Gamma-ray absorptometer for chemical analysis

Abstract

Raw milk from goats and cows (bovine) was combined with sodium selenite (.5 µCi/10 ml milk) and incubated at 39 C to allow selenium binding to milk components. After incubation intervals between .5 and 24 h, samples were separated into cream, whey, and casein fractions by centrifugation and acid precipitation. When samples were incubated up to 6 h, the aqueous whey fraction contained the highest amounts of radioactivity, the casein substantially less, and the cream contained little senenium-75. Twenty-four hours of incubation at 39 C caused the samples to curdle, resulting in a major shift of radioactivity from the aqueous phase to the acid precipitable (casein) fraction. This shift occurred in all curdled samples irrespective of time or temperature of incubation. Acidifying milk samples with lactic acid in the presence of a bacterial inhibitor also increased selenium binding by the casein fraction. Samples of whey fractions were applied to a Sephadex gel filtration column for further separation. Most of the selenium-75 was recovered in low molecular weight fractions. Radioactive selenium appeared to remain largely in the unbound ionic form when added to noncurdled goats’ or cows’ milk. Most of the selenite added to milk does not combine with proteins at normal pH but increasingly does so as pH falls below 6.0.