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.