Selected Articles from Past
Small Farm News Newsletters
(June/July 1997 issue) 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
ppm |
ml/L |
oz/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.
