Freezing of Listeria monocytogenes and Other Microorganisms: A Review

When the temperature of microbes is lowered rapidly, some are injured through thermal shock. Frozen cells can be injured mechanically by intra- and extracellular ice crystals. During freezing, as water is removed, there is a concentration of cell solutes which can lead to dissociation of cellular lipoprotein. Warming of frozen cells can be accompanied by growth of ice crystals which then can physically affect cells. Freeze-thaw injury of microbes is manifested by an increase in fastidiousness and by changes in cellular morphology, release of materials from the micro- and macrostructure of cells, and denaturation of macromolecules. Given the proper environmental conditions, cells can repair such injury. Cryoprotectants minimize damage to cells during freezing and frozen storage. Death and injury of Listeria monocytogenes were greater when cells were frozen and stored at -18°C rather than -198°C. Tryptose broth was more protective of cells than a phosphate buffer solution when freezing and storage were at -18°C; the reverse was true at -198°C. Repeated freezing (-18°C) and thawing (35°C) were more detrimental to cells of L. monocytogenes than were repeated freezing at -198°C and thawing at 35°C. Freezing cells at -198°C and storing them at -18°C caused more injury and death than did freezing and storage at -198°C. Glycerol was an effective cryoprotectant for L. monocytogenes . Less effective were milk fat, lactose, and casein. The extent of injury and death varied among strains of L. monocytogenes given the same treatment. Freezing and thawing increased susceptibility of L. monocytogenes to effects of lipase and lysozyme.