Fate of Listeria monocytogenes During Freezing and Frozen Storage

A cell suspension of Listeria monocytogenes was frozen 30 min at −18°C, or 10 min in liquid nitrogen (LN) at −198°C. Solidification required 15 min at −18°C and ca. 1 min at −198°C. Freezing and storage 1 month in phosphate buffer (PB) at −18°C caused 87% death and 79% injury. These were 54 and 45%, respectively, for cells in Tryptose Broth (TB) at −18°C. Freezing and storage 1 month in LN caused no death or injury of cells suspended in PB, whereas some injury and death occurred in TB. Freezing at −198°C followed by storage 1 month at −18°C resulted in 60% death and 36% injury in PB, and 61 and 44.2%, in TB. Repeated freezing and thawing caused more death/injury than did a single freeze‐thaw cycle.