The safety of sous-vide cooking
Is there a risk of microbial proliferation with low-temperature cooking?
This is a common question, and rightly so for any new technique. However, the unprecedented control over temperature and time that sous-vide offers means that it can be quite safe.
Most microbial contamination arises from poor handling or storage, before or after purchase. It’s the surfaces that become contaminated — the interior of an intact piece of meat is effectively sterile. But, if a piece of contaminated meat were to be blanched in 100C water for just 2 seconds, the surface microbes would all be killed. It’s now a safe piece of meat throughout. So safe in fact, that if it were a skinless chicken breast, you could consider making a tartare out of it.
Lets assume you don’t want to blanch, or the food is not suited to blanching (or blasting with a blow torch or searing in a very hot pan). Lets assume we’re starting with contaminated food, and we want to make it safe by cooking. Note that this post is about microbes, if parasites are a concern then the solution is to freeze the food for at least a week.
Pasteurisation (which is what we’re about to do) is not about temperature; it’s about temperature and time. For high temperatures (e.g. 100C) the process is so quick that it seems instantaneous. But pasteurisation can be achieved by holding at a lower temperature for a longer period of time (holding times can be looked up in scientific tables or those published by the FDA). Time is needed because there is a known ‘death-rate’ for a given temperature above the critical temperature (when microbes switch from multiplying to dying). For example, salmonella begins to die above 48C, and salmonella-contaminated food will be pasteurised if held at say 55C for 1h 30m (or at 60C for 20m, etc). There are other bacteria to consider of course. The most thermophillic (heat resistant) is Clostridium perfringens, and it’s usually used as the standard for pasteurisation. C. perfringens dies above 52.3C.
With sous-vide it’s a simple matter to factor in time — just bring the food up to temperature and hold it there for the designated time. It won’t continue to cook because the temperature doesn’t rise further, and it can be held until it’s pasteurised. This is next to impossible with traditional techniques such as oven roasting or grilling.
With the exception of fish, most foods are usually cooked sous-vide at temperatures above 55C (medium-rare), and usually for an extended period of time to tenderise, and so they will be ‘safe’ from common food microbes. It’s not all about safety though — chicken generally has a better texture when cooked around 62C, which means it would soon be pasteurised, but it’s usually cooked longer for eating quality.
With sous-vide it’s even possible to pasteurise an egg in its shell without cooking it. Egg proteins don’t begin to denature until ~60C, so an egg can be held at 55C for say 2h and be fully pasteurised but still raw. It will behave exactly like a raw egg for cooking purposes — it enables a pasteurised raw-egg yolk mayonnaise to be possible for example. Sous-vide gives us a control over food-safety that we didn’t have before.
Finally, fish is another matter. Pasteurised fish is unpalatable to most, and even with conventional cooking it is lightly cooked and often outside of food safety measures. Food safety is not an absolute though, it’s a compromise. In practise, fish cooked traditionally or sous-vide is considered safe to within acceptable risk. It’s interesting how we are willing to adjust our risk-assessment for selected foods in exchange for taste.
Traditional cooking methods tend to rely on a series of time-honoured heuristics, whereas sous-vide has its roots firmly planted in science and measurement. There is no reason why both methods can’t be safe, however the advantage of sous-vide is that safety can be achieved more reliably, while maintaining better control over doneness, texture and palatability.