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Microbial Food Safety: An Emerging Challenge for Small-Scale Growers

The following article is adapted from a "work in progress" research report by Trevor Suslow, extension specialist in postharvest technology, Department of Vegetable Crops, UC Davis

Farming practices that emphasize the use of raw animal manure, manure slurries or "teas," and animal manure-based compost play an important role in the recycling of organic nutrients and developing a rich soil structure. However, due to the increasing frequency of outbreaks of food-borne pathogens, serious concern has been raised for a different type of recycling through our agricultural production systems. The recycling of bacterial pathogens and protozoan parasites from animals to humans through water, soil and crops has created a serious challenge for producers, processors, and consumers of fresh produce. Researchers at the University of California, Davis, and other academic, government, and private institutions are beginning to address ways to understand the environmental persistence and control points for these pathogens of global concern.

In Japan in 1996, an Escherichia coli (E. coli) outbreak killed, nine people, 30 people were reported in critical condition, and a total of 8,500 cases were recorded. The suspected cause of the outbreak was salad, with sprouted seeds being the primary suspected source of the foodborne contamination. Without question, foodborne illness has emerged as a major worldwide issue impacting production, processing, domestic and export marketing, and consumer confidence in the food supply [Center for Disease Control, 1997]. Wholesale buyers are starting to expect documentation of prevention and critical control point programs for food safety down to the farm level. In the immediate future, agricultural producers will be charged with establishing and documenting methods of risk reduction and prevention. The need exists to specifically address methods of prevention and reduction of microbial risks as a systemic program of postharvest quality and safety of specialty fruits and vegetables.

Although there are several other food pathogens of concern, this article focuses on E. coli O157:H7 in recognition of the elevated consequences of infection from noncooked produce. E. coli O157:H7 is the predominant strain of a group of toxin-producing E. coli. Common E. coli is an ubiquitous intestinal inhabitant. The toxigenic forms, such as E. coli O157:H7, have been an increasing problem since first identified in 1982. Although far fewer cases have been recorded than of Salmonella cases, E. coli O157:H7 is more hazardous, causing the life-threatening condition Hemolytic Uremic Syndrome (HUS), which results in acute kidney failure. An additional risk feature associated with E. coli O157:H7 is the very low number of contaminating cells required for infection. Estimates of one to five bacteria per gram of food may be sufficient to cause infection in sensitive individuals. The population sector most at risk are the very young, the elderly, women during pregnancy, and immunocompromised individuals.

With so few bacterial cells necessary, growth on infested produce is not a requirement for human infection, as with most other pathogens. Therefore, refrigeration of harvested produce is not a sufficient control. In addition, due to the low numbers, absence of detection is not a foolproof assurance of safety. Screening of water sources or harvested produce is not a practical approach to control. Prevention and sanitation are the key tools we must use. Awareness of the characteristics and sources of this pathogen and exposure to the impact of its potential presence in agricultural production and produce distribution are important first elements towards developing local strategies for microbial risk reduction in our food supply.

Known Sources of E. coli O157:H7

E.coli O157:H7 has been found in reservoir and recreational water and in water sources used for overhead irrigation of vegetables. It also has been detected in the feces of many animals including dairy and feedlot cows, poultry (especially chicks), lamb, piglets, children, pets, deer, and waterfowl. Clearly, intermingling farm animals and vegetables during production is not a recommended practice. E. coli O157:H7 has been shown to persist in drying manure and to be present in incompletely composted dairy and feedlot waste. Persistence in manure amended soils is not well characterized and is the subject of current research efforts at UC Davis. In some agricultural systems, raw manure may be surface applied or incorporated into soil at various time intervals prior to planting or harvest. We are in the initial stages of determining what preplant time intervals are required to minimize the risk of crop contamination. Currently, the recommended interval is 60 days.

The transference of E.coli O157:H7 from these sources to the harvested portion of fruits and vegetables may seem logical and predictable, but little documented evidence for their environmental behaviors is available. This information will be critical in the development of guidelines for the safe handling and application of animal manures to farm land, particularly for vegetable production systems.

Unique Characteristics of E.coli O157:H7

What makes E. coli O157:H7 and other related strains of E. coli a threat to humans is E. coli's acquired ability to produce toxins and other virulence factors. In addition, recent research has shown that the E. coli O157:H7 strain is more resistant than standard E. coli to dry conditions, freezing, and acid conditions. Environmental stresses that inactivate conventional E. coli are much better tolerated by E.coli O157:H7.

Growth of E. coli O157:H7 on Vegetables

To date, published research on the fate of E. coli O157:H7, introduced to vegetables as a model for inadvertent contamination from water, soil, or nonhygienic human activities, has dealt primarily with risk assessment analysis of shredded lettuce packaged to create a modified atmosphere condition. Once introduced to lettuce, or other test vegetables including cucumber, cantaloupe, watermelon, alfalfa sprouts, and radish sprouts, survival and growth under permissive temperature conditions is likely to occur.

These permissive temperature conditions are not uncommon in the mainstream distribution and food service chain and may be more likely in small-scale operations and consumer-direct outlets. Elevated CO2 and low O2 had no preventive or curative effect. The best approach, at this time, is a comprehensive prevention and on-farm risk management program.

