February 24, 2013

Sous Vide Part 1: What and Why?

I was introduced to the concept of sous-vide cooking one day while listening to Science Friday. In a nutshell, sous-vide cooking involves vacuum packing your food (usually meats) and placing them into a temperature controlled water bath at lower-than-normal cooking temperatures for longer-than-normal cooking times.

Why would you want to do this? Many reasons. I'll expound with my favorite sous-vide meal: chuck roast.

Chuck roast is known for two things: it's excellent flavor due to a good blend of meat and fat, and it's exceeding toughness due to connective tissues (collagen). Because of how tough it is, it's usually either made into ground beef or slow cooked. It's also fairly cheap (also due to it's toughness).

With conventional cooking methods, pretty much your only option for chuck roast is braising to break down the collagen, which results in a well-done roast. If you were to attempt to cook it medium or medium-rare, the meat would be unbearably tough since the collagen hasn't had a chance to break down. Breaking down collagen is a function of time and temperature. The higher the temperature, the less time needed to tenderize the meat. If, however, you are willing to spend more time, you can use a lower temperature.

It surprised me to learn that meat doneness is purely a function of temperature. If you want a medium-rare steak, cook it to an internal temperature of 56 celsius. Well-done? 70 C. It makes sense when you think about it, just never really thought about it in that way.

So, if breaking down the collagen is a function of time and temperature, but doneness is only a function of temperature, we should be able to select a desired doneness and solve for time. Fortunately we don't actually have to solve anything, there's plenty of information on the internet and in books that people have already figured out.

But with conventional cooking, there's no way you can cook at 55 C. The lowest setting on an oven is usually 170 F (76 C). And even at that, the actual temperature varies wildly. Here's an example from my own oven:

Just your standard electric oven. I set the temperature to 400 F and logged the temperature. The four downward spikes are from opening the oven door (I wanted to see how fast the temperature recovered). Aside from that, the temperature cycles by about 20 F. Assuming you could set your oven to a medium-rare 132 F (56 C), a fluctuation of 20 F is almost the difference between my desired medium-rare roast and a well-done one.

Enter sous-vide. The governing principal behind sous-vide is an accurately controlled cooking temperature, typically in the range of +/- 0.1 C. Water is used as the cooking medium (as opposed to air in an oven) because of its excellent heat transfer properties. The water bath then requires the food to be vacuum packed to avoid the flavors being diluted and the meat becoming water logged with the extended periods of time it spends submerged.

The result of all this is phenomenal. You can transform a cheap cut of meat into something that rivals, and in my opinion surpasses, the best ribeye or t-bone steak. Similar transformations are achieved in other types of meats. Chicken is can easily be dried and turned chewy with too high a temperature. With sous-vide you control the exact temperature it reaches, so the problem is eliminated. Pulled pork can be made just as succulent as with a large and cumbersome smoker on your counter top, and with all the flavors being contained in a bag a single slice of smoked bacon can add all the smoke flavoring you need.

There is a plethora of information on how to cook sous-vide; recipes, times and all that. I'll leave you to read those yourself. One of my favorites is http://meandmytorch.com/. My purpose here is to explain why you would do this, and in part 2 how you control the temperature.

Part 2 Teaser

This graph is one I logged from my first home made sous-vide system. The red horizontal line is the set temperature of 57.5 C. The green line is the actual temperature. You can see that the water temperature is indeed controlled very tightly. The blue line is the power output to the heating element and will get explained more in part 2 of this post.


  1. I found myself on your blog trying to figure out the following question: "Why does it take longer to oven-cook two turkeys than just one?"

    In both cases the surface area to volume ratio is the same, so the only thing that will lengthen the cooking time is a lower average temperature.

    In a heat/cool cycle with two turkeys, the heating leg will be twice as long and the cooling leg will be half as long, but if both the heating and cooling legs are linear, there's no way this affects the average. My initial idea was based on an understanding that the cooling should be exponential, though after seeing this post that's clearly not the case.

    My colleague suggested that maybe radiation from the heating elements had something to do with the cook rate, but given the two-turkey case has a higher on:off ratio, this suggestion lends to the two turkeys cooking faster, not slower.

    I'd really love to test this myself, but don't have the equipment to do it. Does two turkeys somehow lower the average temperature? I don't suppose you're curious enough to cook 3 turkeys?

    1. I've never cooked two turkeys at once, so I don't have any direct knowledge of what you're saying. But here's my thoughts:

      I doubt that the cooler thermal mass of the extra turkey is cooling the oven by any appreciable amount. More likely the heat transfer through the walls/door of the oven is the primary driver of the heating cycle. So the on/off time of the element shouldn't change much with one vs. two turkeys.

      When cooking two turkeys, do you put them side by side, or one over the other? If the latter, and you cook them uncovered, then each turkey is exposed to half the radiation (assuming you alternate positions) since the lower is shadowed by the upper. That may be the radiation comment your colleague was getting at. However, if you cook them covered then the radiation induced cooking is greatly reduced, though there could be a slightly higher temperature inside the cooking vessel if it's not entirely ventilated.

      As for the heating/cooling cycles they definitely are exponential. They look linear on the graph only because their duration is short in comparison to the exponential time constant, and can be satisfactorily modeled as linear because of this. You'll notice that once the oven was shut off it is very obviously exponential. I didn't mention in the post, but I also opened the door when the oven shut off which shortened the time constant significantly.