• Meat consists of muscle fibers, connective tissue, water, and fat
• Sous vide provides precision and prevents moisture loss
• Enzymatic action around 130°F/54.5°C can give meat an incredibly tender texture

Humans learned to cook meat for three main reasons: It gets rid of microbes that could make us sick; it turns bland pink lumps into delicious meals; and it changes meat’s physical structure in crucial ways to make it more eatable and digestible. This is possible because of the microscopic structure of meat and what happens when it is heated—whether with traditional cooking methods or sous vide.

Whether it comes from a cow, pig, or chicken, meat and poultry consists primarily of muscle fibers, water, connective tissue, and fat. Muscle tissue resembles many bundles of wire, each surrounded by a covering of connective tissue. Each bundle is made up of numerous muscle fibers. These fibers are made up of many smaller structures called myofibrils. Cooking changes the structure of those muscle fibers, and whether a piece of meat comes out tough or tender depends on cooking time and cooking temperature. When red meat and poultry are heated, the long protein molecules begin to contract, first (between 104°F/40°C and 145°F/63°C) in diameter, and then (above 145°F/63°C) in length. A single muscle fiber can shrink to half its original length during the cooking process.

When proteins contract, they squeeze out some of the liquid trapped within their structure. The rate of moisture loss becomes significant around 140°F/60°C, the temperature at which the connective tissue surrounding the muscle fibers begins to tighten as well, squeezing the fibers even more firmly. Raw muscle fibers contain a lot of water (around 75 percent!), and this water loss can cause a cooked piece of meat to end up quite tough. We rest meat after cooking via traditional high-heat methods, allowing the contracted proteins to relax and draw some moisture back in.

The connective tissue surrounding the fiber bundles is a membranous, translucent covering that consists of cells and protein filaments. It provides both structure and support to muscles. Collagen, the predominant protein in connective tissue, is composed of three protein chains tightly wound together in a triple-stranded helix and is therefore almost unchewable when raw.

But collagen begins to relax when it hits heat, unwinding into individual strands. This happens very slowly at temperatures as low as 122°F/50°C and far more rapidly between 160°F/71°C and 180°F/82°C. Eventually, the triple helix of collagen turns into gelatin, a single-stranded protein able to tenderize meat, retain up to 10 times its weight in moisture, and add a thick richness to the sauces of a braised dish. Tough, collagen-heavy meats are often held in the higher temperature range for a few hours to encourage the triple helix of collagen to unwind and form gelatin more quickly.

Precision: We know a lot about the science of cooking meat, but that doesn’t make all of our cooking foolproof. A roast comes out dry and mealy instead of succulent, and the middle of a steak is still too rare though the outside is perfectly crusted. Sous vide gives us precision, allowing us to cook meat (and poultry) to an exact temperature all the way through, guaranteeing that you will never again overcook your fancy rib-eye.

Preventing Moisture Loss: Most tender cuts of red meat are best cooked to medium-rare—or around 130°F/54°C—so that cooking is finished before the muscle fibers really begin to squeeze out all of the moisture within. But when cooked to medium-rare in a skillet, the outer layers of a piece of meat soar well above 140°F/60°C—the temperature at which the moisture loss really picks up. Sous vide gives us the ability to cook these cuts to a precise medium-rare from end to end, and with no hot spots. This is why we don’t need to rest meat cooked sous vide in order to retain moisture: We are cooking most meat below 140°F/60°C. That said, we do rest meat before searing in order to let the temperature fall a bit and reduce risk of overcooking when the meat is in the hot skillet.

Turning Tough Cuts Tender: Collagen proteins unwind into moisture-holding gelatin at temperatures as low as 122°F/50°C. Sous vide cooking allows us to hold tough, collagen-heavy cuts of meat at lower temperatures for longer periods of time and get the same tenderizing effect as braising.

Enzymes are also at work during low-temperature sous vide cooking. In living animals, one of the functions of these proteins is the turnover and reprocessing of other proteins around them. In meat, many of the enzymes are still active, and if handled correctly, they can work wonders on the cook’s behalf. Dry-aging beef is a classic example: Beef is held at a steady temperature between 33°F/0.5°C and 40°F/4°C for 30 days or more. In this temperature range, enzymes in the meat work slowly to break down protein, resulting in much more tender steaks.

In meat, there are two important enzymes that work to break down protein: calpain and cathepsin. Calpains break down the proteins that hold the muscle fibers in place. Cathepsins break apart a range of meat proteins, and can even weaken the collagen in the muscles’ connective tissue. Breaking down protein imparts a meatier umami taste (due to the formation of amino acids) and, given enough time, tenderizes the meat—that is, if the environment is right.

The activity of these enzymes is largely based on temperature—and the amount of time held there. The rate at which they break down the protein in a cut of meat increases as the temperature of the meat rises. This is why sous vide cooking allows us to make enzyme-tenderized meat in hours, not days. Calpains cause proteins to fall apart around 105°F/40°C, so they’re not very helpful in sous vide, but cathepsins are. Although they begin to break down proteins around 122°F/50°C degrees, the breakdown is a long process, and cathepsin activity is still going on during a lengthy cook at 130°F/54°C. (This is also why you would not want to cook fish for a long period of time sous vide. These enzymes are also active in fish, and too much time in the presence of tenderizing enzymes can make fish protein—which is quite tender to start with—mushy.)

So, as you cook your perfect seared steaks, skinless boneless chicken breasts, or prime rib, think about what’s going on under the surface: the deliberate movement of proteins, enzymes, and water, working together to create the ultimate finished dish.