Changes in Aerobic Plate and Escherichia coli–Coliform Counts and in Populations of Inoculated Foodborne Pathogens on Inshell Walnuts during Storage

Abstract

After harvest, inshell walnuts are dried using low-temperature forced air and are then stored in bins or silos for up to 1 year. To better understand the survival of bacteria on inshell walnuts, aerobic plate counts (APCs) and Escherichia coli?coliform counts (ECCs) were evaluated during commercial storage (10 to 12°C and 63 to 65% relative humidity) over 9 months. APCs decreased by 1.4 to 2.0 log CFU per nut during the first 5 months of storage, and ECCs decreased by 1.3 to 2.2 log CFU per nut in the first month of storage. Through the remaining 4 to 8 months of storage, APCs and ECCs remained unchanged (P > 0.05) or decreased by <0.15 log CFU per nut per month. Similar trends were observed on kernels extracted from the inshell walnuts. APCs and ECCs were consistently and often significantly higher on kernels extracted from visibly broken inshell walnuts than on kernels extracted from visibly intact inshell walnuts. Parameters measured in this study were used to determine the survival of five-strain cocktails of E. coli O157:H7, Listeria monocytogenes, and Salmonella inoculated onto freshly hulled inshell walnuts (∼8 log CFU/g) after simulated commercial drying (10 to 12 h; 40°C) and simulated commercial storage (12 months at 10°C and 65% relative humidity). Populations declined by 2.86, 5.01, and 4.40 log CFU per nut for Salmonella, E. coli O157:H7, and L. monocytogenes, respectively, after drying and during the first 8 days of storage. Salmonella populations changed at a rate of -0.33 log CFU per nut per month between days 8 and 360, to final levels of 2.83 ± 0.79 log CFU per nut. E. coli and L. monocytogenes populations changed by -0.17 log CFU per nut per month and -0.26 log CFU per nut per month between days 8 and 360, respectively. For some samples, E. coli or L. monocytogenes populations were below the limit of detection by plating (0.60 log CFU per nut) by day 183 or 148, respectively; at least one of the six samples was positive at each subsequent sampling time by either plating or by enrichment.

Effect of water activity and storage of tahini on the viability of stressed Salmonella serovars

“…The S. Tennessee viability dropped by 0.15 to 0.65 or 0.34 to 1.29 log 10 CFU/g in samples stored 14 d at 4 or 22 °C, respectively. In another study, the initial population of Salmonella inoculated on the shell of walnuts was reduced by 0.29 log 10 CFU per nut after the first 8 d storage at 10ºC and an RH of 65% after drying, but no change in the population was noted after 3 months of storage (Frelka et al, 2016). Salmonella possess strategies to survive long times in a desiccated state.…”

“…The variation between these and the results presented in the current study may have been due to serovar differences, variation in culture cultivation procedures, product composition or the variability in the storage time of the products that was mostly compatible with anticipated shelf-life of the product. In the studies by Zhang et al (2017) and Frelka et al (2016), bacteria used were taken from solid media and were sessile cells, while in the present study bacteria used were planktonic in nature, grown in liquid media. In addition, tahini samples used in the Zhang et al (2017) study were prepared by grinding sesame seeds with olive oil in a food processor while commercial tahini samples were used in the present study.…”

Microbiological Quality of Nuts, Dried and Candied Fruits, Including the Prevalence of Cronobacter spp.

“…Such a level of a w prevents growth of many pathogens and lends to the microbiological safety of the LMFs. Although bacteria of the Enterobacteriaceae family ( Escherichia coli, Salmonella ) cannot propagate on plant products with a low a w , they can remain alive on/in such products for a long time (up to 1 year), especially when products are stored in cold [ 3 , 4 ].…”

Survey of Salmonella in raw tree nuts at retail in the United States

“…However, Salmonella has been detected in almonds, cashews, hazelnuts, macadamia nuts, peanuts, pecans, pine nuts, pistachios, and walnuts in the past worldwide (Bansal, Jones, Abd, Danyluk, & Harris, 2010; Bedard, Kennedy, & Weimer, 2014; Blessington, Theofel, Mitcham, & Harris, 2013; Brar, Strawn, & Danyluk, 2016; Calhoun, Post, Warren, Thompson, & Bontempo, 2013; Danyluk et al., 2007; Davidson, Frelka, Yang, Jones, & Harris, 2015; Little, Jemmott, Surman‐Lee, Hucklesby, & De Pinna, 2009; Miksch et al., 2013; Zhang, Hu, Melka, et al., 2017). Salmonella has been shown to survive for a long period of time on tree nuts at ambient temperature (Abd, McCarthy, & Harris, 2012; Frelka, Davidson, & Harris, 2016; Kimber, Kaur, Wang, Danyluk, & Harris, 2012; Santillana Farakos, Pouillot, & Keller, 2017). In addition to Salmonella , Escherichia coli O157:H7 has been found in hazelnuts, peanuts, and walnuts (Canadian Food Inspection Agency, 2011; Centers for Disease Control and Prevention, 2011a; Miksch et al., 2013; Yada, 2019), and was reported to survive on in‐shell hazelnuts, almonds, pistachios, and walnuts (Blessington et al., 2013; Feng, Muyyarikkandy, Brown, & Amalaradjou, 2018; Kimber et al., 2012) .…”