Inactivation of Listeria monocytogenes ATCC 7644 on tomatoes using sodium dodecyl sulphate, levulinic acid and sodium hypochlorite solution
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Listeria monocytogenes have been implicated as a public health concern worldwide. The study explored the survival of non-adapted, heat adapted and chlorine adapted L. monocytogenes on tomatoes; as well as the survival of non-adapted, heat adapted and chlorine adapted biofilms after exposure to sodium dodecyl sulphate (SDS), levulinc acid, sodium hypochlorite solution. Contact time of 1, 3 and 5 minutes was used. The survival of L. monocytogenes was monitored at 0, 24, 48 and 72 hours. The sanitizers were used individually or combined as follows; 1% sodium dodecyl sulphate individually; 0.5% levulinic acid individually; 200 ppm sodium hypochlorite solution individually and 0.5% levulinic acid/0.05% sodium dodecyl sulphate in combination (mixture). The samples were kept at 4 °C throughout the period of assessment. The effect of these sanitizers on pH, total soluble solids (TSS) and titratable acidity (TA) was also determined. Furthermore, the attachment of L. monocytogenes on tomatoes was investigated using a scanning electron microscope. Highest log reduction of non-adapted L. monocytogenes were observed on tomatoes treated with 1% SDS and least log reduction was achieved when tomatoes were treated with sodium hypochlorite solution. Though the log reduction achieved by 0.5% levulinic acid was higher that sodium hypochlorite solution, it was lower than log reduction achieved when 0.05% SDS / 0.5% levulinic acid mixture was used for all contact times. Using non-adapted L. monocytogenes, SDS was able to destroy all L. monocytogenes at 1, 3 and 5 minutes contact time. The trend was the same when heat adapted and chlorine adapted L. monocytogenes were used. There was no significant log reduction observed with biofilms. More favourable results were observed as contact time was increased from 1 to 5 minutes. Though there was a decrease in surviving bacteria from 1 to 3 minutes contact time, this decrease was not significant. The study investigated if exposure to sanitizer has an effect on pH, titratable acidity (TA) and total soluble solids (TSS) of the tomatoes. It was revealed that levulinic acid and mixture can have detrimental effect on pH, TA and TSS of tomatoes. The TA and TSS of samples treated with levulinic acid and mixture varied significantly (P ≤ 0.05) compared to the control sample. Although the TA and TSS of samples treated with SDS and sodium hypochlorite solution were different from the control, the differences were not significant. As much as sanitizers have the potential to reduce the bacterial population in fresh produce they may not completely destroy pathogens. Chlorine based sanitizers such as sodium hypochlorite though frequently used in the fresh produce industry, are not the best sanitizer to be used against food borne pathogens. Other sanitizers such as SDS used alone or in combination with another sanitizer can achieve better results than the widely used sodium hypochlorite solution as observed in this study. Stress adapted pathogens become less responsive to sanitizers during subsequent treatments. Through this research, it was established that biofilms are resistant to sanitizers. Though application of sanitizers in fresh produce is cheaper and simpler to apply, there is need to monitor varying concentrations of sanitizers, contact time and minimise contact with sub-surfaces as this could lead to sensory quality losses.