INFLUENCE OF FAT-CONTENT ON SOME HEAT-INDUCED CHANGES IN MILK AND CREAM

Several heat-induced changes of milk components are reported as suitable indicators of severity of the heat treatments. Conflicting literature exists about the effect of fat on thermal isomerization of lactose to lactulose, a parameter proposed for distinguishing the different types of heat-treated drinking milk. Bulk milk ( > 20 t) with different contents of fat was submitted to pasteurization and both direct and indirect UHT processing under industrial conditions in 6 plants of 7 Italian manufacturers. UHT cream (fat content 230-310 g/kg) and corresponding whole milks were studied as well. The processed samples were analysed for lactulose content, furosine level (evaluating the extent of early-stage Maillard reaction), and whey protein denaturation. All data indicated that fat protected the tested milk components against heat-induced changes. Lactulose formation was 26 % less in direct UHT whole milk than in skimmed milk and only 4% less with indirect UHT processing. The high correlation (r = 0,896, n = 10) between the relative decrease in level of furosine and content of lactulose in the presence of fat showed that both these two heat-induced modifications measured the same phenomenon. Denaturation of whey protein evaluated on both alpha-lactalbumin and beta-lactoglobulin confirms this behaviour. Thus the heat load on milk seems to depend on the fat content of that milk. Fat content would affect viscosity of milk and so hinder transfer of heat.