Free Maillard Reaction Products in Milk Reflect Nutritional Intake of Glycated Proteins and Can Be Used to Distinguish Organic and Conventionally Produced Milk

Using LC-MS/MS and isotopically labeled standard substances, quantitation of free Maillard reaction products (MRPs), namely, N-epsilon-(carboxymethyl)lysine (CML), 5-(hydroxymethyl)-1H-pyrrole-2-carbaldehyde (pyrraline, PYR), N-delta-(5-hydro-5-methyl-4-imidazolon-2-yl)-ornithine (MG-H), and N-epsilon-fructosyllysine (FL), in bovine milk was achieved. Considerable variations in the amounts of the individual MRPs were found, most likely as a consequence of the nutritional uptake of glycated proteins. When comparing commercial milk samples labeled as originating from organic or conventional farming, respectively, significant differences in the content of free PYR (organic milk, 20-300 pmol/mL; conventional milk, 400-1000 pmol/mL) were observed. An analysis of feed samples indicated that rapeseed and sugar beet are the main sources for MRPs in conventional farming. Furthermore, milk of different dairy animals (cow, buffalo, donkey, goat, ewe, mare, camel) as well as for the first time human milk was analyzed for free MRPs. The distribution of their concentrations, with FL and PYR as the most abundant in human milk and with a high individual variability, also points to a nutritional influence. As the components of concentrated feed do not belong to the natural food sources of ruminants and equidae, free MRPs in milk might serve as indicators for an adequate animal feeding in near-natural farming and can be suitable parameters to distinguish between an organic and conventional production method of milk.