To be effective, a postharvest food safety program must rely on pest control, traceability and recall, employee health and hygiene, and effective cleaning programs. In addition, suppliers and distributors must have a food safety program with the development of ingredient specifications and a quality assurance system for vendors. Food safety issues should be addressed at multiple stages from cultivation and field production, through harvest, and throughout the postharvest production process. Reducing food safety risks at each stage of food production can result in a very large cumulative reduction in risks from farm-to-consumer.

Concerns about food safety affect all aspects of postharvest engineering. When food is unsafe to consume, all postharvest efforts along the supply chain are lost. When an outbreak occurs, there are significant negative effects on public health, as well as significant costs to society from tracking and eliminating the threat to food safety. Unfortunately, huge amounts of perfectly safe produce must be destroyed because it is suspected of being contaminated.

Fresh produce is generally grown outdoors where foodborne pathogens are everywhere and difficult to eliminate. Since much produce is eaten raw or with minimal processing, the risk of an outbreak is greatly multiplied. The first step in food safety is understanding the nature of the hazards.

A food safety hazard can be either a biological, chemical, or physical agent that is reasonably likely to cause illness or injury. The term “illness” is used to describe a condition in which the health of individuals is adversely affected from the ingestion of a contaminated food.

Biological Hazards

Biological hazards are pathogenic microorganisms that create a potential health hazard. Many pathogens are enteric, which attack the gastrointestinal (GI) tract via the fecal-oral route of exposure. Some pathogens are also zoonotic, which means they are capable of infecting animals as well as humans and have wild or feral animal reservoirs.

There are three classes of microorganisms that may be pathogenic: bacteria, viruses, and parasites. All three might attack the host directly and cause adverse health effects. Bacteria can also produce toxins that harm the host and weaken the host’s natural defenses. Not all microbes are pathogenic and harmful. Some, like yeast or molds, are organisms that spoil food and can adversely affect product quality but are not considered biological hazards.

One of the key characteristics of bacterial pathogens is that they reproduce in the environment. Viruses and parasites are “obligate intracellular parasites,” which means that they can only reproduce in a susceptible host. The infectious dose of bacteria is generally much higher than that for viruses and parasites. However, with time and temperature abuse, bacterial pathogens populations can grow to levels that can easily meet the infectious dose.

There are two types of foodborne illnesses that are caused by pathogens. The first is infection that results from ingesting the organism, which then multiplies inside the body (usually starting in the gastrointestinal tract) and causes adverse health effects in the host. These “invasive” infections include E. coli (Escherichia coli), Salmonella, and Listeria (Listeria monocytogenes). The second type of illness associated with foodborne pathogens are caused by toxins produced by bacteria, which are either toxico-infections or intoxications. Toxico-infections occur when the pathogen invades a susceptible host, multiplies, and produces toxins in the body. Intoxication occurs when the bacteria produce a toxin while in the environment prior to ingestion by a susceptible host. The host ingests the toxin directly, which then causes adverse health effects. Clostridium botulinum is an example of a bacterial pathogen that produces a deadly toxin, while yeast can produce ethanol, which can lead to intoxication.

Outbreaks of foodborne disease are a significant health issue. Despite continuing public health efforts, the US Center for Disease Control and Prevention (2023) estimated in 2011 that each year 48 million people become ill from a foodborne illness, 128,000 are hospitalized, and 3,000 die.

The Foodborne Diseases Active Surveillance Network (FoodNet) of CDC’s Emerging Infections Program monitors cases of laboratory-diagnosed infection caused by eight pathogens transmitted commonly through food in 10 US sites (Connecticut, Georgia, Maryland, Minnesota, New Mexico, Oregon, Tennessee, and selected counties in California, Colorado, and New York.). More recent data from FoodNet show the cases, hospitalizations, and deaths from foodborne illnesses (see Table 11.1). According to FoodNet, Campylobacter is responsible for the greatest number of foodborne illnesses in 2018, and has been the most commonly identified infection by FoodNet since 2013.

Table 11.1. Foodborne Illnesses in the US — 2018*

*Tack, et al. 2018. Mortality and Morbidity Weekly Report 68, no. 16: 369–373.

