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Factors Influencing Mean Lethal Rates of Microbial Cells in Soft-serve Ice Cream during Freezing¹

Department of Dairy Science, University of Georgia, Athens

ABSTRACT

The mean lethal rates and configurations of the lethal rate curves of microbial cells as influenced by different environmental and physical factors during the freezing of soft-serve ice cream were studied. Statistical analyses of the destruction rate data indicated agitation during freezing had a pronounced lethal effect on microbial cells (P < .01). The lethal rate curves were influenced by agitation, mix pH, and type of organism.

Agitation in the absence of ice crystals at –1.7 C, and temperature variation of frozen unagitated mix from –4 to –10 C had no significant effect on the counts of Escherichia coli. The mean lethal rate of E. coli was greater in mixes containing 4.9% protein than in protein-free mixes (P < .01). Fat levels of 0 and 6% did not influence cellular destruction.

Mean lethal rates of microbial cells were related to the consistency of the frozen mixes and free movement of ice crystals. Indirect evidence suggested that death of cells was due to physical disintegration by the abrasive action and collision impacts with rapidly and freely moving extracellular ice crystals.

¹ Journal Paper Number 380 of the College Experiment Station, College of Agriculture Experiment Stations, University of Georgia, Athens, Georgia.