What You Need to Know About Cross Contamination

Using the same cutting board and knife to cut raw meat and vegetables can lead to cross contamination.

cross contamination

Edited and resized image of Changyoung Koh, is available on Unsplash

Cross-contamination is the transfer of bacteria or other micro-organisms from one substance to another. Each year, an estimated 600 million people around the world suffer from foodborne illness. While there are many causes, an important and preventable one is cross-contamination. Other types of cross contamination include the transfer of food allergens (such as gluten), chemicals or toxins (see studies on this: 1, 2)

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Many people assume that foodborne illnesses are primarily caused by eating in restaurants, but there are many ways in which cross-contamination can occur, including (see studies on this: 3, 4, 5 here):

  • Primary food production - from plants and animals on farms;
  • During harvest or slaughter;
  • Secondary food production - including food processing and manufacturing;
  • Transport of food;
  • food storage;
  • Food distribution - grocery stores, farmers markets and more;
  • Food preparation and service - at home, restaurants and other food service operations.

Since there are many points where cross-contamination can occur, it's important to learn about the different types and how you can avoid it.

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Types of cross contamination

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

from food to food

The addition of contaminated food to uncontaminated food results in cross contamination. This allows harmful bacteria to spread on uncontaminated food (see study about it here: 6).

Raw, undercooked or poorly washed food can harbor large amounts of bacteria, such as salmonella, Clostridium perfringens, Campylobacter, Staphylococcus aureus, E. coli and Listeria monocytogenes - all of them, when ingested, can harm health (see study about it here: 6).

Foods that present the highest risk of bacterial contamination include vegetables, bean sprouts, leftover rice, unpasteurized milk, cheese, meat, eggs, poultry and raw seafood (see study on this: 7).

Adding unwashed, contaminated lettuce to a fresh salad can contaminate the other ingredients. This was the case with an outbreak of E. Coli in 2006, which affected 71 Taco Bell customers (see study about it here: 8).

Also, leftovers kept in the refrigerator for too long can result in bacterial growth. So, eat leftovers within three to four days and cook them at proper temperatures. If you plan to mix leftovers with other foods, the new meal should not be stored again as leftovers.

From equipment to food

Cross-contamination of food equipment is one of the most common types of contamination, albeit unrecognized. Bacteria can survive for long periods on surfaces such as countertops, knives, utensils, cutting boards, storage containers and food manufacturing equipment (see study about it here: 6)

When equipment is not properly washed or unknowingly contaminated with bacteria, it can transfer large volumes of harmful bacteria to food. This can happen at any time during handling - both at home and in factories and restaurants (see study about it here: 6). A 2008 incident at a Canadian sliced ​​meat company resulted in the death of 22 customers due to listeria-contaminated meat cutters (see study here: 9).

At home, using the same board and knife to cut meat and raw vegetables can be harmful if the vegetables are eaten raw (see study about it here: 10).

One study found that older participants were less likely to use soap and water to clean their boards after working with raw meat, while younger participants were not aware of the risks of cross contamination (see study here: 10).

Inappropriate food preservation methods can lead to cross contamination. In 2015, potatoes used in a salad made 22 participants of a party get sick from botulism due to inappropriate canning practices (see study about it here: 11).

From people to food

Humans can easily transfer bacteria from their bodies or clothing to food during many preparation steps (see study on this: 12). A person may cough their hand or touch raw poultry and continue to prepare a meal without washing their hands in the middle of the preparation process (see study on this: 12).

In a 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 (see study about it: 13).

Other common examples include using a bacteria-laden cell phone when cooking or wiping your hands with a dirty apron or towel. These practices can contaminate your hands and spread bacteria to food or equipment (see studies about it here: 12, 14, 15).

While this is a concern, a 2015 meta-analysis found that food safety education at home and at work can significantly reduce the risk of cross-contamination and unsafe eating practices (see study on this: 16).

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 (see studies on this here: 12, 17).

Side effects

Side effects of cross-contamination can be mild to severe. Mild side effects include stomach pain, loss of appetite, headache, nausea and diarrhea. Generally, they appear within 24 hours after ingestion of contaminated food, although they may appear weeks after exposure, making it difficult to determine the specific cause (see study about it here: 18).

In cases of vomiting or diarrhea, it is important to hydrate properly (see study about it here: 18). Serious side effects include diarrhea for more than three days, bloody stools, fever, dehydration, organ failure and even death (see study about it here: 18).

Seek immediate medical attention if side effects worsen or last longer than a day or two, as well as if you are part of an at-risk group.

Who is at risk?

Everyone is at risk of getting sick from cross-contamination (see study about it here: 19).

However, certain groups are at much greater risk, including:

  • pregnant women
  • Children under 5 years old
  • Adults over 65 years old
  • People with weakened immune systems - for example, people with HIV/AIDS, uncontrolled diabetes or cancer

Considering that these groups make up a large segment of the population, it is crucial to practice safe food handling when at home or working in a food service establishment (see study on this: 19).

How to avoid cross contamination

There are many ways to avoid cross contamination.

Buying and storing food

  • Avoid buying food close to its expiration date unless you plan to eat it right away;
  • Store raw meat in a closed glass container inside the refrigerator to prevent liquids from leaking into other foods;
  • Use separate grocery bags for raw meat and eggs;
  • Use leftovers from refrigerated food within two to three days and cook them at appropriate temperatures.

food preparation

  • Wash your hands with soap and water for at least 20 seconds after touching raw meat, petting an animal, using the bathroom, coughing or sneezing, or using the phone;
  • 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 cloths;
  • Cook food to the proper temperatures using a food thermometer.

Finally, keep up-to-date with the restaurants you frequent by visiting your country's food and disease control board website, such as the Centers for Disease Control and Prevention.



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