alkali degradation. Alkaline treatment is used in food industry for peeling, solubilization, and texturization of food proteins and manufacture of gelatin, sausage casings, and tortillas.

The most well-studied reaction is /^-elimination in which the a-hydrogen of an amino acid residue is abstracted by the hydroxide ion (37) (Fig. 7). In protein-bound cysteine, the resulting product is a dehydroalanine which is an a,fi-unsaturated compound. Nucleophilic side groups, such as the lysyl e-amino and cysteinyl sulfur, react with dehydroalanine via Michael addition, leading to new cross-linkings in the proteins and loss of certain essential amino acids.

Alkaline treatment also causes hydrolysis of the amide groups in asparagine and glutamine, and the guanidino group in arginine. Racemization of amino acids is also detected. Protein-bound amino acids are more susceptible to a-hydrogen abstraction and hence recemization to the d-form (38). Prolonged heating results in isopeptide formation between the e-amino group of lysine and the carbonyl group of aspartic or glutamic acid, or the amide groups of glutamine or asparagine.

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