How you can safely cook foods at such low temperatures for so long was the most perplexing question to me when I first started cooking sous vide. I soon discovered that there is no greater food safety risk to sous vide cooking than there is to any other cooking method. Let me explain.
We cook foods so that harmful bacteria cannot grow or multiply and are indeed killed. Food scientists have determined that by cooking food to over 140ºF, we kill most harmful bacteria. What is less known is that when you hold foods at a lower temperature but for a longer period of time, the bacteria are also eliminated and bacterial growth is prevented. It’s not just a matter of temperature, but of temperature and time. Holding foods at a lower temperature is exactly what happens in the pasteurization process, which is what is done to most dairy products that we buy.
With that issue resolved, it is still important to pay attention to two additional food safety precautions. First, it is imperative that the food be fully submerged in the water when cooking sous vide, so please make sure you remove as much air as possible from the bag and seal it properly so that it sinks in the water bath. Secondly, if you are planning on chilling foods for serving at another time, cool the food properly. The best way to cool foods is to use an ice bath. Fill a bowl with ice and water and simply submerge the sous vide bag in the ice bath until it has cooled completely. Then refrigerate or freeze.