Food Chemistry

4.2 Monosaccharides 285

(4.94)

(4.95)

4.2.4.4.9 ProteinModifications....................................

The side chains of proteins can undergo post-

translational modification in the course of

thermal processes. The reaction can also result

in the formation of protein cross-links. A known

reaction which mainly proceeds in the absence

of carbohydrates, for example, is the formation

of dehydroalanine from serine, cysteine or serine

phosphate by the elimination of H 2 O, H 2 Sor

phosphate. The dehydroalanine can then lead

to protein cross-links with the nucleophilic side

chains of lysine or cysteine (cf. 1.4.4.11). In the

presence of carbohydrates or their degradation

products, especially the side chains of lysine and

arginine are subject to modification, which is ac-

companied by a reduction in the nutritional value

of the proteins. The structures of important lysine

modifications are summarized in Formula 4.95.

The best known compounds are theAmadori

product Nε-fructoselysine and furosine, which

can be formed from the former compound via

the intermediate 4-deoxyosone (Formula 4.96).

To detect of the extent of heat treatment, e. g., in

the case of heat treated milk products, furosine

is released by acid hydrolysis of the proteins and

quantitatively determined by amino acid analy-

sis. In this process, all the intermediates which

lead to furosine are degraded and an unknown

portion of already existing furosine is destroyed.

Therefore, the hydrolysis must occur under

standardized conditions or preferably by using

enzymes. Examples showing the concentrations

of furosine in food are presented in Table 4.13.

Table 4.13.Concentration of furosine in heated milk

products

Product Furosine (mg/kg protein)

Raw milk 35–55

Milk (pasteurized) 48–75

Milk (ultrahigh heated) 500–1800

Sterile milk 5000–12, 000

Milk powder 1800–12, 000

Baby food (powder) 9300–18, 900

Noodles 400–8500

Bakery products 200–6000