Quantity and carbohydrate content of glycomacropeptide fractions isolated from raw and heat-treated milk

To determine the effect of heat treatment of milk on the release and carbohydrate content of glycomacropeptide (GMP), raw milk was subjected to five heat treatments of varying severity, cooled, incubated with chymosin and separated into whey and curd fractions. GMP was isolated from each whey fraction using an ion-exchange method and carbohydrates were released from each GMP sample using an optimised acid hydrolysis procedure. The carbohydrates released were derivatised with 1-phenyl-3-methyl-5-pyrazolone, separated and quantified by capillary electrophoresis. The severity of heat treatment significantly influenced the amount of GMP released and the residual glycosylation of GMP. Removal of small ionic species using dialysis was able to fractionate GMP into water-soluble and insoluble forms, which contained high and low quantities of carbohydrates, respectively. These results indicate that the amount of GMP released and its level of glycosylation can be influenced by the severity of milk heat treatment, and possibly the technique employed to isolate the GMP.