What Controls Are Needed?

Until more specific information is available about the environmental dissemination and persistence of E. coli O157:H7 and other key pathogens, common sense approaches to on-farm microbial safety will go a long way to minimizing the risk of foodborne illness. Some farming practices that were considered safe in "the good-old-days" are a current liability. Some new practices developed as a source of supplemental organic nutrients and pest control (foliar applied manure slurries) seem ill-advised without greater process control information. Awareness of the known traits of these microbes that make them a threat will help each individual grower and handler of fresh produce design prevention and control measures specific to their cropping situation or postharvest system. Common-sense controls include:

Don't apply raw dairy or chicken manure or slurries to an existing vegetable crop such as leafy lettuces. Don't apply manure to an area immediately adjacent to a field nearing harvest maturity. Don't apply manure to one area of the farm and then move equipment or personnel to a field in production without a cleanup procedure. Don't apply recycled water from a farm pond to crops such as "spring mix" greens. Don't harvest fruit from the orchard floor for human consumption as whole fruit or nonpasteurized juices; most importantly where manure has been spread or animals are allowed to graze. Don't accumulate harvested product in areas where birds roost. Editor's note: If you have on-farm control suggestions, send them to: Small Farm News, Small Farm Center, University of California, Davis, CA 95616, or call (916) 752-8136.

Like most E. coli, type O157:H7 is sensitive to chlorine when the disinfectant can physically contact the bacterial cell. Preharvest contamination-prevention programs and postharvest sanitation are key tools to preventing outbreaks. On-farm prevention programs should include basic sanitation practices for all harvest containers, contact surfaces, and postharvest washing. Washing fruit and vegetables with clean, domestic (potable) water removes many undesirable surface contaminants. Although not an assurance of complete safety, disinfestation is an essential process when produce intended for commercial sale is washed ( to remove soil, debris or reduce decay on surfaces wounded or cut during harvest. Some pathogens such as Cryptosporidium are very resistant to chlorine and even sensitive ones such as Salmonella and E.coli may be located in inaccessible sites on the plant surface. Wash water for the majority of vegetables should be maintained in the range of 75-150ppm (parts per million). Registered hypochlorite liquids (5.25% and 12.75% active ingredient) are inexpensive and readily available.

Table 1. Amounts of hypochlorite to add to clear, clean water for disinfestation.

target ppmml/Loz/5 gal approx.cup/50 gal
Sodium Hypochlorite (5.25%)
50 .95 .55 .5
75 1.43 .80 .75
100 1.9 1.1 1.0
125 2.4 1.4 1.25
150 2.9 1.7 1.50
Sodium Hypochlorite (12.75%)
50 .2 .12 .1
75 .3 .17 .15
100 .4 .23 .2
125 .5 .29 .25
150 .6 .35 .3

Maintain a neutral solution pH (6.5 to 7.5). The active antimicrobial form, hypochlorous acid, is most available in this range.

Effective chlorine concentrations are reduced by temperature, light, and interaction with soil and organic debris. The wash water should be tested periodically with a monitoring kit, indicator strips, or a swimming pool-type indicator kit. Concentrations above 200ppm can injure some vegetables [such as leafy greens and celery] or leave undesirable off-flavors. For certified organic growers, a maximum of 4 ppm residual chlorine is permissable, measured downstream of the product wash.

What Can Researchers Do?

Our objective as researchers is to obtain baseline and directional information to identify where data gaps exist and could be addressed in future near-term research. Immediate-term information is needed to guide growers, Cooperative Extension, the diagnostic service industry, shippers, and processors in the development of on-the -farm management practices to prevent these microbial pathogens from being introduced during production and at harvest. Areas we are beginning to address include:

  • information on sources and persistence
  • manure compost process control
  • timing of incorporation relative to crop seeding and harvest
  • depth of incorporation into soil to minimize persistence
  • potential for establishment on plant parts during production
  • postharvest prevention programs

We are particularly interested in understanding the environmental behavior of E. coli O157:H7 in relation to farming practices that use animal manure-based organic amendments.

A Team Effort

Agriculural and produce industry associations are working together to establish voluntary guidelines for microbial risk reduction. Research in UC Davis departments is providing the information database to assist in the development of these good farming practices. Small-scale farmers and distributors must make food safety a top priority and take an active role in creating practical guidelines that will allow the continued flow of quality produce to the world.

UC Resources You Should Know About

Dean Cliver, professor, School of Veterinary Medicine, Microbial Food Safety and Population Health, UC Davis, (530) 754-9120.

James Cullor, director, Veterinary Medicine Teaching & Research Center, Dairy Food Safety Lab, Tulare, CA. (209) 688-1731.

Linda Harris, extension specialist, Microbial Food Safety, Department of Food Science, UC Davis, (530) 754-9485.

Deanne Meyer, extension specialist, Animal Waste Management, Department of Animal Sciences, UC Davis, (530) 752-9391.

About the author: Trevor Suslow, who joined the Small Farm Program Workgroup in 1996, received his doctoral degree in plant pathology from UC Berkeley. He is currently developing methods of control of postharvest pathogens for both fresh and fresh-pack produce.