The relationship between lactulose and furosine indices as a quality parameter of sterilized milk

Due to severe heating conditions of milk sterilization processeses, the lactulose and furosine indices, describing the extent of lactose isomerization and early Maillard reaction (MR), respectively, are adopted in the heat-load evaluation. Because of the different kinetics and activation-energy values of the two reactions, the combination of furosine and lactulose values in sterilized milk could provide additional information about the thermal hystory of the end-product and raw milk quality. Furosine and lactulose were determined by HPLC in UHT milk and in-bottle sterilized milk processed under defined conditions or collected at the market. The effect of raw milk pH, addition of reconstituted milk powder, processing conditions, and prolonged storage were investigated. A high correlation between lactulose and furosine levels was found for all the milk samples of known origin and freshly prepared under GMP. When lactulose (y) was expressed as mg L-’ and furosine (x) as mg IOOg protein-‘, the function of the correlation line is y = 3.02x -41.51, with r = 0.99 (n=40). High values of milk pH enhanced lactose isomerization but did not affect MR. On the contrary, milk drying only promoted intensive MR. This behaviour was also found after prolonged preheating or long storage of sterilized milk. In all the cases shifting of the lactulose/furosine ratio from the established function was observed, pointing out bad qaulity of raw milk or abnormal conditions of processing. Lactulose/furosine ratio looks promising to assess the quality of commercial sterilized milk where the possibiliity to recognize reconstituted milk powder is pointed out for the first time.