63. The Mode of Combination and Distribution of Traces of Heavy Metals in Dairy Products

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Abstract
Small amounts of heavy metal are distributed between cream and separated milk in proportion to the curd nitrogen, but on completely centrifuging the fat a higher concentration of metal occurs in the cream, showing adsorption of complex proteinate at the fat-globule surface. The heavy metal entering into butter from cream of different acidities is not affected by the acidity but by the curd-nitrogen content of the butter, but the copper associated with the curd in butter is roughly twice that associated with the protein nitrogen of the buttermilk. This is explained by adsorption of heavy metal on the fat-globule surface, which holds alike in water in oil as well as oil in water emulsion. The entry of heavy metal into the fat phase in the absence of protein is greatest at neutrality but decreases with increasing acidity. The presence of protein increases slightly the amount of heavy metal entering into the fat owing to adsorptive and buffering effects of the protein at the oil-water interface in the emulsion. The amount of diffusible heavy metal increases directly with the acidity and runs parallel with the diffusion of hydrogen ions. The concentration of the ionic form of heavy metal in milk is of a low order, but increase in acidity causes more ionic metal to form. Evidence from potentiometric measurements in milk containing known amounts of heavy metal, or from the addition of milk to a standard copper solution at various acidities, confirms this finding.