Introduction to Vegetable Science

Pesticide Residues

• Monoculture production systems were established to allow greater mechanization, helping farmers manage larger areas with less labor.
• Monoculture systems are more prone to pest outbreaks due to ecological imbalance.
• Pesticides were developed to control insects, plant diseases, and weeds in monoculture production systems.
• From the 1940s through the 1970s pesticide usage increased steadily as new chemicals were made for pest control.
• Public concern grew in the 1970s and 1980s regarding pesticide residues on vegetables and their negative effects on human health.
• Serious illnesses and death were traced to pesticide residues, focusing attention on the issue.
• Poisoning was caused by improper use of aldicarb (Temik) on watermelons and cucumbers.
• In 1985, the largest pesticide-related foodborne outbreak in the USA occurred with 1,373 illnesses reported after eating aldicarb-treated watermelons. 78% of those people had probable or possible pesticide-related illnesses.
• The pesticide manufacturing disaster in Bhopal, India, also helped focus attention on agricultural pesticides in the 1980s.
• These incidents fueled public interest in organic and naturally grown produce without synthetic chemical inputs.
• Recent data shows pesticide usage is reducing due to sustainable production practices.
• Pesticide use in California decreased for a fourth consecutive year in 2009
• Factors leading to decreased pesticide use include better education, greater use of biological controls, reduced use of soil fumigants, adoption of integrated pest management, loss of approved pesticides, increased pesticide costs, and more effective pesticides.
• Pesticide awareness is increasing in developing countries, but misuse and overuse persist.

Biological Contamination of Vegetables

• Increased availability and health benefits of fresh fruits and vegetables have increased demand.
• Consumer demand has increased quantity and a variety of convenience packaged products are available to consumers.
• Eating fresh vegetables is not without risk
• Since the 1990s, the public has been informed of a new threat to vegetable safety, illness caused by biological contamination.
• Biological contamination is the presence of living organisms such as viruses, bacteria, fungi, or agents from these organisms, and mammal or bird antigens affecting human health.
• Outbreaks of biological contamination are often associated with Cyclospora, Hepatitis A, Salmonella enterica, Escherichia coli O157:H7, and Listeria monocytogenes, particularly on salad greens like spinach and lettuce.
• Biological contamination of vegetables is a significant health risk for consumers.
• Producers must work to prevent infection by biological contaminants.
• Escherichia coli O157:H7 and S. enterica are enteric pathogens that infect a broad range of hosts, including humans.
• Due to its severity and low infective dose, E. coli O157:H7 is considered one of the most serious foodborne pathogens.
• Biological contamination of fresh vegetables has likely always existed, viewed as "food poisoning” before modern detection techniques.
• Produce-related illnesses accounted for only 0.7% of food-borne illnesses in the 1970s, compared to 6% in the 1990s.
• During that 20-year period, 54% of the produce-related illnesses were associated with known pathogens.
• Of the pathogens, 60% were bacteria, of which 48% were Salmonella spp., while Cyclospora and E. coli O157:H7 were newly recognized.
• Since 1982, E. coli O157:H7 has been identified in many countries as a cause of hemorrhagic colitis resulting in sickness or death.
• A US study between 1991 and 2004 showed that outbreaks were most commonly associated with lettuce (34%), salad mixes (11%), coleslaw (11%), melons (8%), and sprouts (2%), while fruit crops accounted for 44% of reported contamination events.
• Biological contamination cases have continued to be numerous and widespread between 2000 and 2010.
• A serious E. coli O157:H7 outbreak in spinach in the USA caused 171 illnesses and three deaths in 2006.
• The spinach contamination event was related to a smaller outbreak in lettuce shortly thereafter.
• Understanding how biological contaminants survive in nature and become established on vegetables is important to vegetable production.
• Native benign bacteria are common on vegetables, adapted to the conditions specific to the microenvironment on vegetable leaves.
• This environment provides low nutrient and water availability, high exposure to UV radiation, and rapid environmental changes.
• The interactions between the plant and native microorganisms and microbes are pivotal for establishment of pathogenic bacteria on vegetables.
• Enteric human pathogens also interact with other phyllosphere bacteria, affecting pathogens establishment within vegetable leaves and its spread to humans.
• Once a field is contaminated by E. coli O157:H7 or other human pathogens, attachment is the first step in colonizing a vegetable surface.
• Escherichia coli O157:H7 may use its attachment mechanism to attach to animal and human epithelial cells to attach to vegetables.
• Vegetables become biologically contaminated with E. coli O157:H7 when they come in contact with contaminated manure, tainted irrigation waters, contaminated sewage sludge, or farm workers who practice poor sanitation.
• Escherichia coli and other pathogenic organisms may find niches in the phyllosphere where they can proliferate, complicating their removal from fresh produce.
• Consumption of contaminated, fresh vegetables and minimally processed ready-to-eat vegetables are likely vehicles for foodborne disease.
• The food industry has developed its own strategies to reduce contamination risk in minimally processed vegetables.
• Strategies include the use of gamma irradiation, chlorination, and modified atmosphere packaging, all in combination with storage at low temperatures.
• Decontamination techniques for minimally processed bagged vegetables may not always adequately control microbial populations.
• Refrigeration, shrink-wrapping, or bagging may multiply human pathogens under certain conditions.
• Bacteria may colonize around cut vegetable surfaces or damaged plant tissues using leaked nutrients to feed their growth.
• E. coli O157:H7 is able to live inside lettuce plant tissues, not only on leaf surfaces, as was shown after its recovery from surface-sterilized leaves.
• E. coli O157:H7 inoculated on to lettuce persisted for up to 20 days.
• Studies showed that when human pathogens were introduced on to spinach seeds they concentrated on root junctions shortly after germination, even though E. coli O157:H7 was not necessarily detected in mature leaves.
• Leafy vegetables are capable of supporting biological contaminants because they have a pH greater than 4.6 and high water content
• Avoidance of contamination, improved sanitation, and preservation treatments are primary strategies for avoiding human disease transmission.

