Bacterial cross-contamination

Each year, an estimated 600 million people worldwide experience a foodborne illness.

While there are many causes, a major and preventable one is cross-contamination.

This article explains all you need to know about cross-contamination, including how to avoid it.

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What is cross-contamination?

Bacterial cross-contamination is defined as the transfer of bacteria or other microorganisms from one substance to another.

Other types of cross-contamination include the transfer of food allergens, chemicals, or toxins — though these are not the focus of this article.

Many people assume that foodborne illness is mostly caused by eating at restaurants, but there are many ways in which cross-contamination can occur, including:

• primary food production — from plants and animals on farms
• during harvest or slaughter
• secondary food production — including food processing and manufacturing
• transportation of food
• storage of food
• distribution of food — grocery stores, farmer’s markets, and more
• food preparation and serving — at home, restaurants, and other foodservice operations

Given that there are many points at which cross-contamination can occur, it’s important to learn about the different types and how you can prevent it.

Summary: Cross-contamination is defined as the transfer of bacteria or other microorganisms from one substance to another. It can happen during any stage of food production.

Types of cross-contamination

There are three main types of cross-contamination: food-to-food, equipment-to-food, and people-to-food.

Food-to-food

Adding contaminated foods to non-contaminated foods results in food-to-food cross-contamination. This allows harmful bacteria to spread and populate.

Raw, undercooked, or improperly washed food can harbor large amounts of bacteria, such as Salmonella, Clostridium perfringens, Campylobacter, Staphylococcus aureus, E. coli, and Listeria monocytogenes — all of which can harm your health if consumed.

Foods that pose the highest risk of bacterial contamination include leafy greens, bean sprouts, leftover rice, unpasteurized milk, soft cheeses, and deli meats, as well as raw eggs, poultry, meat, and seafood.

Suggested read: How long does chicken last in the fridge?

For example, adding unwashed, contaminated lettuce to a fresh salad can contaminate the other ingredients. This was the case in a 2006 E. Coli outbreak that affected 71 Taco Bell customers.

What’s more, leftovers kept in the fridge for too long can result in bacterial overgrowth. Therefore, eat leftovers within 3–4 days and cook them to proper temperatures. If you plan to mix leftovers with other foods, the new meal should not be stored again as leftovers.

Equipment-to-food

Equipment-to-food is one of the most common yet unrecognized types of cross-contamination.

Bacteria can survive for long periods on surfaces like countertops, utensils, cutting boards, storage containers, and food manufacturing equipment.

When equipment is not washed properly or unknowingly contaminated with bacteria, it can transfer large volumes of harmful bacteria to food. This can happen at any point during food production — both at home and in food manufacturing.

For example, a 2008 incident at a Canadian-based sliced meat company resulted in the death of 22 customers due to listeria-contaminated meat slicers.

A common example of this occurring at home is using the same cutting board and knife to cut raw meat and vegetables, which can be harmful if the vegetables are then consumed raw.

One study found that older participants were less likely to use soap and water to clean their cutting boards after working with raw meat, while younger people weren’t aware of the risks of cross-contamination. Thus, more food safety education seems to be needed across all age groups.

Suggested read: Is it safe to eat raw salmon?

Finally, improper food preservation techniques can lead to cross-contamination. In 2015, home-canned potatoes used in a potato salad made 22 potluck attendees sick with botulism due to improper canning practices.

People-to-food

Humans can easily transfer bacteria from their bodies or clothes to food during many steps of food preparation.

For example, a person may cough into their hand or touch raw poultry and continue to prepare a meal without washing their hands in between.

In a 2019 study of 190 adults, only 58% of participants reported washing their hands before cooking or preparing food, while only 48% said they washed their hands after sneezing or coughing.

Other common examples include using a cellphone that’s loaded with bacteria while cooking or wiping your hands with a dirty apron or towel. These practices may contaminate your hands and spread bacteria to food or equipment.

Although this poses a concern, a 2015 meta-analysis found that food safety education both in the home and at work can significantly lower the risk of cross-contamination and unsafe food practices.

By far, the most effective way to reduce the risk of cross-contamination is to properly wash your hands with soap and water for at least 20 seconds.

Summary: There are three main types of cross-contamination: food-to-food, equipment-to-food, and people-to-food. In each type, bacteria are transferred from a contaminated source to uncontaminated food.

Side effects of cross-contamination

The side effects of cross-contamination can be mild to severe.

Minor side effects include upset stomach, loss of appetite, headache, nausea, and diarrhea. Usually, these side effects present within 24 hours, although they can appear weeks after exposure, making it difficult to determine the specific cause.

In cases involving vomiting or diarrhea, it’s important to rehydrate properly — for example with a sports beverage — to restore hydration, blood sugar, and electrolyte levels.

Suggested read: How long does pasta last in the fridge?

Severe side effects include diarrhea for more than 3 days, bloody stools, fever, dehydration, organ failure, and even death.

Seek immediate medical attention if your side effects worsen or last longer than 1–2 days, as well as if you’re considered to be in an at-risk population.

Summary: Side effects of cross-contamination range from stomach upset to more severe aftereffects, including dehydration, organ failure, and even death.

Who is at risk?

Everyone is at risk of becoming sick from cross-contamination.

However, certain groups are at a much higher risk, including:

• pregnant women
• children under the age of 5
• adults over the age of 65
• those with weakened immune systems — for example, people with HIV/AIDS, uncontrolled diabetes, or cancer

Considering these groups make up a large segment of the population, it’s crucial to practice safe food handling when at home or working in a food service establishment.

Summary: Anyone is at risk of becoming sick from cross-contamination. However, certain groups, including pregnant women, children, older adults, and those with weakened immune systems, are at the highest risk.

How to avoid cross-contamination

There are many ways to avoid cross-contamination.

Food purchasing and storage

• Avoid purchasing food close to its expiration date, unless you intend to eat it right away.
• Store raw meat in a sealed container or plastic bag on the bottom shelf of the refrigerator to prevent juices from leaking onto other foods.
• Use separate grocery bags for raw meat and eggs.
• Use refrigerated leftover food within 2–3 days and cook it to proper temperatures.

Food preparation

• Wash your hands with soap and water for at least 20 seconds after touching raw meat, petting an animal, using the washroom, coughing or sneezing, using your phone, or related instances.
• Wash your utensils, countertops, cutting boards, and other surfaces with soap and warm water, especially when handling raw meat.
• Use separate cutting boards for meat and vegetables.
• Use clean sponges and dishcloths.
• Cook foods to their proper temperatures by using a food thermometer.

Finally, be sure to stay up to date with food recalls by visiting the website of your country’s food and disease control board, such as the Centers for Disease Control and Prevention (CDC) in the United States.

Summary: Proper food safety practices can significantly reduce your risk of cross-contamination. Thoroughly wash your hands and surfaces, properly store foods, and stay up to date with food recalls.

Summary

Bacterial cross-contamination can have serious and even fatal consequences, but thankfully, it’s easy to prevent.

Suggested read: 11 foods and beverages to avoid during pregnancy

Practice good hygiene, wash and sanitize your equipment, and properly store and serve food to prevent cross-contamination. Plus, it’s a good idea to stay up to date with food recalls, which are available online.

By practicing safe food handling, you can protect yourself and others from getting sick.