Bactericidal Effect of Freezing With Chemical Agents

When cells of Salmonella typhimurium or other Gram‐negative bacteria were suspended in nutrient broth and frozen with certain chemical agents in concentrations to which the unfrozen cells are not sensitive, their viability significantly decreased. Of the various chemical agents tested, laurate and the derivatives [e.g. sodium laurate, sodium lauryl sulfate, monolaurin, polyoxyethylene sorbitan monolaurate (Tween 20), or Sorbitan monolaurate (Span 20)] showed the highest activities in reducing viability. These chemicals were not effective when they were added after thawing. Slow freezing (3°C/min in the cooling rate) and storage at ‐20°C for 24 hr with 0.2 mM laurate reduced the population of bacteria in nutrient broth to less than five orders. Fast freezing in liquid nitrogen (100 °/min in the cooling rate), slow freezing in the presence of 10% dimethyl sulfoxide or 10 mM Mg ++ failed to reduce the viability effectively. Freezing with laurate was also found to be lethal against Gram‐positive bacteria.