Study on the Effect of Heavy Metal Contamination of Milk on the Coagulation Process

This study investigated how Pb, Cd, and Cu are distributed between curd and whey during milk coagulation in milk from different animal species, and how the level of metal addition and the coagulation method influence metal retention. Raw milk from buffalo, cow, donkey, goat, and sheep was supplemented with Pb, Cd, and Cu under controlled laboratory conditions at two levels corresponding to the regulatory maximum level (ML) and ten times this level (10 × ML). All three metals were added simultaneously to the same milk aliquot, and coagulation was induced either enzymatically or by acidification at pH 4.6. Metal concentrations in curd and whey were determined by atomic absorption spectrophotometry. In all milk types, Pb, Cd, and Cu were retained mainly in the curd fraction. At ML, curd retention generally ranged from about 77% to 97%, whereas at 10 × ML, retention decreased and transfer to whey increased. Donkey milk consistently showed lower metal retention in curd than ruminant milk. Statistical analysis of curd retention showed that metal type, milk species, the level of metal addition, and their interactions significantly influenced metal retention, indicating that the effect of coagulation method depended on the experimental conditions rather than being uniform across all cases. Overall, the results show that milk coagulation favours the association of Pb, Cd, and Cu with the curd fraction, highlighting the importance of the milk protein phase in determining metal distribution during dairy processing. These findings improve our understanding of heavy-metal behaviour during milk processing and help clarify their potential transfer into curd-based dairy products.