Thermal Resistance of Acid Adapted and Non-Adapted Stationary Phase Escherichia coli O157:H7, Salmonella enterica Typhimurium and Listeria monocytogenes in Pomegranate Juice

Abstract

The purpose of this study was to investigate the thermal resistance of acid adapted and non-adapted stationary phase Escherichia coli O157:H7, Salmonella enterica Typhimurium and Listeria monocytogenes in pomegranate juice. In addition, the performance of generic E. coli was evaluated as an indicator. Non-adapted stationary phase cells were grown by incubating inoculated tryptic soy broth without glucose (TSB-NG) at 36±1°C for 18±2 hours. Tryptic soy broth with 1% glucose (10 g/l; TSBG) was used for acid adaptation. All media used for L. monocytogenes was supplemented with 0.6% yeast extract. After washing the cells with peptone, 5 ml of pasteurized pomegranate juice was added onto the pellet to obtain inoculated juice with a initial concentration of 107-1010 log CFU/ml. Inoculated pomegranate juice was sealed into the microcapillary tubes. Microtubes were heat treated in waterbaths at 50, 52 and 54±1°C by immersing at pre-determined time intervals. Survived populations were counted on tryptic soy agar (TSA). S. Typhimurium had the lowest thermal resistance in pomegranate juice. At 50°C, E. coli O157:H7 was the most resistant, whereas L. monocytogenes was more thermally tolerant at 52 and 54°C. Acid adaptation decreased the thermal resistance of E. coli O157:H7, but increased the heat resistance of L. monocytogenes at all tested temperatures significantly. Thermal tolerance of S. Typhimurium increased only at 50°C. The most resistant microorganism was non-adapted generic E. coli at 50 and 52°C; acid-adapted L. monocytogenes had the most thermal tolerance at 54°C. Thermal inactivation of microorganisms in pomegranate juice could be tested at lower temperatures compare to other fruit juices. This may be due to the natural antimicrobial effect and more acidic content of pomegranate juice.