THE EFFECT OF ACID SHOCK ON THE HEAT-RESISTANCE OF LISTERIA-MONOCYTOGENES

The effect of acid shock on the heat resistance of Listeria monocytogenes was investigated. After growth for 24 h at 30-degrees-C in tryptic soy broth containing 0-6% yeast extract, cell culture suspensions of L. monocytogenes were acidified with HCl or acetic acid over various time periods before being heated in whole milk to a temperature of 58-degrees-C. When cells were acid-shocked immediately with HCl for 1, 2 or 4 h, those acid-shocked for 1 h demonstrated the largest increase in thermotolerance as compared to control cells, when heated at 58-degrees-C in whole milk. In fact, cells acid-shocked for longer than 1 h with HCl demonstrated in some instances a decreased recovery as compared to control cells. Other types of acid-shock treatments included lowering the pH gradually either over a 4 h or a 24 h period. However, regardless of the type of acid-shock treatment, cells acid-shocked with HCl (but not acetic acid) prior to heating had significantly greater heat resistance as compared to control (non-acid-shocked) cells. It appears that acidification with HCl prior to final heating can enhance the heat resistance of L. monocytogenes.