Today in the US, many foodborne illnesses go unreported because our country has a relatively passive surveillance system for tracking such illnesses. Since there are only a small number of reportable illnesses, much of the information needed for tracking illnesses is lost at the level of the physician, hospital, or local health department. In addition, many foodborne illnesses are attributed to viral pathogens, with about 50% of all foodborne illness associated with norovirus (Steele et al. 2020). Until recently, most physicians and local health departments lacked the type of sophisticated equipment or trained personnel needed to detect foodborne viruses. Retrospective studies are also difficult to conduct because few individuals who are infected can recall what they have eaten during the potential incubation period of many pathogens. However, new and improved isolation and identification methods for detecting and quantifying microbial pathogens in complex foods are now available.

Outbreaks of foodborne illness are somewhat difficult to control and prevent because there are many different sources and routes for microbial pathogens to move around in the environment. The routes of microbial contamination for pre-harvest food are often described as the “Four W’s”: water, worker, waste, and wildlife.

Since most fresh produce is grown outdoors, fecal contamination from birds and wildlife is a major concern. For example, birds sitting on a power line that crosses a produce field may be responsible for Salmonella contamination. Many outbreaks of foodborne disease have been associated with contaminated irrigation water or with animal manure that has been applied as a fertilizer for organically grown produce. Microbial contamination from feces can remain viable for months or even years in the soil or on harvesting and handling equipment.

Another major source of contamination is the poor personal hygiene of workers. One of the most common and most contagious foodborne pathogens is norovirus, which is also known as the stomach flu or the “cruise ship disease.” According to the National Center for Environmental Health (2022), norovirus is the most frequent (over 90%) cause of outbreaks of diarrheal disease on cruise ships, which often attract attention in the press. However, in reality, norovirus outbreaks on cruise ships account for only a small percentage (1%) of all reported norovirus outbreaks. One large 2020 study by Steele et al. of more than 7000 outbreaks found that 75% occurred in long-term care and assisted living facilities and 57% occurred in the winter. The National Center for Immunization and Respiratory Diseases (2023) reported that every year in the United States, norovirus causes on average:

• 900 deaths, mostly among adults age 65 and older
• 109,000 hospitalizations
• 465,000 emergency department visits, mostly in young children
• 2,270,000 outpatient clinic visits annually, mostly in young children
• 19 to 21 million illnesses

A very small amount of the pathogen (less than 100 viruses) is needed to pass the disease from person to person. Worldwide, norovirus is the leading cause of acute gastroenteritis that leads to diarrhea and vomiting. Many large outbreaks have been traced back to only one ill individual who worked in a food-service setting. Fresh produce, which is often harvested and packed by hand and then eaten raw by the consumer, is the perfect vector for the disease. Norovirus contamination has been found in a wide variety of fruits and vegetables, although the largest number of outbreaks has been associated with leafy greens, which are eaten raw and have a large surface area with creases and folds that can trap microbial organisms.

Below is a list of common foodborne pathogens. Although they all can be transmitted by contaminated fresh produce, some are much more commonly found in meats.

Table 11.2. List of common foodborne pathogens

Controlling Biological Hazards

There are three primary strategies for controlling biological hazards. Taken together and practiced diligently, these strategies can be very effective in reducing outbreaks.

1. Prevent contamination of foods. (Keep them out.) This begins with a full review of the food production system from farm inputs, such as seeds and fertilizer, and all the way through shipping and retail. Knowing where and how foods become contaminated can identify weak spots in the system and lead to improvements. Continual environmental monitoring is the key to prevention.
2. Inactivate foodborne disease agents. (Kill them.) Many foodborne pathogens are simply difficult to kill. These agents have developed ways to survive, although heat and chemicals can be the most effective means of control. UV light and ionizing radiation are effective in some, but not all cases. Any efforts to inactivate disease agents must be thorough to be successful, because serious outbreaks have been traced to incomplete efforts.
3. Prevent multiplication of pathogens. (Stop them from multiplying.) The key to preventing multiplication is understanding the specific conditions required for their growth. These conditions include a susceptible host, a virulent pathogen, and favorable environmental conditions. This is known as the disease triangle, as shown in Figure 11-1.

