Microbiological risks in fruits and vegetables

Although microbiological food outbreaks have historically been related to animal-origin products, the reality is that cereal, fruits, and vegetables are responsible for many food safety alerts. Contaminated foods or beverages cause foodborne illnesses and can be caused by chemicals, microorganisms, or toxins produced by some microorganisms.

Foodborne illnesses are a danger to the general public as they lead to illness and, in some cases, hospitalization and even death, especially for most vulnerable groups like children, pregnant women, and the elderly. Food outbreaks are also damaging for the business responsible as they face legal and economic repercussions and damage to the reputation and the potential loss of businesses. It is, therefore, paramount to understand what are the food safety risks of your products and what strategies can be adopted to manage these risks and have a safe product. This article focuses on the microbiological hazards of agricultural products, possible contamination routes, the most common microorganisms, and strategies to mitigate these risks.

To take effective measures against microbial contamination, it is crucial to understand the most likely sources of contamination and the conditions that allow the growth of pathogens.

Sources of microbiological contamination on agricultural products.

Microorganisms can colonize cereals, fruits, and vegetables by attaching themselves to their surface or ingressing inside them via plant wounds or stomata; some microorganisms can also produce toxins that are poisonous substances that can cause illnesses. It is important to know the routes and sources of contamination.

The most common sources of contamination are soil, water, and animals; consideration also needs to be given to the contamination via people working with the crops and to the storage and transport conditions.

• Soil. The soil where the products are farmed is the main source of contamination of farming products. Some microorganisms can be naturally present in the soil, but some of them find their way into the soil through the use of organic fertilizers. The use of organic materials like manure or compost to increase soil nutrients can be a source of pathogens. The manure can find its way onto the vegetables by direct contact with the vegetable surfaces or splashing contaminated soil onto vegetables and fruits. The survival of microorganisms in manure depends on a wide range of factors, including the source of the manure, processing techniques, and environmental conditions, among other factors. Organic farmers normally use more compost and/or manure than their non-organic counterparts, so it is important to understand the potential risks of these practices. Regarding the naturally occurring pathogens present in the soil, several factors affect their growth and survival, like the soil’s nutrients, moisture, pH, or temperature. It has to be considered that some of the microorganisms have adapted to the environment and are capable of surviving under harsh conditions (e.g., low moisture) with dormant structures called spores, thriving again when the conditions are ideal for growing.
• Irrigation water. Water is essential for growing farming products, and in most of the crops, irrigation water is used as water from rainfall is not enough. The type of irrigation system and the water source and wastewater treatment are important factors influencing the potential contamination of irrigation water. It is also of paramount importance the condition of the irrigation equipment used for irrigation and the storage of the irrigation water as these steps may lead to the contamination of the water by external factors, e.g., animal feces or microbial growth in the equipment.
  The type of water application is very important as some systems are riskier, like sprinkling water onto the plants, while others, like applying water directly onto the soil, pose a lesser risk of contamination of the crops. The water source is also important; potable water is safer as treatment has been applied to reduce and eliminate microorganisms. Likewise, rainfall water is usually safe from a microbiological point of view. Other water sources, like underground water, can be considered safe unless leaks from the surface have contaminated the aquifer. The application of surface water carries more risks as lakes, ponds, or rivers are more exposed to environmental contamination from animals, wastewater, storm drains, and contamination from farms and/or industries. Wastewater is sometimes used for irrigation with minimal or no treatment, and it can be hazardous from a microbiological point of view. Different factors like temperature, pH, time, light, and the load and type of pathogens will determine the survival of pathogens in the water.
• Animals. Domestic and wild animals are important reservoirs of pathogenic microorganisms, and the role of manure in the contamination of soil, water, and crops is explained above. The farming animals may access the fields or adjacent areas and contaminate crops mainly via their feces. Wild animals are more difficult to control; birds, reptiles, rodents, or insects can easily find their way into the fields attracted by plants, water, or nearby waster or manure piles. As will be explained in the next section, many animals are capable of harboring different pathogens that can cause foodborne illnesses if humans consume contaminated products.
• People. Food handlers can be an important route of contamination that is sometimes overlooked in farming. Poor hygiene of food handlers can lead to the transfer and contamination of crops and fruits in the hands of the food handlers. Likewise, people with viral or bacterial infections can contaminate crops, water, or even colleagues, and the infectious vector could potentially find its way into the food and potentially the final consumer.
• Storage and transportation. As discussed above, environmental conditions (time, temperature, moisture, etc.) affect the growth and survival of microorganisms.

Therefore it is important to consider these when storing and transporting fruits, vegetables, and cereals. It is also important to consider that these conditions can also lead to mold growth which is a spoilage sign and not a food safety risk, but molds can produce toxins that may cause food poisoning. The hygienic conditions of transport and storage are also important to prevent the contamination of foods and the access and/or development of pests that may contaminate the products.

