The percentage of cadmium or lead present in the fat fraction of bovine milk is not affected by heating or freezing. In human skimmed milk, cadmium is mainly associated with a fraction with molecular weight lower than 10,000. Storage at -20°C for 10 days does not have any effect on the distribution of cadmium when milk is incubated with this metal before freezing. This treatment causes only a small increase in the amount of cadmium associated with the low molecular weight fraction when the metal is added after freezing. In bovine milk, 64% of cadmium is associated with a fraction with molecular weight above 70,000. Freezing causes a 37% decrease of the cadmium present in this fraction when the metal is added after thawing. When bovine milk was incubated with cadmium before freezing there was not a marked change in its distribution as when added after thawing. Heating at 63°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 heating in the distribution of lead in human and bovine milk.
Keywords: Milk; cadmium; freezing; heating; lead.