Food Biochemistry and Food Processing

224 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking

PROTEIN CROSS-LINKS IN FOOD

Protein cross-linking refers to the formation of cova-
lent bonds between polypeptide chains within a pro-
tein (intramolecular cross-links) or between proteins
(intermolecular cross-links) (Feeney and Whitaker
1988). In biology, cross-links are vital for maintain-
ing the correct conformation of certain proteins, and
may control the degree of flexibility of the polypep-
tide chains. As biological tissues age, further protein
cross-links may form that often have deleterious
consequences throughout the body, and play an im-
portant role in the many conditions of ageing (Zarina
et al. 2000). Similar chemistry to that which occurs
during ageing may take place if biological tissues are

removed from their natural environment—for exam-

ple, when harvested as food for processing.

Food processing often involves high tempera-

tures, extremes in pH, particularly alkaline, and ex-

posure to oxidizing conditions and uncontrolled en-

zyme chemistry. Such conditions can result in the

introduction of protein cross-links, producing sub-

stantial changes in the structure of proteins, and

therefore the functional (Singh 1991) and nutritional

(Friedman 1999a,b,c) properties of the final product.

A summary of protein cross-linking in foods is giv-

en in Figure 9.1, in which the information is organ-

ized according to the amino acids that react to form

the cross-link. Not all amino acids participate in

protein cross-linking, no matter how extreme the

Figure 9.1.A summary of the cross-linking reactions that can occur during food processing, from Gerrard (2002).
Further details are given in the text.