Development and application of a multiple reaction monitoring mass spectrometry method for absolute quantification of lysinoalanine and lanthionine in dairy products

Abstract When milk is heat-treated during the production of long shelf-life products glycation and the Maillard reaction cascade can result, but also side reactions where serine, phosphoserine, glycoserine, cysteine or cystine can be converted into dehydroalanine, which can further react with lysine, histidine or cysteine to yield lysinoalanine (LAL), histidinoalanine or lanthionine (LAN) cross-links. The nutritional value, digestibility and functionality of the proteins may be affected by the introduction of covalent cross-links, modified amino acid residues and eventually aggregation. In this study, a liquid chromatography triple quadrupole based method was developed to directly quantify LAL and LAN using multiple reaction monitoring. The method was validated and applied to investigate the content of these cross-links in different dairy products. The level of LAL and LAN was below quantification in raw milk and pasteurised milk, but increasing amounts were measured in ultra-high temperature milk and micellar casein isolate.