Influence of temperature, humidity, and diluent type on survival of Salmonella spp. on the surface of raw tomatoes

Oleksandr Tokarskyy,Keith Schneider

doi:10.5219/1121

Abstract

Tomatoes are an important commodity, placing fourth among most popular vegetables in the U.S. However, fresh tomatoes lack a final pathogen elimination step and have been implicated in Salmonella-related outbreaks. The purpose of the study was to evaluate survival of Salmonella post-drying in three diluents on the surface of green mature tomatoes at 12 °C or 25 °C. Additionally, low and high air relative humidity influence was evaluated at 25 °C on pathogen survival. A five Salmonella rifampin-resistant strain cocktail was double-washed in buffered peptone water (BPW) and resuspended in 0.1% peptone, BPW, or fresh tomato serum. Inoculum (0.1 mL) was allowed to dry on the surface of tomatoes. For study I, tomatoes were placed in 12 °C and 25 °C incubators with no humidity control and sampled on days 0, 1, 3, and 5. For study II, tomatoes were sampled on days 0, 1 (biosafety hood storage) and on day 5 after storage in two 25 oC incubators (low and high relative humidity). Salmonella was recovered from tomatoes (20 mL BPW) and plated (TSA-rif80, 37 °C, 48 hours). Post-drying Salmonella counts (ca. 4.5 - 5.0 log10 CFU.mL-1) remained at 4.03 and 4.40 log10 CFU.mL-1 in serum after 5 days of storage at 12 °C and 25 °C, respectively. Conversely, corresponding counts in BPW and peptone were lower at ca. 1.4 to 1.8 and 2.2 to 2.8 log units at 12 °C and 25 °C, respectively. At low humidity, post-drying Salmonella counts showed highest decline for peptone (final 1.98 log10 CFU.mL-1) compared to BPW (3.79 log10 CFU.mL-1) and tomato serum (4.75 log10 CFU.mL-1) on day 5. Counts declined rapidly to 0.03, 0.56, and 0.44 log10 CFU.mL-1 for peptone, BPW, and tomato serum, respectively, at high humidity on day 5. To summarize, it was shown that increased solutes have protective effect on Salmonella in desiccated conditions, while high humidity storage causes accelerated death of stationary culture within five days storage period.

Highlights

Tomatoes are an important commodity, placing fourth among most popular vegetables in the U.S According to FAOSTAT (2017), top ten tomato producing countries in the world were China, India, Turkey, USA, Egypt, Iran, Italy, Spain, Mexico, and Brazil, with Slovakia present in the top twenty and producing as much as 21,964 tonnes in 2017 alone
Scientific hypothesis It is hypothesized that high organic solute concentration in inoculation diluent, as well as low humidity and low temperature, may improve survival of Salmonella on the surface of undamaged tomatoes
A two-factorial design was used to test the effects of treatment-storage factor and diluent type (BPW, 0.1% peptone, and tomato serum), as well as their interaction, on Salmonella counts on tomato surface

Summary

Introduction

Tomatoes are an important commodity, placing fourth among most popular vegetables in the U.S According to FAOSTAT (2017), top ten tomato producing countries in the world were China, India, Turkey, USA, Egypt, Iran, Italy, Spain, Mexico, and Brazil, with Slovakia present in the top twenty and producing as much as 21,964 tonnes in 2017 alone. Wei et al (1995) showed that Salmonella Montevideo die-off at

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