Sources of Biological Contamination of Vegetables

• Livestock, particularly cattle, are considered to be natural reservoirs for bacteria that cause human disease, such as E. coli.
• Infected cattle can spread E. coli O157:H7 to humans through their feces.
• Other species can also asymptomatically carry and shed this bacterium in fecal material, potentially contaminating water, soil, and crops.
• Other sources of E. coli O157:H7 contamination may include animal excreta or workers.
• Escherichia coli O157:H7 has been isolated from deer, dogs, ducks, kangaroos, and wild pigs.
• Feces from feral swine may have caused contamination of spinach by E. coli O157:H7.
• Feral swine, containing the same strain of E. coli as cattle from a nearby dairy, likely contaminated spinach fields while they passed through.
• Insects like the lesser mealworm, fruit fly, and housefly can transmit E. coli O157:H7.
• Animal manure is sometimes used as an organic fertilizer in vegetable crop production.
• Vegetable contamination by human pathogens may occur following application of poorly composted or raw manure.
• Biological contamination may occur in conventional or organic vegetable production systems.
• Animal feces may serve as a reservoir for E. coli O157:H7, proper handling of manure for vegetable production is critical for preventing bacterial transmission.
• Proper composting reduces or eliminates bacterial pathogens, parasites, fly larvae, and weed seeds while converting manure and other materials into safe fertilizer, soil amendments, or potting media.
• Internationally, organic production guidelines call for composted manure to reach a critical temperature to kill harmful organisms and restrict the application of uncomposted manure to vegetable crops.
• However, if these procedures are not carefully followed, the risk of contamination is great.
• During composting, temperatures increase due to bacteria and yeast metabolism.
• During the heating phase of composting, human pathogens are inactivated, but if the composting process fails to reach the threshold temperature or the duration of composting is insufficient, the compost may harbor biological contaminants.
• Contaminated livestock and poultry manure may degrade water quality, particularly if manure is over applied and enters irrigation water supplies through runoff or leaching.
• Large-scale commercial livestock and poultry operations in areas where fresh vegetables are grown have increased risk of microbial contamination due to improperly handled animal wastes polluting irrigation water.
• Irrigation water may be contaminated by the leaching of pathogens through soil into ground water.
• Irrigation method affects vegetable contamination risk.
• Drip or trickle irrigation systems have lower risk because water is applied directly to the soil.
• Sprinkler irrigation wets the entire vegetable surface, increasing the risk of crop contamination.
• Soil is directly contaminated by application of animal waste or indirectly through rainwater or irrigation runoff.
• Soil type, tillage practice, and rainfall amounts determined the degree of leaching.
• Ammonia and nitrate concentrations in soil were positively correlated with the leaching of E. coli O157:H7, which persisted in soils for more than 5 months after application of contaminated compost or irrigation water.
• Soil microcosms inoculated with E. coli O157:H7 showed long-term persistence in the phyllosphere, rhizosphere, and non-rhizosphere soils.

