Food Biochemistry and Food Processing

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

biological processes such as blood clotting and
wound healing. -N-(-glutamyl)lysine cross-links
can also be produced by severe heating (Motoki and
Seguro 1998), but are most widely found where a
food is processed from material that contains natu-
rally high levels of the enzyme. The classic example
here is the gelation of fish muscle in the formation of
surumi products, a natural part of traditional food
processing of fish by the Japanese suwari process,
although the precise role of endogenous transgluta-
minase in this process is still under debate (An et al.
1996, Motoki and Seguro 1998) and is an area of
active research (Benjakul et al. 2004a,b). -N-(-
glutamyl)lysine bonds have been found in various
raw foods including meat, fish, and shellfish. Trans-
glutaminase-cross-linked proteins have thus long
been ingested by man (Seguro et al. 1996). The in-
creasing applications of artificially adding this en-
zyme to a wide range of processed foods are dis-
cussed in detail below.

OTHERISOPEPTIDEBONDS

In foods of low carbohydrate content, where Mail-

lard chemistry is inaccessible, severe heat treatment

can result in the formation of isopeptide cross-links

during food processing, via condensation of the -

amino group of lysine with the amide group of an

asparagine or glutamine residue (Singh 1991). This

chemistry has not been widely studied in the context

of food.

MANIPULATING PROTEIN

CROSS-LINKING DURING FOOD

PROCESSING

A major task of modern food technology is to gener-

ate new food structures with characteristics that

please the consumer, using only a limited range of

ingredients. Proteins are one of the main classes of

molecule available to confer textural attributes, and

Figure 9.4.Trifunctional cross-links reported to result from the spontaneous reaction of allysine with lysine (Brady
and Robins 2001, Eyre et al. 1984, Yamauchi et al. 1987).