Reduction or limitation of any single factor can control foodborne pathogens. Since pathogens are everywhere in the environment, their complete elimination is nearly impossible. Likewise, the same might be said for susceptible hosts since these are humans and animals. The most productive approach to controlling pathogens is creating favorable environmental conditions that prevent growth and spread. Factors that affect pathogen growth include temperature, time, oxygen, pH, and water activity. High or low temperatures can control some but not all foodborne pathogens. Cooking eliminates many pathogens but in general, freezing does not eliminate pathogens. With fresh produce, neither freezing nor cooking make a difference. For example, psychotropes such as Listeria can live and grow from 32°F to 70°F. Esophiles such as Campylobacter and Salmonella can grow and survive at temperatures as high as 110°F. Thermophiles such as Clostridium botulinum can survive temperatures above 180°F for short periods, which is the reason consuming improperly canned foods can be so deadly.

A bacterial pathogen that is established can multiply exponentially in favorable conditions. Some pathogens are aerobic and can only grow in the presence of oxygen, while others are anaerobic and grow only without oxygen. Still others are facultative, which means they can grow with or without oxygen. Campylobacter are microaerophilic and require very low but specific amounts of oxygen to survive.

Unfortunately, the range of approved means to prevent multiplication of pathogens in fresh produce handling is limited. These include chemical means such as acidified sodium chlorite, ozone, and chlorinated water. A low pH (acidification) environment can also retard the growth of many spoilage microorganisms. However, the most effective strategies for limiting foodborne bacterial pathogens are developing and adhering to a good food safety plan, which includes frequent cleaning and sanitation of all food contact surfaces, along with good record keeping that tracks processes and food lots.

Chemical Hazards

These include chemicals such as pesticides, herbicides, other crop protection agents, lubricants, sanitizers, heavy metals, or naturally occurring toxins that are harmful to consumers. Problems are rare but can be serious. The effects of chemical hazards depend on levels and time of exposure. There are long and short-term effects associated with chemical exposures. For most chemicals that are used on or around foods, safe levels have been established by toxicological studies that are enforced by the US Food and Drug Administration (FDA), the US Environmental Protection Agency (EPA), or the US Department of Agriculture (USDA).

Chemicals frequently used in the handling and processing of fresh produce may include:

• Pesticides, herbicides, defoliants, fertilizers, and growth regulators used on growing crops.
• Food additives and processing aids used in postharvest handling and preparation. This might include sulfites, other preservatives, and food dyes.
• Lubricants, paints, cleaners, sanitizers, and pesticides used on the equipment and around the facility.

Another important consideration is the naturally occurring mycotoxins found in decayed produce. These mycotoxins are secondary metabolites of micro-fungi that are capable of causing sickness and death in both animals and humans. One of the most well-known is furanoterpenoid, which is produced by soft rot (Rhizopus stolonifera) in sweetpotatoes.

When different types of produce are processed in the same line, there can be cross contamination with products that are allergens. These plant products include peanuts, tree nuts, soybeans, and wheat. (See the “Food Allergy Hazards” section.)

Physical Hazards

Each year, there are a number of recalls because of physical hazards. Any item or material that, when eaten, can cause cuts, punctures, broken teeth, or choking is a physical hazard. These include foreign materials such as plastic, wood, glass, metal, or stones. Generally, any foreign object size 7mm to 25 mm is considered a choking hazard for the general population. Objects less than 7mm may be considered hazards for certain types of food items, particularly baby foods. Items such as hair, sand, and insect fragments are considered aesthetic contaminants and typically do not cause injury.

Most instances of physical contaminants involve relatively few consumers. Metal parts such as fasteners, chips, and glass shards are the most common physical contaminants. These result from poor packing line design and maintenance practices, as well as poor employee training. Control measures include magnets, x-ray equipment, metal detectors, screens, and filters.

Food Allergy Hazards

A food allergy is a medical condition in which exposure to a specific food triggers a harmful immune response. This response occurs because the immune system of certain individuals attacks proteins in the food that are harmless to others. These proteins are called allergens. The symptoms of an allergic reaction to food can range from mild such as an itchy mouth and minor hives to severe difficulty breathing. A more serious allergic reaction is anaphylactic shock, which is sudden in onset and may result in death.

