Fate of Salmonella Typhimurium, Listeria monocytogenes, and Surrogates on butternut squash during aerobic storage

This study aimed to evaluate the survival of foodborne pathogens and bacterial surrogates on butternut squash during storage. Fresh squash, either untreated or pre-treated with 70% ethanol and UV light, was inoculated with Salmonella Typhimurium, Listeria monocytogenes , L. innocua , and Enterococcus faecium , then stored at 22.5 o C (40.6% RH) for 30 days. Microbial counts were periodically quantified from day 0–30 using tryptic soy agar or bile esculin agar supplemented with 200 ppm of nalidixic acid. By day 30 of storage, cell counts of S . Typhimurium, L. monocytogenes , and L. innocua significantly decreased ( P < 0.05) from 5.59-6.87 to 1.83–2.68 log 10 CFU/cm 2 on untreated squash, and from 5.23-7.35 to 1.30–2.86 log 10 CFU/cm 2 on pre-treated squash. Cell counts of E. faecium decreased ( P < 0.05) by only 0.85-0.87 log 10 CFU/cm 2 across all samples throughout the entire storage period. The kinetic inactivation parameters, calculated from the Linear, Linear with Tail, and Biphasic models, indicated a lower k max value, a higher LogN res value, and a larger subpopulation resistant to dry conditions compared to the other 3 tested bacterial strains. These results suggest that E. faecium is a suitable surrogate for Salmonella and L. monocytogenes on butternut squash during storage. • Salmonella and Listeria decreased from 5.6 to 6.8 to 1.8–2.7 log 10 CFU/cm 2 during storage. • E. faecium decreased by < 1.0 log 10 CFU/cm 2 during storage. • Survivors fit Linear, Linear with Tail, and Biphasic models. • E. faecium is a suitable surrogate for Salmonella and L. monocytogenes on squash during storage.