Use of gamma radiation for inactivating Salmonella spp., Escherichia coli O157:H7 and Listeria monocytogenes in tahini halva

Abstract

Tahini halva is a traditional sweet product that is consumed with bread in different countries. It is a low water activity (a_w) product basically made by mixing and cooking tahini, sugar, citric acid and Saponaria officinalis root extract together. Tahini halva maybe contaminated with foodborne pathogens during any stage of production from tahini and other raw ingredients, workers, environment or contact surfaces. The objectives of the study were to i) investigate the efficacy of gamma radiation to inactivate Salmonella spp., Escherichia coli O157:H7 and Listeria monocytogenes in tahini halva, ii) evaluate the effect of pre-irradiation storage (0, 7 and 30 days at 21 °C) of tahini halva on the sensitivity of these microorganisms toward gamma radiation, and iii) evaluate the effect of post-irradiation storage of tahini halva for up to 6 months on the their survival characteristics. Tahini halva samples were inoculated with Salmonella spp., E. coli O157:H7 and L. monocytogenes separately then stored at 21 °C for 0, 7 and 30 days prior to irradiation at 0-4 KGy and for up to 6 months after irradiation at 4 KGy. Salmonella spp. were the most irradiation resistance among the tested microorganisms. Irradiation (0.8-4.0 KGy) reduced the bacteria in samples stored for 0, 7 and 30 days pre-irradiation in the range of 0.43-2.11, 0.45-2.68 and 0.52-2.7 log₁₀ CFU/g for Salmonella spp., 0.55-3.08, 0.66-3.00 and 0.60-2.80 log₁₀ CFU/g for E. coli O157:H7, and 0.69-2.96, 0.86-4.30, 0.62-3.29 log₁₀ CFU/g for L. monocytogenes, respectively. The D₁₀-value, the irradiation dose needed to inactivate 1 log₁₀ of pathogen, was 1.83, 1.47 and 1.50 KGy for Salmonella spp., 1.28, 1.32 and 1.48 KGy for E. coli O157:H7, and 1.33, 0.94 and 1.27 KGy for L. monocytogenes in pre-irradiation stored samples for 0, 7 and 30 days, respectively. Post-irradiation storage was efficient in decreasing the levels of the microorganisms ca. ≥2 log₁₀ CFU/g in the first month and to undetected level after the second month of storage but enrichment results showed that Salmonella spp. and L. monocytogenes were detected in the samples until of the end of storage period. The study demonstrates that gamma radiation can be applied to inactivate of foodborne pathogens in tahini halva. Irradiation dose at 4 KGy can reduce Salmonella spp., E. coli O157:H7 and L. monocytogenes in tahini halva by 2-3 log₁₀ CFU/g. Storage of tahini halva before or after irradiation may reduce the risk of foodborne pathogens in the product.

Keywords: Foodborne pathogens; Irradiation; Low water activity; Post-irradiation storage; Pre-irradiation storage; Tahini halva.