Bacteria Thrive on Unwashed Knives, Greater on Graters


Reusing knives, graters, and other cutting tools in the kitchen is pretty common practice. Most get a quick rinse to remove anything seemingly left on the surface and are thrown back in the drawer. But that practice may not be so good for your health. A new study out this week had a look at how poor cleaning practices affect the bacterial contamination left on them after use. Their findings demonstrate why it’s so important to give cutting tools a thorough wash before they find their way back into the cupboard.

How does bacteria get into our food?

Bacteria are everywhere. They live on the surface of our skin, fill our oceans and water supplies, and thrive in the soil of the earth. For the most part, these bacteria don’t bother us. They either live in harmony with our bodies or are easily fought off by our immune system. But every now and then, a strain of bacteria with the ability to make us sick finds its way into our body. One source is from food. There are many ways food becomes bacteria-laden. When food preparers don’t wash their hands or wear gloves, for example, they transfer a variety of bacteria to the food they make, which then gets transferred to diners. Bacteria can also come from the soil the food grows in. This is especially true for vegetables. Using utensils like knives to cut multiple items can mean transferring potentially harmful bacteria from one contaminated food to many other clean ones.

How did the researchers study contamination?

The researchers performed their test with off-the-shelf steak knives and graters. They made bacterial liquids that contained E. coli O157:H7 and Salmonella from two separate food outbreaks. These bacteria were given a special marker that made them glow under UV light so that the researchers could easily find contamination. They then put these bacteria in carrot juice, strawberry juice, and grated carrots to simulate the leftovers from contaminated cutting. They applied these residues to the knife and grater and left them overnight. The next day, they used these utensils to cut up and grate some fruits and vegetables and tested the results for contamination. They also put some of the bacterial fruit and vegetable juice on some previously clean fruits and vegetables and used a clean knife and grater to cut them up to study transfer in the other direction.

What did the researchers find?

The team found that contamination occurred in all cases, knife or grater regardless of what was contaminated first, but that dirty graters were the worst offenders. They tended to harbor higher amounts of bacteria and passed on more bacteria when they were used on clean foods. The team thinks this might be because graters often hold onto small particles of the food, which contain bacteria that can be more easily passed on. In contrast, knives tend to hold less residue and are in contact with food for less time. Interestingly, the researchers found that bacterial transfer wasn’t equal for all foods. Contamination from cucumbers and melons was lower than tomatoes and strawberries, but it’s unclear why this would be the case.

How does this apply to me?

With the holiday season looming, you’re probably starting to make a mental list of all the foods you’ll be preparing. While it can be tempting in a food prep time crunch to rinse your knives and other cutting utensils for use later, remember that you might be contaminating future food in addition to what you’re currently making. Give them a thorough wash, especially graters, with soap and hot water or, better yet, throw them in the dishwasher for a heavier clean. Doing so might keep your next holiday meal from going sour.