Epimerization of lactose to free lactulose in heated model milk solutions

Abstract

The kinetics of the epimerization of lactose to lactulose were determined for two ' model milk' solutions, one containing lactose and the other lactose and lysine. The lysine was added to lactose to determine whether amino groups of proteins act as base catalysts promoting the production of free lactulose. The presence of lysine actually reduced the rate of production of free lactulose, presumably by the formation of amino-sugar complexes. The rate of production of free lactulose over the range of processing conditions was shown to be first order for both systems. Thermal effects on lactose were shown to be complex. The epimerization of lactose to lactulose was reversible and both sugars were thermally decomposed to their monosaccharides. The difference in the rate of production of free lactulose between the lactose, and the lactose and lysine solutions was also observed in processed milk and its ultrafiltrate (protein free).

Effect of the protein, citrate and phosphate content of milk on formation of lactulose during heat treatment

“…The lactulose content of the ultrafiltrate was found to be much lower than that of milk diluted with ultrafiltrate, suggesting that the milk protein may have had a catalytic effect on its formation. This is in contrast to the results of Greig & Payne (1985), who found that lactulose was formed more rapidly in ultrafiltrate than in milk. However, increasing the protein content of the milk reduced the amount of lactulose recovered so that the sample containing 1-68 % total N contained approximately the same amount of lactulose as the ultrafiltrate.…”

Formation and occurrence of lactulose in heated milk

“…They removed the sugars from processed milk by dialysis and then converted them to their trimethylsilyl (TMS) derivatives with TMS imidazole, which were then separated and quantified by HPLC. Greig & Payne (1985) also determined lactulose by HPLC, but without a derivatizing step. They detected less lactulose in a UHT milk than was measured in a sample of the same milk by the enzymic method of Andrews (1984).…”

“…Andrews (19856) also showed that the UHT presterilization and in-bottle processes for sterilized milk were additive with respect to their production of lactulose in the milk. Greig & Payne (1985) found that when a 4 % (w/v) lactose solution was heated for up to a few hours at 125 °C, the lactulose concentration reached a maximum of 1200 mg/100 ml. They also showed that when a lactulose solution was heated, some was isomerized to lactose.…”

Advanced Dairy Chemistry Volume 3

“…The kinetics of epimerization of lactose to lactulose were studied in model milk solutions (Greig and Payne, 1985). Two milk solutions were studied -one containing lactose and the other containing lactose and lysine.…”

“…There is evidence from studies on milk ultrafiltrates that protein may inhibit the formation of lactulose (Greig and Payne, 1985). Similarly, the presence of amino groups in the form of N-acetyl-L-lysine led to a decrease in lactulose formation (Olano et al, 1989).…”