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Cause of Microbial Death during Freezing in a Soft-Serve Ice Cream Freezer¹

Department of Dairy Science, The University of Georgia, Athens

ABSTRACT

Death of microbial suspensions during freezing with agitation was studied by following the optical density changes of yeast suspensions in frozen-thawed sucrose solutions; and by cytological examination of frozen-thawed yeast suspensions using a light microscope, and frozen-thawed suspensions of Escherichia coli with an electron microscope.

Changes in the optical density values of frozen-thawed yeast suspensions with duration of freezing, and in the supernatant liquid of these suspensions after centrifugation, suggested changes in the size distribution of the suspended cells and the presence of cytoplasmic materials in the supernatant layer.

Photomicrographs established that disintegration of yeast cells occurred during freezing with agitation. Electron micrographs revealed the disintegration of cells of E. coli.

¹ Journal Paper no. 481, College Experiment Station, University of Georgia, Athens, Georgia.

² Present address—Department of Dairy Science, University College, Cork, Ireland.

³ Deceased.