Most common microorganisms on agricultural products.

Below is a list of microorganisms that can produce foodborne illnesses that have been frequently linked to vegetables, fruits, and cereals.

• Escherichia coli. Most of the strains of this bacteria are harmless, but some have been linked to foodborne illnesses, being the most common O157, responsible for a deadly outbreak in Germany in 2011 linked to fenugreek seeds. The origin of this bacteria is the intestinal tract of cattle; therefore, it can contaminate soil, crops, and water via animal feces. The bacteria have the ability to produce heat-resistant toxins that can produce digestive illnesses in humans and animals. This bacteria has been frequently linked to outbreaks in vegetables used in salads and fresh herbs.
• Salmonella. It is a family of bacteria that is capable of producing foodborne illnesses, causing gastroenteritis, and being responsible for many food outbreaks. It can be found in the digestive tract of wild and domestic animals. This bacteria has been found in fresh products like seeds, leaves (like lettuce), vegetables (sprouts and tomatoes), and fruits (melons or mangoes); it is also able to colonize other agricultural products like seeds, nuts, and cocoa beans. Contamination of vegetables can occur via contaminated soil, manure, or water. The bacteria are easily killed through cooking, but it is a risk in uncooked foods like fruits or salads.
• Listeria monocytogenes. This bacteria is responsible for a condition that can be life-threatening for certain groups like children, the elderly and pregnant women, listeriosis. Listeria can be found in water, animal feces, and soil, potentially contaminating crops, and it can survive under difficult conditions like cold, high salinity, and acidity. It is not heat resistant, but it has the capability to cause outbreaks in uncooked products like salad vegetables.
  Bacillus cereus. This bacteria can easily be found in the soil, and therefore, it has the potential to contaminate different types of foods like vegetables, fruits, and cereals. It can form spores to survive adverse conditions, but the spores are inactivated in chilled conditions. It has been associated with ambient foods like cereals, grains, and flour.
• Viruses. There are a variety of viruses that can cause foodborne illnesses being, the most common Noroviruses and Hepatoviruses. Norovirus causes gastroenteritis, and Hepatovirus causes hepatitis. The main routes of contamination of these viruses are infected food handlers and infected water, these viruses are easily killed by heat, but fresh products like fruit and vegetables are susceptible to contamination.
• Mycotoxins. These are toxins produced by molds that grow on a variety of food and are usually related to cereals, nuts, spices, apples, and dried fruits. Usually, molds grow in vegetables after harvesting during storage, and therefore it is very important to store the crops under the right conditions to reduce their growth and toxin production. Mycotoxins can cause a variety of illnesses, from diarrhea and gastroenteritis to severe complications.

How to control microbiological risks in farming?

To prevent the contamination and growth of microorganisms in our harvests, it is important to understand what are the most common microorganisms and routes of contamination in our crops and then produce a series of strategies during the whole growth cycle of our crops.

It is essential to tackle the main routes of contamination to enhance the food safety of our products.

• Clean soil. It is important to rationalize and manage compost and/or manure use to reduce the chances of contamination. As microorganisms can survive in manure for up to 90 days, it is important not to use fresh manure; keep the manure for 60 to 90 days before it is applied to the soil under the appropriate conditions, and preferably store it away from the fields to prevent it from leaking into them.
  It is also essential to carefully select the fields to avoid fills where there could be streaming of water from animal operations or surface water.
• Clean water. As irrigation water is an important source of contamination, carefully select the water used for irrigation and, if possible, test the water or use potable water, avoid the use of wastewater, and assess the quality of any surface water used for irrigation. As the irrigation methods can also pose some risks of contamination, consider using an irrigation system that feeds into the soil and avoid, when possible, the use of sprinkling water. Proper maintenance of the irrigation systems is also important to avoid the creation of biofilms that can contaminate the water.
• Control of domestic and wild animals. Create strategies to minimize the presence of wild animals and prevent the access of domestic animals to the fields and surface waters that may be used for irrigation.
• Promotion of hygienic practices. Promote the good hygiene of food handlers during farming, harvest, and post-harvest, provide adequate facilities for cleaning hands, and provide clean and adequate restrooms. It is important to have a clear procedure for sick employees and exclude them from food-handling activities during their illnesses. Education is also fundamental to creating a food safety culture.
• Controlled conditions during post-harvesting. Maintain good cleaning standards on packing areas, warehouses, and transports used after harvesting. Take measures to control the conditions during storage, packing, and transportation to prevent extreme conditions that can compromise food safety. Ensure that there are adequate systems to prevent the access of animals to these areas.