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Cause of Prefreezing in the Freezing Point Determination of Milk

Department of Dairy Science, University of Alberta, Edmonton, Canada

ABSTRACT

During an investigation on the freezing point of milk in Alberta, some samples of milk were found to show prefreezing, which makes the accurate determinations of the freezing point impossible. A similar difficulty has recently been described by Sargent et al. (1), who attributed it to the action of bacteria. In the present investigation, a Fiske Model I cryoscope was used. The freezing-point determination involves supercooling the milk and subsequent freezing by the violent stirring action of a metal rod. The temperature then rises to the freezing point and remains constant. The abnormal samples could not be supercooled and the milk froze before any supercooling was achieved. As the initiation of freezing requires the formation of centers of crystallization, it was surmised that the abnormal samples were contaminated by solid particles that would act as crystallization centers. The prefreezing phenomenon occurred only in the milk from a small number of producers, occasionally in the milk from a tank truck, but never in retail milk sent out from the local dairy plants.