Effects of combined preservative factors to control Escherichia coli O157:H7 in acidified pickled vegetables

The combined effects of three preservative factors (heat, acetic acid, and salt) used in pickling vegetables on inactivating E. coli O157:H7 were investigated in this study. Each paired combination of heat, acetic acid, and salt showed different effects for inactivating E. coli O157:H7 in laboratory media. The combination of heat and salt was additive. The combination of heat and acetic acid was synergistic and the combination of acetic acid and salt was antagonistic. The same synergistic effect of heat and acetic acid was observed in asparagus with this inhibitory effect also effective during storage. Treatment with high concentrations of acetic acid (i.e. ≥2%) was effective in killing E. coli O157:H7, however it caused significant loss of green color of pickled asparagus. The highly synergistic effect of heat and acetic acid possibly lowers the temperature and amount of acetic acid necessary for pickling vegetables and increases quality as well as maintaining product safety. The antagonistic effect of acetic acid and salt against E. coli O157:H7 was also observed for cucumbers. When storage temperatures were compared, the rate of reduction was more rapid at 22°C than at 5°C. Therefore, the addition of salt at concentration (≥3%) may not be desirable for the preservation of pickled vegetables and room temperature rather than refrigeration may be the recommended temperature for storage of these products to increase safety against E. coli O157:H7. In laboratory media, the antagonistic effect of salt and acetic acid was observed in media with yeast extract and tryptone. The addition of glutamate significantly increased survival of E. coli O157:H7 in phosphate buffer. The addition of salt increases the acid resistance of E. coli O157:H7 because the addition of salt may increase its acid resistance response and consequently, prevent the acidification of cytoplasm by organic acids. Increased cytoplasmic pH in E. coli O157:H7 after the addition of salt was observed.