Sous Vide, Revisited
In a previous post, I had recommended the 4130 NIST-Traceable Temperature Controller to control the temperature in a slow cooker. Unfortunately, that particular controller has a range that tops out at 60 degrees C / 140 degrees F, which is enough for cooking beef for long periods of time, but not enough for say, cooking duck confit, which for which a sous vide temperature of 80 degrees C is recommended. In addition, the 4130 is pretty expensive; almost $150. It’s possible to add a resistor to change the range of the 4130, but the temperature displayed by the controller is no longer correct, and you have manually create a conversion table between true temperature and the temperature as seen by the controller.
I’ve recently come across a cheaper and better possibility, the Ranco ETC-111000-000 Temperature Controller which is only half the price and comes with a much larger working range (-30 to 220 degrees F). The price with the AC cord already wired in is $75; and the version which just has a 120VAC SPDT relay is only $60.
A bit more about food safety. There has always been a lot of concern about bacteria growth and botulism, for good reason — and so therefore the recommendations for cooking temperature have a lot of safety margin in them — to the point now that the USDA recommends that steaks be cooked to at least 145 degrees F, which is well within what had traditionally been called “medium”, and chicken to at least 165 degrees F, which is enough to really destroy taste and texture. Sous vide cooking, especially some of the more low temperature variants, have raised a lot of concerns, to the point where a few years ago New York City (temporarily) banned it, causing a great outcry in the foodie community, since many top restaurants use sous vide techniques.
First of all, any recommendation about internal temperatures and food safety that doesn’t also factor in time is massively oversimplifying the problem. Here is a table taken from “Food Safety Hazards and Controls for the Home Food Preparer”, published by the Hospitality Institute of Technology in 1994:
This table lists the time to reduce bacteria concentrations of Salmonella and E. coli from 100,000 to 1 (5D) or 3,162,277 to 1 (6.5D). The FDA and USDA recommend cooking hamburger to 5D destruction. Since it is extremely unlikely for there to be more than 100 Salmonella organisms per gram of meat, a 5D kill will reduce Salmonella concentrations to no more than 1 organism per kilogram. So whether you cook a piece of meat to an internal temperature of 165 degrees for 1.64 seconds, or hold it at 130 degrees for 90 minutes, the effect on Salmonella and E. coli bacteria will be the same.Of course, one concern is that in many forms of cooking, particularly oven roasting and grilling, the temperature of the food as it heats up may not be even; so how do you guarantee that all parts of the food product has been brought up to the requisite 165 degrees? One way that recipe authors, fearful of liability concerns, have done so is to tell people to cook meat to much higher internal temperatures, to provide that extra safety margin at the expense of dessicated, horribly tasting turkey or chicken. But the advantage of sous vide cooking is by immersing the food in a water bath, the excellent heat conductivity of water helps guarantee that the entire body of meat will get raised to desired temperature relatively quickly. (How quickly depends on the thickness of the meat, obviously). So if you hold a roast beef that has been vacuum packed in a Foodsaver bag for five or six hours, there should be no question that all of the common bacteria has been inactivated, and that amount of time at 130 degrees F should be sufficient to inactivate 99.9 percent of all botulism toxin molecules (not there should be any in a fresh piece of meat, of course!)
However, sous vide temperatures are not enough to kill bacterial spores, in particular C. botulinum, which is responsible for botulism. This requires temperatures far in excess of boiling water at sea level. For example, home canning protocols recommend holding the food product at 250 degrees F for at least 15 minutes. This is an issue in the restaurant business because very often food would be cooked sous vide, and then stored in the vacuum sealed bags for potentially weeks (yes, you could be eating an extremely expensive meal at a top-end French restaurant that had been cooked several weeks ago, and reheated just before serving; yummy, no?). If the food packages aren’t cooled quickly enough, and then allowed to warm to the danger zone, it’s possible that in the anerobic environment the C. botulinum spores could germinate and then start producing toxin. But, if you are cooking home sous vide where you are serving the food right after it has been cooked, this shouldn’t be a concern.
Update (7/7/2009): On re-reading this post, one thing which I struck me in the last paragraph is that people might get the wrong idea when I talked about the “danger zone”, which for normal foods is traditionally regarded as between 40 and 140 degrees Farenheit. (Although some spoilage bacteria can multiple at temperatures as low as 29 degrees Farenheit; the fact that you shouldn’t keep food between 40 and 140 for more than a four hours cumulative doesn’t mean that you can store foods for long periods of time at 38 degrees!) The danger zone is however much larger for vacuum-sealed bags where botulism spores might be a possibility; some strains of botulism can germinate and grow at temperatures as long as 3.3 degrees Centigrade. I didn’t mention this originally because I’m not terribly interested in doing restaurant-style storage of foods cooked sous vide. The bottom line is that if you want to store foods for long periods of time, still in the vacuum sealed bag which was used to cook the food sous vide, you need to be very, very, careful. Doing this right requires blast chillers and very strictly controlled and monitored temperature controlled refridgerators. It is not something I would at all recommend for most home cooks; you’re much better off opening up the bag as soon as you pull it from the water bath!