Are organic vegetables (especially spinach) safe to eat?

Fresh, bagged spinach was identified as the source of E. coli infections that sickened people beginning in mid-September of 2006.  For detailed information about the outbreak, visit the website of the Centers for Disease Control.

Some of the spinach initially suspected in the outbreak was organic spinach packaged by the Natural Selection company (doing business as Earthbound Farms). By March of 2007, the Food and Drug Administration had determined that the source of the outbreak was Dole brand “Baby Spinach,” and that it most likely came from a California cattle ranch that leased land to vegetable growers. The suspected field was in its 36-month transition period to certified organic production, and the spinach had been sold as conventional spinach.  You can download the final report on the investigation here (PDF, 6 Mb).

Between September 2006 and March 2007 there were articles in the media suggesting that organic spinach, and organic vegetables in general, were more dangerous to people’s health than conventionally-grown vegetables. The reason for this, it was said, is because manure is used for fertilizer on organic vegetables. Not surprisingly, some people became worried about eating organic vegetables.

Should you worry about eating organic vegetables?  The short answer is no.

  • Organically grown vegetables are no more likely to be contaminated than conventional vegetables.

  • Accidental contamination can happen to both organic and conventional vegetables.

  • Accidental contamination may be more likely to happen with leafy greens such as Swiss chard and spinach, and with lettuce, because they are in direct contact with the soil.  Washing these vegetables before serving is always a good idea, regardless of whether they are packaged or not.

  • Certified organic farmers must follow strict rules on manure handling that are designed to minimize the chance that disease organisms will wind up on vegetables.

  • If you are buying from a farmer who claims to be organic but who is not certified, ask about her or his manure-handling practices. Long aging of manure, or composting, or adding manure to soil several months before the harvest, are best practices for reducing the chance of contamination of the vegetables.

The scoop about manure

Animal manure can be used for fertilizer on any vegetables, conventional or organic.  There are no restrictions on manure use on conventional farms. Certified organic growers, however, must follow strict rules for the handling of manure. Manure used on organic farms can be composted according to exacting procedures that cause the compost to heat up to a temperature that kills disease organisms. If manure is not composted, it must be tilled into the soil at least 120 days before the harvest of a food crop that comes in contact with soil (such as spinach); or at least 90 days before the harvest of a food crop that does not come in contact with the soil (such as sweet corn). These rules for manure use are designed to minimize the chance that disease organisms will contaminate organic vegetables.

Read the National Organic Program Standards on manure handling:
Scroll down the web page to find:
§ 205.203 Soil fertility and crop nutrient management practice standard

University of Minnesota Research

Researchers at the University of Minnesota sampled vegetables and fruits on Minnesota farms, both organic and conventional, and tested for the presence of several types of bacteria.

The citation for the article is below. Contact Jane Jewett to request a copy by e-mail.

Avik Mukherjee, Dorinda Speh, Elizabeth Dyck, and Francisco Diez-Gonzalez. 2004. Preharvest Evaluation of Coliforms, Escherichia coli, Salmonella, and Escherichia coli O157:H7 in Organic and Conventional Produce Grown by Minnesota Farmers. Journal of Food Protection: Vol. 67, No. 5, pp. 894-900.

Not all organisms found on vegetables are dangerous to humans.  Some coliform bacteria (“coliform” means similar to E. coli) are frequently found on vegetables, and are not a health risk, but they may indicate contact with manure. The presence of E. coli is generally considered to be an indication that the vegetables did come in contact with manure, but most E. coli are harmless. E. coli O157:H7 is the dangerous variety responsible for food-borne illness in humans. Salmonella can also cause food-borne illness in humans.

The researchers recruited organic and conventional vegetable growers in Minnesota for this study.  They allowed growers to self-identify as not certified organic but using organic practices.  They actually analyzed three groups:  conventional growers, growers who self-identified as organic but were not certified, and certified organic growers.  They found no significant differences in bacteria levels between vegetables from conventional and certified organic growers.  They found that growers who used manure or compost aged less than 12 months had vegetables with coliform bacteria levels 19 times higher, on average, than those who used a longer aging process.

Unfortunately, the study was not clear enough about separating certified organic farmers from non-certified farmers.  The certified farmers in the study followed National Organic Program rules for handling manure, and some of the non-certified farmers did not.  This made the results confusing, and the study has been misrepresented in the media as proving that organic vegetables are more contaminated than conventional vegetables.  In fact, the study provided some evidence that certified organic methods reduce the chance of contamination of leafy greens:
  • One non-certified farm had an extremely high percentage (92%) of all vegetables testing positive for E. coli.  This farm had spread fresh manure on vegetable fields during the growing season, in clear violation of National Organic Program standards.
  • E. coli prevalence on lettuce was zero for certified organic farms (zero out of 10 samples), 16.7% for conventional farms (one out of six samples), and 30.8% for non-certified farms (12 out of 39 samples).
  • Similarly, E. coli prevalence on leafy greens was zero for certified organic farms (zero out of 19 samples), 25% for conventional farms (one out of four samples), and 13.8% for non-certified farms (9 out of 65 samples).
  • No E. coli O157:H7 and no Shiga-toxin producing E. coli were found on any produce samples.  E. coli O157:H7 is the dangerous variety of E. coli that is responsible for outbreaks of food-borne illness.  The E. coli varieties found in this study were non-dangerous varieties.
  • Salmonella was found on two produce samples from non-certified farms.
Some of the same researchers were involved in a follow-up study with more farms and more samples of produce.  The citation for this article is below, or read the full article (PDF, 150 kb :

Avik Mukherjee, Dorinda Speh, Aaron Jones, Kathleen Buesing, and Francisco Diez-Gonzales. 2006. Longitudinal Microbiological Survey of Fresh Produce Grown by Farmers in the Upper Midwest. Journal of Food Protection, Vol. 69, No. 8, 2006, Pages 1928–1936

In this study, the farm types were clearly separated into organic, “semiorganic” (meaning self-identified as organic but not certified), and conventional.

Samples were taken from farms in 2003 and 2004. As in the previous study, none of the dangerous E. coli O157:H7 was found on any produce sample. This study also found no Salmonella.

E. coli was found on a greater total percentage of semiorganic and organic vegetable samples than conventional samples. However, this was partly because the greatest percentage of leafy greens and lettuce samples came from the semiorganic and organic farms. Leafy greens and lettuce made up 27% to 35% of all samples from the semiorganic and organic farms, but only 5% to 8% of all samples from the conventional farms.

The researchers found that leafy greens, lettuces, and cabbage were the types of produce most susceptible to E. coli contamination. They took samples directly from the field, and there was sometimes visible dirt on the samples. They noted that other studies at the retail level have found little contamination of these produce types, and that these items are typically washed prior to sale.

Additional Resources

The Organic Center published a detailed report by Dr. Charles Benbrook called “E. coli O157:H7 Frequently Asked Questions.” (link to webpage housing the PDF, 1.35 Mb).

The report documents sources and spread of E. coli O157:H7, and provides an extensive literature review on pathogenic strains of E. coli. It also covers published research that suggests how farming systems and food safety practices need to be changed in order to prevent similar episodes in the future.