Cooking science

Collagen conversion to gelatin

Collagen is the connective tissue that makes a hard-working muscle tough. Given heat, moisture and enough time it converts into gelatine — soft, rich and dissolved into the surrounding liquid. That single conversion is why braising exists.

Tighter first, then soft

Collagen is a tough, fibrous protein arranged in triple helices. Heat it and the first thing it does is contract, squeezing the muscle fibres it surrounds and forcing out moisture. That is why a piece of shin cooked like a steak comes out tougher and drier than it started.

Held above roughly 70°C in the presence of water, and given hours rather than minutes, the helices unwind and the protein dissolves into gelatine. Gelatine is soft, slippery and holds a great deal of water, so the meat becomes tender and the cooking liquid becomes rich and lip-sticking.

The conversion is slow and it is not accelerated usefully by higher temperature — above a gentle simmer the muscle fibres contract harder and squeeze out moisture faster than the gelatine can compensate, which gives meat that is falling apart and dry at the same time.

What it means for cuts

High collagen — shin, chuck, brisket, oxtail, shoulder, cheek

Braise or slow roast for hours. These become the best-eating cuts there are.

Low collagen — fillet, sirloin, loin

Cook fast and briefly. There is nothing to convert and long cooking only dries them.

Thin, worked cuts — skirt, bavette, onglet

Cook fast and slice thinly across the grain. Too worked to be tender, too thin to braise.

Fish

Its connective tissue converts at a much lower temperature and in minutes, which is why fish is tender from the start and flakes so readily.

Poultry legs versus breast

Legs have collagen and reward longer cooking; breast has almost none and does not.

Why a stew is better the next day

Gelatine sets on cooling and gives the sauce body, and the flavours continue to diffuse.

Getting it right

• A bare simmer, not a boil. Boiling tightens the fibres faster than the collagen converts.
• Time, not temperature. A braise that is tough at two hours is often perfect at three.
• Moisture. The conversion needs water, which is why braising works and dry roasting a lean cut does not.
• Acid helps. Wine, tomato or vinegar speeds the breakdown and brightens a long-cooked dish.
• An oven is easier than a hob, because it surrounds the pot and there is no hot spot on the base.
• Judge by feel. A fork should go in with no resistance. That is a state, not a time.

Why a stew is dry and falling apart at once

It sounds contradictory and it is the standard result of a braise that was boiled rather than simmered.

Two things happen to the meat under heat and they happen at different rates. The collagen converts to gelatine, which softens the structure — that is what makes the meat fall apart. Meanwhile the muscle fibres themselves contract and squeeze out moisture, and they do that harder and faster the hotter the pot gets.

At a bare simmer the conversion gets ahead and the gelatine holds enough moisture to compensate. At a rolling boil the fibres win: the meat is structurally collapsed, because the collagen went, and dry, because the fibres wrung themselves out. The fix is not more liquid but less heat.

Common questions