Infiltration and survival of a rifampicin-resistant five-serovar Salmonella cocktail was investigated in laser-etched tomatoes (Lycopersicum esculentum) and smooth (untreated) and punctured (dye solution only) tomato surfaces in storage for 14 days at 25 °C/60% relative humidity. Surviving Salmonella populations were enumerated on tryptic soy agar supplemented with the antibiotic rifampicin. In the first survival study (laboratory-etched tomatoes), the population of Salmonella spp. in wounds increased to 6.8 log cfu/fruit, whereas cells on smooth surfaces decreased to undetectable levels during 14 days of storage. For etched tomatoes, the storage reduced 2.7 log cfu/fruit after the first 3 days; however, an increase was observed at 7 days, followed by a population decreased to 2.9 log cfu/fruit at 14 days. In the second survival study (pilot plant-etched tomatoes), the populations decreased a total of 3.5 log cfu/fruit and 2.5 log cfu/fruit comparing 1 day with 14 days for smooth and etched surfaces respectively. Infiltration of the dye solution or Salmonella beyond the area of immediate tissue damage was not observed on any tomato surface tested.
Hyun-Gyun Yuk, Benjamin R. Warren and Keith R. Schneider
Michael J. Mahovic, Rajya Shukla, Renée M. Goodrich-Schneider, Michael V. Wood, Jeffrey K. Brecht and Keith R. Schneider
It has been reported that netted muskmelons (Cucumis melo var. cantalupensis) treated with moist heat (steam or hot-water immersion) have reduced populations of vegetative surface organisms that may be responsible for spoilage, or that may be pathogenic to consumers. It is unknown, however, what affect a similar heat treatment may have on infesting bacterial endospores (which are dormant, nonreproductive structures that are resistant to environmental stress). Also, any heat treatment used must be effective without exceeding the treated melon's thermal damage threshold. In this study, natural microflora on muskmelon rind pieces treated from 75 to 95 °C for 3 minutes and whole fruit rinds inoculated with Bacillus atrophaeus spores and treated at 85 °C for 3 minutes were observed as a model system to explore the efficacy of moist heat in reducing surface populations of bacterial spores. There were significant reductions in populations of aerobic, nonspore-forming microbes, although the treatments had little to no effect on either the recoverable populations of inoculated B. atrophaeus spores or indigenous spore-forming bacteria. Recovery studies suggested a less than 2 log10 unit reduction of inoculated B. atrophaeus spores after a 3-minute, 85 °C moist heat treatment, and no heat injury symptoms developed on melons during storage for 2 weeks at 5 °C. Increasing treatment temperature from 75 to 95 °C resulted in no increase in efficacy in terms of recovery of indigenous vegetative bacteria. The results of this study suggest that aqueous heat treatment is not a suitable method for reducing populations of the resting structures of spore-forming bacteria from the surface of netted muskmelons.
Raina L. Allen, Benjamin R. Warren, Douglas L. Archer, Keith R. Schneider and Steven A. Sargent
Multi-state outbreaks of salmonellosis due to the consumption of contaminated fresh tomatoes (Lycopersicon esculentum) have recently occurred in the United States. This study investigated the survival of a five-serovar (serotype) Salmonella cocktail artificially inoculated onto tomato and packing line surfaces when held at various temperature and relative humidity (RH) combinations over 28 days. Packinghouse surfaces included stainless steel, polyvinyl chloride (PVC), sponge rollers, conveyor belts, and unfinished oak wood surfaces. Packinghouse climates were generated to simulate conditions in Florida during late spring (30 °C/80% RH) and fall/winter (20 °C/60% RH) months. Additionally, survival of Salmonella on tomatoes in standard ripening room conditions (20 °C/90% RH) was evaluated. Recovery of inocula was by a vigorous shake/hand rub method. After 28 days, Salmonella populations remained detectable on tomato surfaces regardless of environmental conditions. Inoculated Salmonella populations tested at spring conditions declined to undetectable levels on all packing line materials by day 11, with the exception of the unfinished oak, which reached undetectable levels by day 21. In contrast, inoculated Salmonella populations tested at fall/winter conditions declined to undetectable levels on sponge rollers and conveyor belts by day 7 and day 21, respectively. Stainless steel, PVC, and wood surfaces supported the survival of detectable populations of Salmonella over the 28-day sampling period. Results of this study demonstrate the potential for Salmonella to persist on tomato and packing line surfaces under common environmental conditions.
Michael J. Mahovic, Keith R. Schneider, Kim Cordasco and Jeffrey K. Brecht
The effect of ethylene on the survival of Salmonella cells residing on unwounded surfaces of tomato (Solanum lycopersicum) fruit was investigated in this study. Inoculated fruit were stored in flow-through chambers that were adjusted to maintain an environment simulating a tomato ripening room. Fruit were held at 20 °C and ≥95% relative humidity after surface inoculation with the low virulence and rifamycin-resistant pathogen S. enterica ssp. enterica serovar Typhimurium strain LT2 (S. Typhimurium). Tomato fruit were treated either with a continuous flow (rate, ≈200 mL·min−1) of air or with ≥150 μL·L−1 ethylene in air. Bacterial recovery at 0, 24, 48, and 72 h after initiation of treatment showed that total populations of the S. Typhimurium cells declined in both the air and ethylene treatments during the first 24 to 48 h of storage, then increased to near initial levels by 72 h, similar to decline and recovery reported by other researchers in nonethylene treatment trials. These results suggest that although Salmonella can survive on the surfaces of tomato fruit in typical ripening rooms, proliferation of Salmonella is neither promoted nor inhibited by ethylene exposure.
Jaysankar De, Aswathy Sreedharan, You Li, Alan Gutierrez, Jeffrey K. Brecht, Steven A. Sargent and Keith R. Schneider
Cooling procedures used by blueberry (Vaccinium sp.) growers often include delays up to 24 hours that can damage the fruit through rough handling and adverse temperatures, thereby potentially compromising quality and, subsequently, safety. The objectives of this experiment were to compare forced-air cooling (FAC) compared to hydrocooling without sanitizer (HW) and hydrocooling with sanitizer (HS) regarding the quality and shelf life of southern highbush blueberry [SHB (Vaccinium corymbosum)] and to determine the efficacy of these treatments for reducing Salmonella in SHB. Freshly harvested SHB that were inoculated with a five-serovar cocktail of rifampin-resistant Salmonella were rapidly chilled by FAC or hydrocooling (HW and HS) using a laboratory model system. FAC did not show any significant reduction (P > 0.05) in Salmonella or in the effects on the microbiological quality of blueberries. HW and HS reduced Salmonella by ≈2 and >4 log cfu/g SHB, respectively, on day 0. These postharvest treatments was also evaluated for their ability to help maintain fruit quality throughout a storage period of 21 days at 1 °C. Hydrocooling (both HS and HW) provided more rapid cooling than FAC. Hydrocooled blueberries showed significant weight gain (P < 0.05), whereas FAC resulted in a slight, but insignificant (P > 0.05), reduction in final weight. The results of hydrocooling, both HS and HW, shown in this study could help to extend the shelf life while maintaining or increasing the microbiological quality of fresh market blueberries. Information obtained by this study can be used for developing the best temperature management practices to maintain the postharvest safety and quality of blueberries.