Good Agricultural Practices/Good Handling Practices

• Good Agricultural Practices (GAP) and Good Handling Practices (GHP) refer to any collection of specific methods, which when applied to agriculture, produce the objectives of the proponents of those practices.
• There are numerous competing definitions of what production and handling methods constitute GAPs and GHPs.
• GAP concept has changed because of rapidly changing agriculture, globalization of world trade, food crisis, water pollution, pesticide resistance, and soil erosion.
• One of the primary objectives of most GAP programs is to help preserve water and soil resources from biological contamination.
• Certifications such as GlobalGAP, EurepGAP, and ISOGAP are designed to improve food security, particularly of internationally traded commodities.
• Some governments have developed their own national GAP standards.
• Anyone in vegetable production or handling should be familiar with the applicable national and local GAP/GHPs.
• "Good Agricultural Practices” defined by the Food and Agriculture Organization (FAO) of the UN pertain to broad-based principles applying to on-farm production and post-production, resulting in safe and healthy food and non-food agricultural products, while taking economic, social, and environmental sustainability into account.
• FAO GAPs may be applied to diverse farming systems, including both organic and traditional, and can be applied to both large and small farms.
• The four overarching principles of the FAO GAP program are:
  • Economically and efficiently produce sufficient, safe, and nutritious food.
  • Sustain and enhance natural resources as a component of food production.
  • Maintain viable farming enterprises and contribute to sustainable livelihoods of people involved in agricultural production.
  • Meet cultural and social demands of society.
• FAO GAPs that apply to soil include reducing erosion, use of fertilizers when the plant needs the fertilizer, avoiding runoff, maintaining/restoring soil organic content, reducing soil compaction, and in situ green manure applications by growing legumes.
• Agricultural water management GAPs include scheduling irrigation based on plant needs, reducing soil salinization by limiting water input, recycling water, avoiding crops with high water requirements, avoiding drainage and fertilizer runoff, maintaining soil covering, managing water tables, maintaining wetlands, providing water points for livestock away from irrigation water sources, preventing erosion, and strip-cropping across slopes.
• The United States Department of Agriculture (USDA) has developed an audit/certification program to verify that farms use GAPs and/or GHPs that do not harm the environment and control the spread of biological contaminants in our food supply.
• Unlike the FAO guidelines, the USDA guidelines focus on food safety, and do not address animal welfare, social sustainability, biodiversity, or the use of antibiotics and hormones.
• The USDA GAP/GHP guidelines and principles grew out of the 1998 Food and Drug Administration publication entitled, “Guide to Minimize Microbial Food Safety Hazards for Fresh Fruits and Vegetables.”
• The program was developed after state governments petitioned the USDA to create a GAP and GHP audit program due to wholesale buyers asking farmers to demonstrate their adherence to recommendations.
• “GAPs metrics” allow those who monitor GAPs to determine their effectiveness, by using threshold values.