Individual and combined efficacies of mild heat and ultraviolet-c radiation against Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes in coconut liquid endosperm

This study determined the inactivation kinetic parameters of selected pathogens in heat, ultraviolet-C and combined heat-UV-C treated coconut liquid endosperm. Separate cocktails of Escherichia coli O157:H7, Salmonella enterica serovars, and Listeria monocytogenes strains were inoculated into coconut liquid endosperm (pH 5.15, TSS 4.4oBx, TA 0.062% malic acid, extinction coefficient (ε) at 254 nm of 0.0154 cm−1) for inactivation studies. Result showed that all organisms generally exhibited a log-linear heat inactivation behavior (R2 0.81-0.99). The E. coli O157:H7 cocktail (D55 = 19.75 min, D57 = 10.79 min, D60 = 3.38 min, and D63 = 0.46 min) was found to be significantly more resistant (P > 0.05) than the tested cocktail of L. monocytogenes (D55 = 11.68 min, D57 = 4.53 min, D60 = 1.82 min and D63 = 0.26 min) and S. enterica cocktail (D55 = 3.08 min, D57 = 2.60 min, D60 = 0.89 min and D63 = 0.25 min). Despite the differences in DT values, computed z values for L. monocytogenes cocktail (5.12 ± 0.43 °C) and E. coli O157:H7 cocktail (4.95 ± 0.12 °C) were not significantly different (P > 0.05), but were both significantly (P < 0.05) lower than that of S. enterica cocktail (7.10 ± 0.15 °C). All test organisms also exhibited a generally log-linear UV-C inactivation behavior (R2 0.90-0.99) with E. coli O157:H7 cocktail (DUV-C = 25.26 mJ/cm2) demonstrating greatest resistance to UV-C than S. enterica (DUV-C = 24.65 mJ/cm2) and L. monocytogenes (DUV-C = 17.30 mJ/cm2) cocktails. The D55 values of each organism cocktail were used to calculate for the 3-log reduction heating process schedules, during which UV-C treatments were simultaneously applied. Lethal rates (F values) calculations in the combined processes revealed that within the 3-log reduction heating processes, co-exposure of UV-C resulted in 5.62 to 6.20 log reductions in the test organism populations. Heating caused 69.3, 97.2, and 67.4% of the reduction in E. coli O157:H7, S. enterica and L. monocytogenes cocktails, respectively. These results can be used as baseline data in the establishment of mild heat treatment in combination with UV-C process schedules for coconut liquid endosperm and other similar products.