Protein reactions during food processing and storage--their relevance to human nutrition.

The main reactions of protein components during food processing and storage were presented and discussed. Since most data are available from the reactions of lysine with other food components these results are mainly demonstrated. Using furosine as indicator the formation and presence of the early Maillard products fructoselysine, lactuloselysine or maltuloselysine in several food systems was measured. Also lysinoalanine is mentioned. A further, recently detected compound, N-epsilon-carboxymethyllysine, appears to be also of biological and technological interest. Several foods contained up to 2,000 ppm carboxymethyllysine in the protein. Experiments with rats and human volunteers have shown that the fructoselysine moiety is poorly digested and absorbed and apparently not metabolized but soon excreted via the kidneys. While being on a normal diet the 20 volunteers excreted 3.3 +/- 1.4 mg fructoselysine per day, which indicates that our food contains always some of these products. Less is known about the absorption and excretion of lysinoalanine or carboxymethyllysine. These reactions cause undoubtedly drastic reductions in the availability of lysine and in this way also in the biological value (and in higher grades of damage additionally of the protein digestibility). Moreover some adverse effects-e.g. interference with the transport of other amino acids, increased renal losses of zinc and copper and cytomegaly in the pars recta of the outer medullary stripe of rat kidneys-were observed in connection with the products of the early or advanced Maillard reaction. These partially negative influences, however, appear not to be of significance under practical conditions. Compounds, possibly carcinogenic, are some degradation products of tryptophan and imidazoquinoline- or imidazoquinoxalin-2-amine derivatives (IQ compounds), which are formed during broiling of meat or fish by the reaction of Maillard products (pyridines or pyrazines and aldehydes) with creatinine.