Occurrence of galactosyl isomaltol and galactosyl β-pyranone in commercial drinking milk

Occurrence of galactosyl isomaltol (GAI) and galactosyl β-pyranone (GAP), two advanced glycosylation end products arising from the Maillard reaction of lactose via 1-deoxyosone pathway, was studied in commercial drinking milk. Galactosyl isomaltol was extracted from milk spiked with this isomaltol glycoside avoiding usage of any deproteinizing agent and was determined by a sensitive and interference-free HPLC method. No quantifiable amount of GAI proved to be present in any type of drinking milk, suggesting that some data reported in literature arise from uncontrolled conversion of GAP into GAI. The standard molecule of GAP was produced by heating a model system containing lysine and [U-14C]lactose, purified by solid phase extraction (SPE) on a C18 cartridge eluting with water, separated by the inverse distance function (IDF) standard HPLC method specified for lactulose determination, and characterized by both spectroscopic data and tandem mass spectrometry. The behaviour of formation of GAP and GAI in model systems containing lysine and lactose, heated under conditions of in bottle sterilization of milk was studied in a wide range of values of the molar ratio lysine to lactose. While GAP easily forms as soon as lysine is present in the system GAI does not form below a value of 0.1 of this molar ratio, so explaining why this compound is not present m commercial drinking milk. Amounts of GAP varying from 0.04 to 43.1 μmol/1 were found in the different types of drinking milk ranging from high temperature pasteurized to in bottle sterilized, proving that this compound is a stable and sensible marker for evaluating the extent of the advanced Maillard reaction, hence the heating severity of commercial drinking milk. Moreover, GAP can be determined after conversion into GAI under acid warm conditions with a yield of 0.5 mol GAI from 1 mol GAP. Values of GAP obtained on commercial milk samples either by the direct HPLC method or after conversion into GAI were rather comparable, but the latter method needs further study in view of routine application.