According to FARE (Food Allergy Research and Education), there are approximately 32 million Americans with one or more food allergies. This includes almost six million children under age 18, or one in 13 children, and 40% who are allergic to more than one food. FARE also reported recent research on allergies to specific foods: shellfish: 8.2 million; milk: 6.1 million; peanut: 6.1 million; tree nuts: 3.9 million; egg: 2.6 million; fin fish: 2.6 million; wheat: 2.4 million; soy: 1.9 million; and sesame: 0.7 million.

Although more than 170 foods have been known to cause allergic reactions, most reactions are related to the following eight foods: milk, eggs, peanuts, tree nuts (such as almonds, cashews, pecans, pistachios, and hazelnuts), wheat, soy, fish, and shellfish such as shrimp, crab, and lobster. Children are particularly susceptible to food allergies. Some food allergies can be outgrown such as milk, eggs, wheat, and soy. Others such as peanuts, tree nuts, fish, and shellfish may last a lifetime.

Figure 11-1. The disease triangle indicates the three factors needed for disease.

Source: J. Simmons.

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Figure 11-1. The disease triangle indicates the three factors needed for disease.

Source: J. Simmons.

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Cleaning and sanitizing are paramount for fresh produce that is harvested and packed in a packing shed. The packing shed serves as a “funnel” for all produce grown on the farm. When a hazard is introduced on the farm, all produce that is packed will be affected. Further, if a hazard is introduced at this stage, the grower has little control of that hazard before shipment of the produce from the farm and into the retail food chain.

Cleaning and sanitizing are two distinct operations. Cleaning is the physical removal of any microbial contaminants that may be stuck on food contact surfaces (any surface that the produce touches, such as harvest bins, sorting and packing conveyors, optical sorters, and final produce packaging). After microbial contaminants, along with other particles, are removed, a sanitizer can be applied to inactivate any microbial contaminants that may remain. Detergents are used along with physical removal during the cleaning step, while chemical sanitizers sanitize surfaces to inactivate any remaining microbial contaminants.

Areas within a packinghouse are divided into zones based on their proximity to the produce items being packed. Zone 1 areas are the food-contact surfaces that are in direct contact with the produce being packed. Zone 2 areas are those that produce does not technically touch, but are in close proximity to the produce. Zone 3 are areas such as floors, trashcans, walls of the packinghouse, and other surfaces. Zone 4 areas are farthest away from the food-contact surfaces, such as the break areas and bathrooms. It is important to understand which are the most important areas for cleaning and sanitation. This should be considered when developing a cleaning and sanitizing schedule. Obviously, Zone 1 areas must be cleaned and sanitized more regularly than the Zone 4 areas.

Bacteria can form biofilms on food-contact surfaces if adequate cleaning and sanitation are not done on a regular basis. The bacteria that colonize on these surfaces may or may not be microbial pathogens. Bacteria begin to grow and form an exo-polymer slime layer that helps connect them to the surface and protects them. Biofilms are extremely difficult to remove with routine cleaning procedures because the slime layer is extremely resistant to chemical sanitizers. The goal for a cleaning and sanitation program is preventing the formation of biofilms on all food-contact surfaces throughout the packing environment.

The time when a grower stops the packing line to clean and sanitize is considered a “clean break,” which then defines specific “lots” of produce that are shipped from the farm and that can be followed through the retail food chain. This is called traceability. When there is a food safety incident associated with a particular lot of produce, only that specific lot must be withdrawn from the market.

The layout and flow of produce and employees in the packinghouse, as well as the design of the packing equipment, are very important for food safety. Product flow should always move from the “dirtiest,” or raw commodities from the field, to the cleanest, which means, washed, graded, and packed product that will soon be shipped off the farm. If possible, the most efficient packinghouse design promotes a linear flow from raw commodity through washing, sorting, grading, packaging, refrigeration, holding, and then to shipping. Likewise, the flow of employees depends on their job and the location of offices, bathrooms, and break areas. When designing the flow within the packinghouse, it is important to allow for sufficient space around each piece of equipment that allows for properly cleaning and sanitizing.

