Microbial Safety of Organic Fruits and Vegetables

2004 Annual Report for LNC03-231

Project Type: Research and Education
Funds awarded in 2003: $139,650.00
Projected End Date: 12/31/2006
Region: North Central
State: Minnesota
Project Coordinator:
Francisco Diez-Gonzalez
University of Minnesota

Microbial Safety of Organic Fruits and Vegetables


A comparative microbial risk assessment of fresh produce grown by organic and conventional farms in the Upper Midwest was conducted by testing fruits and vegetables collected from fields for pathogenic and indicator bacteria. No Salmonella or enterohemorrhagic Escherichia coli were detected in any of the produce types and this result indicated that there was no imminent safety concern with any specific farm type. However, the prevalence of generic E. coli was significantly greater in produce grown by organic farmers than that in conventional ones, and this increased prevalence appeared to be linked to the use of animal manure or compost.

Objectives/Performance Targets

A. Project Objectives
The specific objectives of this proposal are to:
1) Determine the presence of fecal indicator organisms (coliforms, Escherichia coli) and pathogens (E. coli O157:H7, Salmonella) in organic and conventional fruits and vegetables produced by farmers in Minnesota and Wisconsin at the pre-harvest stage.
2) Conduct trace-back investigations in participating organic farms by comparisons of bacterial strains isolated from environmental samples and those isolated from produce.
3) Identify potentially high-risk management practices and provide recommendations for improvement.
4) Disseminate results and findings among the agricultural community.

B. Project Outcomes
1) A quantitative and comparative microbial risk assessment of fresh fruits and vegetables produced by organic farms.
2) A series of improvements in management practices such as modifications in manure type, proper use of composting and time of application of organic fertilizers, that will eventually reduce the risk of microbial contamination and produce safer fruits and vegetables.

3) Enhanced farmers’ awareness regarding application of manure fertilizer, irrigation water and the rationale for composting requirements.
4) Increased confidence of those farmers already using practices linked to low microbial loads and to improved safety of their produce.

5) Strengthen organic agriculture by providing the basis for enhanced consumer confidence that might lead to increased demand.
6) Help resolve the long-standing debate on the microbial safety of the organic fruits and vegetables for the scientific community and society in general.


Objective 1:
The activities leading to this objective have been completed and for the most part this objective was accomplished as proposed. During the harvest seasons 2003 and 2004, a total of 64 farmers participated in this project by allowing us to collect samples of lettuce, tomatoes, cabbage, leafy greens, strawberries, raspberries, broccoli, cucumbers, squash, apples, zucchini and other produce from their fields, and by answering farm management practices surveys. From that total, 14 farmers reported using organic practices that were certified by an accredited agency (organic farmers), 24 growers indicated using organic practices but they were not certified (non-certified farmers), and 26 farmers indicated that they used conventional agricultural practices (conventional farmers).

During the harvest seasons from May to September, each farmer was visited from 1 to 4 times depending on the availability of crops, and three individual samples of each produce variety were collected. The number of samples from certified organic, non-certified, and conventional farmers were for 2003: 178, 372 and 321, respectively; for 2004: 289, 539 and 348, respectively. The total number of samples was 2,047. The produce varieties that accounted for most of the samples were: leafy greens (other than lettuce), 14.5%; peppers (green, yellow, bell), 13.8%; tomatoes, 11.7%; cabbage, 9.8%; cucumber, 8.9%; zucchini, 7.6%; and lettuce, 7.4%.

The results of the microbiological analyses of these samples were as follows:

A. Coliform determination
Almost 28% of the certified organic and conventional and about 22% of non-certified produce tested negative for coliforms. In those samples that tested positive, the average coliform count was 150 Most Probable Number per gram (MPN/g) in certified organic and non-certified samples and 50 MPN/g in conventional samples, but this difference was not statistically significant.

