Furosine formation

The concentration of furosine in some enteral formulae has been determined, ranging from 245 to 441 mg per 100 g protein.40 It seems to decrease on further processing and does not appear to be a useful indicator of quality. Furosine content seems to be stable in enteral formulae on storage for >36 weeks at 4 °C, 24 weeks at 20 °C, and >12 weeks at 30 °C.41 Stability at 55 °C was down to 3-8 weeks, depending on protein content, whereafter furosine content declined. Reactivity of lysine residues with ophthaldialdehyde declined roughly in parallel with furosine formation. 

Figure 9.8 Initial steps of the Maillard reaction with the formation of furosine (after hydrolysis with 7.8 M HCi) as well as of. Y-s-carboxymethyl lysine and erythronic acid (from Erbersdobler...

Soya products contain much less reducing carbohydrate and also lower bound lysine than those from milk moreover, only small amounts of free lysine and glucose were added in the production of soya-crumb. The smaller furosine values from soya products can be explained by these facts. The effect of intensive heat treatment on the formation of furosine is demonstrated by roasted coffee. 

R. Krause, K. Knoll, and T. Henle, Studies on the formation of furosine and pyridosine during acid hydrolysis of different Amadori products of lysine, Eur. Food Res. Technol, 216 (2003) 277-283. 

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 H2O, H2S or 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 -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. 

Figure 4,102 Formation of furosine and pyridosine from lactose and milk proteins.

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