CHANGES IN THE DISTRIBUTION OF CADMIUM AND LEAD IN HUMAN AND BOVINE-MILK INDUCED BY HEATING OR FREEZING

The percentage of cadmium or lead present in the fat fraction of bovin e milk is not affected by heating or freezing. In human skimmed milk, cadmium is mainly associated with a fraction with molecular weight low er than 10,000. Storage at -20 degrees C for 10 days does not have any effect on the distribution of cadmium when milk is incubated with thi s metal before freezing. This treatment causes only a small increase i n the amount of cadmium associated with the low molecular weight fract ion when the metal is added after freezing. In bovine milk, 64% of cad mium is associated with a fraction with molecular weight above 70,000. Freezing causes a 37% decrease of the cadmium present in this fractio n when the metal is added after thawing. When bovine milk was incubate d with cadmium before freezing there was not a marked change in its di stribution as when added after thawing. Heating at 63 degrees C for 30 min caused a slight decrease in the amount of cadmium present in the casein fraction. The distribution change of cadmium after freezing or heating is probably due to the formation of complexes between the whey proteins and the metal, or to the disaggregation of the cadmium bound to casein micelles. Lead is mainly associated with caseins in bovine and human milk. No significant changes were caused by freezing or heat ing in the distribution of lead in human and bovine milk.