2009_Book_FoodChemistry

4.2.4.4.9 Protein Modifications
4.2 Monosaccharides 285
(4.94)
The side chains of proteins can undergo posttranslational
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 accompanied
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 the Amadori
product N ε -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 analysis.
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)
(4.95)
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