In addition, the hygienic design of equipment should be considered when setting up or replacing equipment. It is often more cost efficient to spend more money initially for better designed equipment in contrast to the time and effort required over the life of that equipment to properly clean and sanitize it on a routine basis. To judge the hygienic design of equipment, it is important to consider if all food contact surfaces are visible, reachable, smooth, and cleanable; that there are no collection points for microbial contaminants (in water or from product); that the equipment is made of materials compatible with cleaning and sanitation; and that the equipment is designed in such a way that it will prevent the product from being contaminated from outside sources. The purpose of hygienic design is to intentionally create or improve spaces and equipment so that they can be cleaned and sanitized. Hygienic design should incorporate features in buildings and equipment that will minimize or eliminate the harboring of pathogenic microorganisms.

Despite vigilance, food safety problems inevitably occur. The actions taken in response are dictated by the severity of the problem. The least severe problems generally initiate a voluntary market withdrawal, which occurs when a product has a minor problem that would not be subject to FDA legal action and which the firm voluntarily removes the product from the market or corrects the minor violation. For example, a firm discovers that one of its products, although perfectly safe for the consumer, does not comply with their own quality standards. (See Class III recall below.)

A higher level food safety problem is called a recall, in which a firm removes or corrects a product that the FDA considers to be in violation of the laws it administers. The FDA has three classes of recalls:

Class I recall: A situation in which there is a reasonable probability that the use of or exposure to a product will cause serious adverse health consequences or death. An example is when a food contains a food hazard or tampering has occurred.

Class II recall: A situation in which use of or exposure to a product may cause temporary or medically reversible adverse health consequences or where the probability of serious adverse health consequences is remote. For example, when the manufacturer changes the ingredients in a product without changing the label, the product becomes mislabeled.

Class III recall: A situation in which use of or exposure to a product is not likely to cause adverse health consequences. For example, the product does not meet the manufacturer’s quality standard or the product does not have an appropriate label with all necessary information needed for consumer packaging.

The most severe food safety problem is an outbreak. By definition, a food safety outbreak can be declared when two events have occurred:

1. Two or more persons have experienced a similar illness after ingesting a common food.
2. A laboratory analysis of specimens has identified a common causative agent and epidemiologic analysis has implicated a food as the source of the illness.

Different agencies in the US government regulate different aspects of food safety. In general, the areas of responsibilities are separate, although there is some occasional overlap. The USDA regulates some on-farm activities associated with growing crops, which includes the regulation of fertilizers, herbicides, pesticides, and worker safety. With the 2011 Food Safety Modernization Act Produce Safety rule, the FDA gained jurisdiction over the growing, harvesting, holding, and packing of fresh produce. For many states, the farm inspections are conducted by state regulatory personnel, rather than the FDA. Regulatory oversight also includes all classic postharvest activities, such as cleaning, grading, storing, cooling, processing, packaging, and shipping. In addition, the EPA regulates any food additives and postharvest chemicals.

Centers for Disease Control and Prevention, National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), Division of Foodborne, Waterborne, and Environmental Diseases (DFWED). 2022.). “Foodborne Germs and Illnesses.” Accessed December 18, 2022. ↲

Food Allergy Research and Education (FARE). 2023. “The Food Allergy Epidemic.” ↲

National Center for Environmental Health. 2022. “Facts about Noroviruses on Cruise Ships.” Accessed May 4, 2022. ↲

National Center for Immunization and Respiratory Diseases, Division of Viral Diseases. 2023. “Burden of Norovirus Illness in the United States.” Accessed May 8, 2023. ↲

Steele, M. et al. 2020. “Characterizing Norovirus Transmission from Outbreak Data, United States.” Emerging Infectious Disease 26, no.8: 1818-1825. ↲

Tack, Danielle M., Ellyn P. Marder, Patricia M. Griffin, Paul R. Cieslak, John Dunn, Sharon Hurd, Elaine Scallan, et al. 2019. Preliminary Incidence and Trends of Infections with Pathogens Transmitted Commonly Through Food — Foodborne Diseases Active Surveillance Network, 10 U.S. Sites, 2015–2018. Mortality and Morbidity Weekly Report 68, no. 16: 369–373. ↲