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A Solubility Method for the Determination of Alpha and Beta Lactose in Dry Products of Milk

The American Dry Milk Institute, Inc., Chicago, Illinois

ABSTRACT

A method has been developed for the determination of the two forms of lactose in dry products of milk based upon the maximum rate of solution of lactose and the difference in solubility of the alpha and beta modifications. The essential steps consist of (a) adding an excess of alpha lactose hydrate to a known quantity of the sample, (b) determining the solubility at several time intervals and (c) extrapolating to zero time to obtain the total initial solubility. The beta lactose content may be ascertained easily, since below 55 m.mols. per 100 ml. of water the total initial solubility is the sum of the initial solubility of the alpha hydrate and of the quantity of beta lactose present. With the beta lactose known, the alpha modification is calculated by difference from the total lactose.

For a sample of spray process nonfat dry milk solids, it has been found that size of the sample, within the range of 10 to 50 g., seems to have no effect on the results obtained, but with increasing sample size the suspension becomes progressively more viscous and difficult to handle.

The method has been applied to nonfat dry milk solids and dry whey solids. Results for the nonfat dry milk solids are in agreement with those reported in the literature. For the samples of dry whey solids in which the alpha lactose is present as the chief form, water of crystallization has been calculated, assuming complete crystallization of all alpha lactose present. Results are in satisfactory agreement with those determined by the moisture desorption method and the indirect method previously published (2) for the estimation of water of crystallization of alpha lactose.