Cross contamination is a fancy name for bacteria on one thing getting onto another thing via direct contact. In the culinary arts, one of the things usually tends to be a knife or a cutting board, and the other thing is food.
And it's not just bacteria. It could be a virus or a toxin of some kind, or even a cleaning product. But whatever it is, if it comes into contact with someone's food, it's cross contamination.
If they eat the food and it makes them sick, we call it food poisoning.
Where Does Cross Contamination Occur?
Cross contamination can happen on a very large scale, because of equipment at processing facilities not being cleaned properly, for instance, or any of the other numerous and sundry ways your food can be mishandled as it makes its way to you.
That is why from time to time we all read about outbreaks of food poisoning, product recalls, restaurant closures and the like. And unfortunately, there's not much you can do to protect yourself at that level, other than keeping track of the news and using good sense in deciding where to eat out.
As a home cook, though, there are quite a few steps you can take and habits you can build, to help reduce the likelihood of cross-contamination in your kitchen.
Preventing Cross Contamination on Knives and Cutting Boards
In nearly all cases, cross contamination is going to be caused either by your kitchen knife, your cutting board, or your hands.
But once it's on your hands, it's on everything else as well.
Those two items really are the major culprits, since everything touches your cutting board, especially if you're planning on cutting it, which is where the knife comes in. Cutting up food on cutting boards is, after all, a big part of cooking.
Since dangerous bacteria are killed by high heat, the risk of cross contamination is highest with food that doesn't need to be cooked. That's why outbreaks of salmonella poisoning are increasingly found to be linked to foods like sprouts and bagged salads, foods you might think of as innocuous or "safe" but are risky because they customarily aren't cooked.
Ultimately that means that preventing cross contamination requires building habits such as frequent washing of hands, utensils, cutting boards and work surfaces. For instance, if you prep a raw chicken on a cutting board, don't use the same cutting board later to slice tomatoes for the salad. At least not without washing it first. And the same goes for your knife.
It goes for your food, too! Even if that bagged salad says it's been washed three times, wash it again anyway. Same with sprouts. It can't hurt to wash vegetables even if you're planning to peel them anyway, like carrots. It is an extra step, but when it comes to preventing cross contamination, you are better safe than sorry.