Critical Management Point: Post Harvest

Critical Management Point: Post Harvest

Transcript (with additional commentary)

[Monica Ponder, Department of Food Science and Technology, Virginia Tech]:
“I am an Associate Professor in the Department of Food Science and Technology, and I am the co-investigator leading the studies to examine how post-harvest washing with sanitizers, particularly peroxyacetic acid and sodium hypochlorite, affect the microbial resistance associated with fresh produce.”

According to Dr. Ponder, depending on the type of vegetable, millions of bacteria normally live on fresh produce.  But how much of the bacterial load is from compost?  And can we use techniques already in use in the food industry to remove these types of bacteria?

[Amy Pruden, Department of Civil and Environmental Engineering, Virginia Tech]:

“The final stage is actually once you’ve harvested the vegetables, how do you wash them? And, there’s a huge market for pre-washed, cut, packaged vegetables in the grocery store. I know that I, myself, as a working mom, appreciate being able to go to the store and buy some fresh lettuce that’s already cut and washed for me.”

Washing fresh vegetables is routinely used to remove soil and debris, improve the quality and appearance, and enhance the product shelf life, at least partially by decreasing the microbial load.

Commercial producers and processors use different sanitizers, concentrations, and contact times dependent on the vegetable, types of equipment used, and the final market for the crop–either fresh, packaged ready- to-eat, or frozen.

Chemical sanitizers are routinely used to minimize the spread of microbial contaminants in recirculating wash water. However, these surface-sanitizing agents are ineffective in reducing bacterial populations that have penetrated into the leafy tissues.

In this study, broccoli, carrots, and lettuce were purchased, and then either inoculated with very small amounts of compost from the previous studies, or left uninoculated. These vegetables represent different parts of plants that we eat – flowers, roots, and leafy greens.  Also, each crop type interacts with bacteria and soil in different ways.

The plants were then washed with either tap water, peroxyacetic acid, or bleach, or left unwashed.  The washes represent commonly used practices by the produce industry to cool and sanitize vegetables after harvest and before sale.  After this step, samples were tested.

To see if storage practices influence bacterial development, the plant samples were then packaged into bags, stored under refrigeration, and then re-tested.

In addition, another set of inoculated samples were irradiated, using FDA-approved practices, prior to testing.  Ionizing irradiation penetrates vegetable tissues and has been shown to reduce or eliminate surface and internalized E. coli in lettuce and spinach.

In 2008, the FDA approved irradiation for lettuce and spinach, and rulings for other vegetables are under consideration. Though irradiation is not possible in on-farm post-harvest cleaning, research is underway in other facilities to look at its effectiveness to reduce bacterial load.

This study will provide guidance on which post-harvest handling and washing techniques are most effective at reducing antibiotic resistant bacteria and resistance genes.

Glossary of Terms

  • Antibiotic resistance genes – Pieces of DNA carried by bacteria that encode functions that allow bacteria to survive and grow in the presence of antibiotics.
  • Antibiotic resistant bacteria – Bacteria that are not controlled or killed by an antibiotic with a spectrum that includes that type of bacteria.  An antibiotic spectrum is the range of bacteria susceptible to that drug. (CDC, Antibiotic Resistance,
  • Bacterial load – Measurable quantity of bacteria on/in an object or organism.
  • Inoculation – Deliberate introduction of small amounts of microorganisms to allow them to grow.  In this experiment, the dip inoculation method was used.  The plants are held in a bacterial suspension for a set amount of time to expose them to certain bacterial levels. (Davidson, G.R., Buchholz, A.L., Ryser, E.T., (2013). Efficacy of commercial produce sanitizers against nontoxigenic Escherichia coli O157:H7 during processing of iceberg lettuce in a pilot-scale leafy green processing line. J. Food Prot. 76, 1838-1845.)
  • Irradiation – Exposing “food, either packaged or in bulk, to carefully controlled amounts of ionizing radiation for a specific time to achieve certain desirable objectives,” namely to reduce disease causing organisms while not changing the nutritional value of the food.  (UW Food Irradiation Education Group,  Tauxe RV. (2001) Food Safety and Irradiation: Protecting the Public from Foodborne Infections . Emerg Infect Dis. Vol 7 Num 7. )


  • Beuchat, L.R. 1999. Survival of enterohemorrhagic Escherichia coli O157:H7 in bovine feces applied to lettuce and the effectiveness of chlorinated water as a disinfectant. J. Food Prot. 62, 845-849.
  • FDA. 2008. 21 CFR Part 179: Irradiation in the production, processing and handling of food. DHHS.
  • Niemira, B.A. 2007. Relative efficacy of sodium hypochlorite wash versus irradiation to inactivate Escherichia coli O157:H7 internalized in leaves of romaine lettuce and baby spinach. J. Food Prot. 70, 2526-2532.