Strains and Suspending Menstrua as Factors Affecting Death and Injury of Listeria monocytogenes During Freezing and Frozen Storage

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Abstract
Cell suspensions of Listeria monocytogenes strains V7, California, and Ohio in phosphate buffer solution, tryptose broth, or milk were frozen and stored at -18 degrees C. At appropriate intervals during storage, a sample was thawed at 35 degrees C and surface-plated on suitable media to allow colony formation by noninjured or noninjured plus injured cells. Degrees of death and injury were calculated from the data. Cells of L. monocytogenes were more resistant to death and injury when they were suspended in milk or tryptose broth rather than phosphate buffer solution. There was a significant (two-way ANOVA) difference in resistance to death and injury during frozen storage among strains of L. monocytogenes suspended in tryptose broth. The difference was nonsignificant when the cells were suspended in phosphate buffer solution or milk. Listeria monocytogenes strain Ohio was more resistant to death and injury during frozen storage when cells were suspended in tryptose broth rather than milk. The opposite was true for strains V7 and California. Death and injury of L. monocytogenes strains V7, California, and Ohio suspended in phosphate buffer solution were 98.7, 97.9, and 91.2% and 77.5, 51.6, and 70.2%, respectively, after 4 wk of frozen storage. The values were 67.3, 91.6, and 42.3% and 44.4, 65.6, and 32.6%, respectively, when cells were suspended in tryptose broth, and they were 37.8, 40, and 60.7% and 10.8, 66.8, and 46%, respectively, when cells were suspended in milk.