Effect of lemon grass oil on the heat resistance of Escherichia coli O157:H7 and listeria monocytogenes in apple juice

Keywords: E. coli O157:H7, L. monocytogenes, lemongrass oil, apple juice, heat treatment, D value, z value The effect of lemongrass oil (LGO) on the heat resistance of Escherichia coli O157:H7 and Listeria monocytogenes in apple juice was investigated. E. coli O157:H7 and L. monocytogenes were inoculated (7-8 log cfu/ml) into apple juice samples added with LGO (0.1 and 0.2%). Then samples were heat treated at isothermal temperatures (55, 57.5, 60, 62.5°C) and the viable counts were determined after sampling at predetermined time intervals. D and z values were calculated by curve fitting to Log-Linear and Weibull models using GInaFit software. According to the Log-Linear model, the D values of E. coli O157:H7 at 55, 57.5, 60 and 62.5°C were 10.84, 2.06, 1.39 and 0.82 min, respectively, in control samples (0%LGO); 10.78, 1.86, 1.25 and 0.48 min, respectively, in 0.1%LGO added samples; and 9.11, 1.19, 1.11 and 0.48 min, respectively, in %0.2 LGO added samples. While, according to the Weibull model, they were 11.18, 2.00, 1.53 and 0.85 min, respectively, in control samples; 9.90, 1.86, 1.28 and 0.60 min, respectively, in 0.1%LOG added samples; and 8.32, 1.19, 1.06 and 0.54 min, respectively, in 0.2%LOG added samples. The survival curves were well fitted to both models (r2>0.95). The z values of E. coli O157:H7 in apple juice with 0, 0.1 and 0.2%LOG were calculated as 7.09°C (r2=0.89), 5.92°C (r2=0.93) ve 6.45 (r2=0.85), respectively, according to Log-Linear model, while they were calculated 7.19°C (r2=0.87), 6.53°C (r2=0.92) ve 6.89°C (r2=0.84), respectively, by fitting to Weibull model. When the data were fitted to the Log-Linear model, the D values of L. monocytogenes at 55, 57.5, 60 and 62.5°C were 12.72, 1.80, 1.56 and 0.62 min, respectively, in control samples, 11.57, 1.24, 1.14 and 0.61 min, respectively, in 0.1%LOG added samples, and 10.41, 1.18, 0.94 and 0.53 min, respectively, in 0.2%LOG added samples. On the other hand, by fitting to the Weibull model, they were 1.63, 1.87, 1.62 and 0.66, respectively, in the control samples, 10.95, 1.31, 1.17 and 0.58, respectively, in 0.1%LOG added samples, and 7.73, 1.16, 0.90 and 0.49 min, respectively, in 0.2%LOG added samples. The survival curves were better fitted to the Weibull model (r2>0.95) than the Log-Linear model (r2>0.91). The z values of L. monocytogenes in apple juice with 0, 0.1 and 0.2%LOG were calculated as 6.27°C (r2=0.87), 6.46°C (r2=0.80) ve 6.28 (r2=0.82), respectively, according to Log-Linear model, while they were 6.57°C (r2=0.88), 6.43°C (r2=0.83) ve 6.74°C (r2=0.86) according to Weibull model.The number of membrane-damaged cells of both pathogens increased with the increasing temperature and LOG concentration. Based on the finding obtained in this study, it was concluded that LOG reduces the thermal resistances of E. coli O157:H7 and L. monocytogenes in apple juice; therefore, it can be used as a preservative in fruit juices preserved by heat treatment.