NOP Compliant Antimicrobial Rinses on Leafy Greens and the Effect on Foodborne Pathogen Indicator Presence

Final Report for FW10-062

Project Type: Farmer/Rancher
Funds awarded in 2010: $14,944.00
Projected End Date: 12/31/2011
Region: Western
State: Washington
Principal Investigator:
Dan Hulse
Tahoma Farms
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Project Information


When we commenced the study and our operation was still in its infancy in 2010, there was a belief that our farm was going to have conform to new regulations under the Food Safety Modernization Act (FSMA) that became law in January 2011, and that we were going to have to do so quickly. Part of our response to this was engaging with the faculty at Washington State University (WSU) and, specifically, the food safety team at WSU to develop this grant proposal. Our project was approved in 2010 and our work commenced, as planned, that summer.

However, our original projections for a timeline turned out to be grossly optimistic: we had a wet, cold growing season that pushed our second round of tests to 2011; our evaluation of the first round of data from the 2010 tests left us feeling ambivalent about the efficacy of our treatments and the design of our study; 2012 was a lost year, at least in terms of the study, as the project got moved further to the back of the burner as other, more pressing farm projects, took precedent; finally, in June of this year, we completed the outreach portion of the project. And, as of the writing of this final report, we are still a couple of weeks out on having the wash basin installed on the farm.

With that said, the evolution of the project and the delay in implementation of the changes legislated by the FSMA were coincidentally helpful in allowing us to get a better end-product, both in the report presented here and the on-farm solution we came up with.


Our project set out to test the effectiveness of various antimicrobial treatments in reducing pathogen indicator organisms on leafy greens. The presence of foodborne pathogens in leafy greens has a higher rate of occurrence than any other fruit or vegetable, and with greater regulation of food safety in the fresh fruit and vegetable industry looming, producers need options to help minimize the risk of selling contaminated food. For organic producers in particular, who are prohibited from using chlorine in production or processing, there is a need for a viable alternative to the standard used in many conventional operations that use an antimicrobial rinse. There is not a more critical issue for small- and medium-sized growers right now than that of food safety.

For this project, heads of leafy greens were assigned to one of six treatments (20 heads per treatment): pre-rinse, water rinse, chlorine antimicrobial rinse, lactic acid rinse, peroxyacetic acid rinse and a vinegar rinse. The pre-rinse treatment will provide an indication of initial microbial load on leafy greens prior to an antimicrobial intervention. After each rinse, heads will be sampled for microbial load to examine the effectiveness of the intervention on reducing indicator organisms. Each head of leafy greens will be enumerated for aerobic plate count (APC) and total coliforms. There were two sampling events for a total test population of 240 heads of lettuce.

Project Objectives:

Our primary objective was to test the effectiveness of anti-microbial rinses for reducing pathogen indicator organisms on leafy greens.

Secondary objectives included: designing a wash basin for application of the rinse, comparing the impact on quality and taste of the various applications, and completing an economic analysis to determine the feasibility of each application against the others.


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  • Andy Bary
  • Dr. Karen Killinger


Materials and methods:
Pathogen Reduction

Heads of red and green leaf lettuce were assigned to one of six treatments: pre-rinse, water rinse, chlorine antimicrobial rinse, lactic acid rinse, peroxyacetic acid rinse and a vinegar rinse.

The pre-rinse sample was meant to provide an indication of initial microbial load on the sample population prior to an antimicrobial intervention.

Each head that went through a rinse was actually dunked into a wash basin with one of the five other treatments (water or water with an additive) before testing microbial load to examine the effectiveness of the intervention on reducing indicator organisms. Each head of leafy greens was enumerated for aerobic plate count (APC) and total coliforms.

The initial round of sampling occurred within the waxed-cardboard box that is standard for the industry and followed a field-pack prescription for testing: individual heads were harvested directly into the box and the box itself was submerged in each one of the five treatments.

In the second round of testing, it was decided that individual heads would be treated so as to mimic a process used by many small- to mid-size farms to harvest in one container and then re-pack into a separate container for sale or transport off the farm.

In both scenarios, the samples were immersed in the treatment for one minute followed by a one minute immersion in water.

Taste and Quality

All samples were tested daily for taste and appearance for a period of two weeks following the harvest and treatment date. Two subjects - myself and an associate - logged our observations of the effect each treatment had on overall quality of the lettuce on a scale of 1 to 10.

Economic Feasibility

To achieve a 2% lactic acid solution, 1.32 gallons were added to 66 gallons of water; to achieve a 2% acetic acid solution, 26.4 gallons of vinegar had to be added to 66 gallons of water! Vinegar is relatively inexpensive but would the volume needed to achieve a 2% solution cost more or less than the 1.32 gallons of lactic acid required also for a 2% solution? How does the 2% citric acid solution compare cost-wise to these other solutions? And, finally, how does the cost of these National Organic Program compliant rinses compare with ones being utilized in non-organic operations?

Research results and discussion:

Based on the primary objective of evaluating the effectiveness of various rinses on reduction of pathogen indicator organisms, the design of the study did not allow us to draw any conclusions. Since initial loads of the indicator organisms were varied, if we were to do this testing in the future, we would definitely inoculate the heads of lettuce at the outset to achieve a baseline by which to evaluate any reductions.

Furthermore, data of actual reductions within each treatment were inconsistent. We actually saw some samples that were treated with one of the rinses return a higher total Aerobic Plate Count (APC) than the sample heads that were not treated at all (NT, in our report, for No Treatment).

We concluded that this was largely due to variability between the heads of lettuce initial load. Thus, a further examination using a single head of lettuce, inoculating it, and then dividing that head into smaller pieces for additional testing would be much better at determining efficacy of treatment. (It is worth noting that the effectiveness of these antimicrobial rinses and others is reported in other studies. These can serve as a guide to an individual producer looking to compare relative effectiveness and cost, which we won't go into here).

That said, the original design of the study was to replicate what was deemed to be an appropriate and efficient way of packing and rinsing leafy greens that was known by the project participants to be the standard within the industry (namely, field-packing).

The outcome that proved to be the most beneficial to this producer is in the way that it forced me to think about our post-harvest handling practices and its many and sometimes conflicting objectives.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

The following was sent out widely throughout the producer, consumer and academic circles prior to our outreach event:

Education and Outreach Outcomes

Recommendations for education and outreach:

Potential Contributions

We intend to hold another outreach event in early 2014 to showcase the new wash station and to review our farm's progress towards obtaining GAP certification.

We obtained $25,000 in funding through the Farm Service Agency (FSA) microloan program and believe that the wash station could be a piece of equipment that could serve as a template for other producers that are interested in:

  • A cleaner product
    Using an antimicrobial rinse
    More efficient and automated workflow in the packing house
    Better hydro-cooling
    Harvest and reusable tote cleaning

Future Recommendations

As mentioned before, a more refined study to test these specific antimicrobial rinses could be constructed. Alternately, information from existing studies could be used to determine adaptability of different rinses and application procedures specific to each operation. This would also aid in the economic feasibility analysis that was not conducted for this project based on the inconclusive nature of the testing.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.