Food Biochemistry and Food Processing (2 edition)

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10 Protein Cross-linking in Food – Structure, Applications, Implications for Health and Food Safety 213

N

O O

O CHO

O

Protein backbone

HN

N

R

H 2 N N

N

N

O

R

Protein backbone

(A) (B) Bisarg

N N

(C) Crosspy

O N

N

O

OH

OH

HO

OH

(D)

(E)

N

OH

O

O

H 3 C

N

O O

O

CH 3

CH 3

Figure 10.3.Formation of a melanoidin-type colourants on reaction

of (A) protein-bound lysine with furan-2-aldehyde (Hofmann 1998b);

(B) two protein-bound arginine residues with glyoxal in the presence

of carbonyl compound (Hofmann 1998a); (C) two protein-bound

lysine residues with glycolaldehyde or glyoxal (in the presence of

ascorbate). R, remainder of carbonyl compound (Hofmann et al.

1999); (D) product of reaction of propylamine with glucose (Knerr

et al. 2001); and (E) butylaminammonium acetate with glucose

(Lerche et al. 2003).

Encouragingly, when browning was inhibited, radical formation

was completely blocked.

A model system containing propylamine and glucose was

found to yield a yellow cross-link product under food-processing

conditions (Fig. 10.3D; Knerr et al. 2001); however, this prod-

uct is still to be isolated from foodstuffs. In another model study

undertaken by Lerche et al., reacting butylaminammonium ac-

etate (a protein bound lysine mimic) with glucose resulted in the

formation of a yellow product (Fig. 10.3E); this molecule is also

still to be isolated from foodstuffs (Lerche et al. 2003).

The hypothesis that formation of melanoidins involves reac-

tion of protein with carbohydrate is supported by Brands et al.,

who noted that following dialysis of a casein-sugar (glucose

or fructose) system with 12,000 Da cutoff tubing, around 70%

of the brown-coloured products were present in the retentate

(Brands et al. 2002).

MAILLARD-RELATED CROSS-LINKS

Although not strictly classified under the heading of Maillard

chemistry, animal tissues such as collagen and elastin contain

complex heterocyclic cross-links, formed from the apparently

spontaneous reaction of lysine and derivatives with allysine,

an aldehyde formed from the oxidative deamination of lysine

catalysed by the enzyme lysine oxidase (Feeney and Whitaker

1988). A selection of cross-links that form from this reaction

is outlined in Figure 10.4. The extent to which these cross-

links occur in food has not been well-studied, although their

presence in gelatin has been discussed, along with the presence

of pentosidine in these systems (Cole and Roberts 1996, 1997).

Cross-links Formed via Transglutaminase

Catalysis

An enzyme that has received extensive recent attention for

its ability to cross-link proteins is transglutaminase. Trans-

glutaminase catalyses the acyl-transfer reaction between the

N+

N

H 3 C R^2

N+

OH

OH

H 2 N

CO 2 H

CO 2 H

H 2 N

H 2 N CO 2 H

Hydroxypyridinium

(pyridinoline)

N

H

N CO^2 H

NH 2

NH

OH

HO 2 C

H 2 N

HO 2 C

NH 2

Histidinohydroxylysinonorleucine

Figure 10.4.Trifunctional crosslinks reported to result from the

spontaneous reaction of allysine with lysine (Eyre et al. 1984,

Yamauchi et al. 1987, Brady and Robins 2001).