Injury and Death of Frozen Listeria monocytogenes as Affected by Glycerol and Milk Components

¹ Department of Food Science and The Food Research Institute, University of Wisconsin-Madison, Madison 53706

A cell suspension of Listeria monocytogenes strain Scott A in phosphate buffer solution alone or with added glycerol, milk fat, lactose, or casein was frozen and stored at –18°C. At suitable intervals, samples of cell suspensions were thawed at 35°C and plated on suitable media to distinguish between surviving injured and noninjured cells of L. monocytogenes. Glycerol (2 or 4%) protected L. monocytogenes from death and injury during frozen storage for up to 6 mo; however, when 2% glycerol was present, 30 min of frozen storage had to elapse after completion of freezing before protection against death was evident. During short-term (2 wk or less) frozen storage, lactose, milk fat, and casein, each at 2%, provided better protection to L. monocytogenes than did 2% glycerol. During long-term frozen storage, milk components, each at 2%, protected L. monocytogenes against death and injury, but less than that provided by glycerol. Protection by lactose and milk fat against death during frozen storage was observed during 4 wk and against injury during 5 mo and 4 wk of frozen storage, respectively. Protection by casein against death and injury occurred during frozen storage for up to 6 mo. Salts that simulate mil k ultrafiltrate provided almost no protection to L. monocytogenes during freezing and frozen storage. Increasing the concentration of milk fat from 2 to 4% resulted in almost no change in death of L. monocytogenes, but in a decrease in injury only during the first 24 h of frozen storage. A similar change in concentration of lactose resulted in an obvious decrease in death of L. monocytogenes during frozen storage for up to 6 mo and in a decrease in injury only during the first 24 h of frozen storage.

Key Words: Listeria monocytogenes • milk components • frozen storage

Submitted on September 24, 1990
Accepted on December 18, 1990

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