B. Escherichia coli prevalence
E. coli was detected in 8.4% of certified organic samples, 8.0% of non-certified samples and 2.6% in conventional samples for both seasons. These results were very similar for each of the seasons with the exception of conventional that was 1.3% in 2003 and 3.6% in 2004. Lettuce, leafy greens and cabbage had the largest percentage of E. coli positive samples with 21, 15 and 12%, respectively.

C. Salmonella and Escherichia coli O157:H7
None of the produce samples analyzed in this project tested positive for any of these bacteria.

Objective 2:
In the spring and fall, samples of soil, manure, compost and water were collected from selected farms that had vegetables positive for E. coli, and were subjected to microbiological analysis. From a total of 80 samples, 8 were positive for this bacterium. The next step will be to compare those strains isolated from environmental samples with those isolated from the product using a fingerprinting method to determine the source of contamination.

Objective 3:
Since there were no samples positive for the pathogenic bacteria, this objective will focus on identifying management practices linked to E. coli prevalence as an indicator of fecal contamination. In addition to the E. coli prevalence data, we have gathered information about the management practices from most of the farmers for each of the seasons. This information was obtained by mailing a detailed questionnaire that asked about: years in production, crops grown (rotation used), markets, certification agency, specific requirements for fertilization and handling, type of fertilizers, manure sources, composting practices (materials, method, duration, amount applied, timing of applications), harvesting method, and handling practices.

The data is being clustered according to management practices and potential links between them and E. coli prevalence will be identified using statistical analysis. We will consider different statistical analysis and at this time, the graduate student working on this project is being trained in advanced statistical analysis. Because of the complexity of the data, it is critical to employ the most suitable statistics. Because there were more certified organic and non-certified farmers that used manure or composted manure (90 and 60%, respectively), as compared to conventional farmers (40%), it appeared that the greater prevalence of E. coli was linked to the increased used of animal compost or manure as fertilizer. This relationship will be, however, more thoroughly evaluated.

Objective 4:
We have communicated directly with each of the participating farmers reporting their individual results and the overall season results after each season. Disseminating the results to the agricultural community will not be done until the risk assessment of management practices is completed. Unfortunately, we have not been very successful in attracting the participating farmers to a meeting. Once we have the risk assessment study based on the statistical comparisons we plan to have a gathering with farmers.

To the scientific community, we have presented partial 2003 results at the World Congress of Organic Foods, a relatively small conference of mostly researchers interested in organic agriculture, last year. The project website is currently under construction and is linked to the Department of Food Science and Nutrition website.

After a previous study similar to this project was published last year (1), our work received considerable attention from the media and especially from organic and sustainable agriculture websites. We expect that once the results of this project are made public, there will be great interest from the agricultural, scientific, and consumer communities.

Project Outcomes
All of the outcomes depend on the completion of objectives 3 and 4 above.

1. Mukherjee, A., D. Speh, E. A. Dyck, and F. Diez-Gonzalez. 2004. Pre-Harvest Evaluation of coliforms, Escherichia coli, Salmonella and E. coli O157:H7 in organic and conventional produce grown by Minnesota farmers. J. Food Prot. 67:894-900.

Impacts and Contributions/Outcomes

Because of the limited dissemination of results described above, the impact of this particular project has only been felt by the farmers who have participated in the study. Several of them have benefited by the increased confidence of knowing that their products have consistently tested free from any pathogenic and even fecal indicator bacteria. The increased level of awareness about food safety among those farmers has also been a positive outcome of this project.


Pauline Nickel

Southwest Research and Outreach Center
23669 130th St.
P. O. Box 428
Lamberton, MN 56152
Dorinda Speh

Research Plot Technician
Southwest Research and Outreach Center
23669 130th St.
P. O. Box 428
St. Paul, MN 56152
Avik Mukherjee

Graduate Research Assistant
University of Minnesota
Department of Food Science and Nutrition
1334 Eckles Avenue
St. Paul, MN 55108