By: JOE HUGHES
Sous vide cooking is much more than a trend; in 1974, French chef Georges Pralus was the first to use the technique in a fine dining setting. He prepared fois gras sous vide at his three-star Restaurant Troisgros after finding that gentle cooking preserved the size and shape of the liver, while also improving its texture and taste.
Sous vide slowly gained popularity among adventurous chefs, but high-end restaurants did not begin adopting the technique en masse until after revered chef Thomas Keller1 wrote his explanatory cookbook Under Pressure: Cooking Sous Vide in 2008. Adventurous foodies have been cooking sous vide – or at least, experimenting with it – over the last decade, and more-affordable immersion circulators are making the technique even more widely known to the general public.
However, even though the technique is gaining traction in the food world, many who regularly cook sous vide aren’t exactly sure how or why it produces such perfectly-cooked food. There are two ways to explain the food science2 of sous vide cooking; the first will make sense to geeks and those familiar with molecular gastronomy. The second (and simpler) explanation is perfect for skeptical family and friends who associate “food cooked in bags” with mediocre dishes such as Minute Rice or veggies reheated in the microwave.
Here are the basic mechanics of sous vide cooking. Food one would like to sous vide is generally vacuum-sealed in a plastic bag before being put into a water bath, where it sits for anywhere from one to three hours at low, carefully-controlled temperatures (usually under 140°D). And, voila! When the food is taken out, it is perfectly cooked, but how does that process work?
Our first example begins with meat3, the food most commonly cooked by the sous vide method. There is approximately 20% actual protein in your “protein”; everything else is mostly water or fat. Heating causes most of the protein within the meat (primarily actin and myosin) to break down, or in scientific terms, become denatured. In this process the muscle fibers begin to shrink at a temperature of around 100°F, and the amount of shrinkage increases in a linear fashion until the temperature of the meat reaches about 175°F.
But remember that the water bath is usually kept below 140°F. Here’s why. Fiber contraction occurs horizontally up to that temperature, so most of the water held between the fibers is retained. As the temperature rises above 140°F, though, the shrinkage becomes longitudinal and the fibers can no longer hold much of the water inside the meat. For that reason, as even home cooks know from everyday life, meat cooked over high heat is likely to shrink and become dry as it loses most of its moisture. Sous vide cooking done below the magic temperature of 140°F which keeps meat juicy.
There’s also one other factor to consider when talking about sous vide cooking, and that’s collagen, the connective tissues that hold the muscle fibers in meat in place. As collagen is heated to temperatures above 130°F it also begins to contract and denature, becoming a gelatinous substance. However, that’s a good thing, since collagen is primarily responsible for the “toughness” experienced when eating meat. Breaking down the collagen makes meat deliciously tender. It’s best done slowly and at low temperatures, however, so the meat doesn’t overcook during the process of tenderizing it with heat. This is the same science behind stewing or cooking lesser cuts of meat in slow cookers to make them soft and tender.
What advantage does the newer sous vide cooking method have over time-honored cooking techniques like stewing, you may ask? The answer to that is precise temperature control. Whether you’re cooking in an oven, a pan or a stockpot, heat has to be radiated at temperatures much higher than the meat’s optimal internal temperature4, because so much heat is “lost” while being transferred to the food being cooked. As an example, you have to set your oven to 300-400° in order to roast meat to a much lower internal temperature.
Sous vide cooking eliminates that problem. Water has the distinct feature of being nearly ten times more efficient than air as a medium for transferring heat to food, so a 140°F water bath will heat vacuum-sealed food to almost exactly 140°F. Add in the ability of a sous vide machine5 or immersion circulator to maintain constant temperatures (high-end machines can keep water temperature within one-tenth of the unit’s setting), and you have a cooking process that is almost perfect.
There’s also another benefit to sous vide. Since heat is transferred uniformly throughout the meat, it doesn’t have the characteristic “well-done on the outside, pink on the inside” look and taste of meat prepared in a hot pan or oven. A sous vide steak, for example, will be cooked to the desired temperature, and will have uniform color, all the way from top to bottom. The one drawback is that the meat will be missing the luscious “crust” created at high heat by what’s known as the Maillard effect – but quickly searing the steak in a pan before serving will add that finishing touch.
As was discussed before, Sous vide is French for “under vacuum,” referring to the food-grade plastic bags into which the meat is vacuum-sealed (although many believe zip-loc bags are just as effective for sous vide cooking6). Some foods don’t require this step before being prepared sous vide, but there are important scientific reasons for taking it. Since there’s no air circulating in the bag, thermal transfer is most efficient, moisture doesn’t evaporate from the food, flavor doesn’t escape, the effect of any spices on the meat or in the bag is enhanced, and the possible growth of bacteria is minimized.
This discussion has focused on the sous vide cooking of meat but the science behind the process remains similar when preparing other foods, although most should be immersed at different temperatures depending on the food being cooked. The cell walls in vegetables, for example, are easily damaged by high levels of heat, causing the nutrients in the veggies to leach out and water to evaporate when boiled or steamed. Additionally, many vegetables contain pectin, the glue which holds the cells together. At very high temperatures that material becomes gelatinous, the reason why cooked vegetables are often mushy. Sous vide cooking keeps the pectin intact so vegetables aren’t overcooked. Temperatures higher than the 140° typically used for proteins are ideal for sous vide vegetable cooking, but should be kept below 175-185° because pectin begins to break down at those temperature levels.
This leads us to one final and very important point. The heat used in sous vide cooking can be rigorously controlled, but that won’t matter if a machine is set to the wrong temperature. Menus or guides are essential for proper results when experimenting or beginning with sous vide7, because as those familiar with molecular gastronomy well know, guessing is the wrong approach to a scientific cooking process. In other words, garbage in, garbage out – literally.
Now, for the promised explanation suitable for friends and family who can’t understand the attraction of fiddling with vacuum sealing and immersion circulators, let alone the longer cooking times required before the food is ready. Try this:
“You know how meat is often tough on the outside but too rare on the inside? That’s because the outside of the meat touching the pan or near the broiler is exposed to much higher temperatures than the inside. Sous vide makes sure that the meat is cooked evenly, so you never get a steak that’s burned on the outside or too red to eat when you cut into it.”
Once they do, they’ll never question the science behind sous vide cooking again.
Joe Hughes is an avid cook, writer, and owner of Sous Vide Wizard, a leading resource in sous vide cooking tips and recipes. Joe enjoys trying out new things in his kitchen and preparing delicious meals for his friends and family. You can reach him at Joe@SousVideWizard.com.