Written by Claire Murphy, Assistant Professor and Extension Specialist, December 5, 2024
Background
The Produce Safety Rule (PSR), part of the FDA’s Food Safety Modernization Act (FSMA), sets standards to reduce contamination risks in fruits and vegetables during growing, harvesting, packing, and storage. Recognizing that agricultural water (water that may come into contact with the harvestable part of produce) is a potential source of contamination, the PSR requires growers to conduct a written risk assessment of preharvest agricultural water. As part of this process, growers must evaluate whether additional measures are necessary to mitigate risks associated with preharvest water use. One such mitigation strategy is increasing the time interval between the last direct application of agricultural water and the harvest, allowing time for microbial die-off. To implement this time-to-harvest interval effectively, producers need scientific data that is specific to their unique conditions, including factors like region, crop type, and water application methods. This data should also account for the biphasic nature of microbial die-off, which consists of an initial rapid die-off phase followed by a slower, long-term reduction in microbial populations.
This article reviews the findings from two key studies by Washington State University and the Washington Tree Fruit Research Commission on the die-off of generic Escherichia coli on apples in the field, providing valuable data to help support time-to-harvest interval recommendations.
Study 1
Survival of Generic Escherichia coli on In-Field Mature and Immature Gala and Golden Delicious Apples with or without Overhead Evaporative Cooling Treatment
This research study examined the survival of inoculated generic Escherichia coli over three growing seasons considering evaporative cooling treatment, canopy location, apple variety, and fruit maturity.
Key Results
- Reduction over time: For mature Gala and Golden Delicious apples, coli levels decreased by 2.1-3.1 log CFU/apple after 10 hours and by 4.2-4.9 log CFU/apple after 18 hours. By 154 hours, E. coli on mature apple surfaces decreased by close to or over 6 log CFU/apple across treatments.
- Evaporative Cooling: The study results indicated that applying evaporative cooling treatments after coli inoculation did not significantly affect the survival or die-off of the E. coli compared to apples that were not treated with evaporative cooling.
- Die-off pattern: Non-linear patterns provided a better fit for explaining the overall die-off of E. coli on apple surfaces compared to a log-linear pattern, showing a faster initial decline followed by a slower tailing.
- Apple Variety: The impact of apple variety was not biologically meaningful, supporting similar timing of harvest practices for both apple varieties after the application of potentially contaminated water.
Study 2
Impact of Overhead Evaporative Cooling, Canopy Location, Sunlight Exposure, Inoculation Level, Region, and Growing Season on the Survival of Generic Escherichia coli on In-Field Fuji Apples
This research study examined the survival of inoculated generic Escherichia coli over two growing seasons on Fuji apples considering evaporative cooling treatment, canopy location, region, inoculation level, and sunlight exposure.
Key Results
- Reduction and die-off by inoculation level: For high-inoculated apples, coli decreased by 1.6 ± 1.1 log CFU/apple in 10 hours, while low-inoculated apples saw a 1.6 log CFU/apple reduction by 18 hours. By 154 hours, E. coli levels on Fuji apple surfaces decreased by 5.5 ± 1.3 log CFU/apple for high inoculum and 3.3 ± 0.4 log CFU/apple for low inoculum apples.
- Evaporative Cooling: Overhead evaporative cooling did not impact the survival or die-off of coli on Fuji apple surfaces.
- Region: The region/orchard location (Douglas County and Benton County) did not impact the reduction and die-off of coli.
- Sunlight Exposure: Apples in full sun demonstrated significantly fast die-off rates than apples in full shade.
Takeaways
- Harvesting apples 10-18 hours after water application, regardless of variety or region in Washington, significantly reduces coli levels on apple surfaces, providing guidance for optimal time-to-harvest intervals.
- Overhead evaporative cooling, a practice beneficial for fruit quality, does not affect E. coli survival on apple surfaces. However, using unsanitary water for cooling could be a potential source of contamination.
- Sunlight, or solar ultraviolet radiation, plays a key role in reducing pathogens on fresh produce.
Full Manuscripts
- Murphy, C.M., Jeong, K.H., Walter, L., Mendoza, M., Green, T., Liao, A., Killinger, K., Hanrahan, I. and Zhu, M.J., 2024. Survival of Generic Escherichia coli on In-Field Mature and Immature Gala and Golden Delicious Apples with or without Overhead Evaporative Cooling Treatment. Journal of Food Protection
- Murphy, C.M., Mendoza, M., Walter, L., Jeong, K.H., Liao, A., Green, T., Killinger, K., Hanrahan, I. and Zhu, M.J., 2024. Impact of overhead evaporative cooling, canopy location, sunlight exposure, inoculation level, region, and growing season on the survival of generic Escherichia coli on in-field Fuji apples. Journal of Applied Microbiology
Contact
Claire Murphy
Assistant Professor and Extension Specialist
Fruit Matters articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: Originally published by Washington State Tree Fruit Extension Fruit Matters at treefruit.wsu.edu and a link to the original article.
