Acid and Heat Tolerance of Acid-stressed Listeria monocytogenes Inoculated in Broth and Shrimp Model

The acid and heat tolerance of Listeria monocyto genes mixed strains were examined after induced to be an acid-stressed cells by culturing in tryptic soy broth supplement with yeast extract pH 5.5 for 1 h. The acid-stressed cells showed better tolerance to acid than that non acid-stressed cells (p<0.05) when tested in medium acidified to pH 3.5. For heat tolerance in broth model, both acid- and non acid-stressed cells were centrifuged, suspended in medium at pH 7.2, and heated in shaking water bath at 55-63 degrees C for 0-180 min. D-value of acid- and non acid-stressed cells at 55, 57, 59, 61 and 63 C were 45.25, 17.83, 7.47, 3.80 and 2.40 min and 34.36, 11.96, 7.37, 3.74 and 2.42 min, respectively. In addition, Z-value of acid- and non acid-stressed cells were different at 6.21 and 7.08 degrees C, respectively. From these results, acid-stressed cells showed cross protection effect higher than that non acid-stressed cells. However, heat tolerance of acid-stressed cells inoculated on Pacific white shrimp flesh using sous vide technique (packed in vacuum pouch, and heated at 60, 62.5 and 65 degrees C for 10 min) did not show a significant difference when compared with non acid-stressed cells (p>0.05). While blanching inoculated shrimp flesh at 70 degrees C for 2 min, non acid-stressed cells survived more than acid-stressed cells. The results revealed that acid-stressed cells showed acid and heat tolerance better than non acid-stressed cells especially in medium, not in shrimp.