Cooking science

Shrinkage

A joint of meat can lose a quarter of its weight in the oven. Spinach loses nine tenths of its volume. Onions reduce to a fraction. Almost all of it is water leaving, and knowing roughly how much saves a great deal of misjudged shopping.

Water leaving, and fat rendering

Meat shrinks because muscle fibres contract and squeeze out the water held between them, and because fat melts and runs off. Both are unavoidable and both increase with time and temperature.

Vegetables shrink because the cells lose turgor and collapse, and the water inside them evaporates or runs out. Leafy vegetables shrink most dramatically because they were mostly air and water to begin with.

Almost nothing shrinks by losing solids. What leaves is water, and to a lesser extent fat — which is why concentrated flavour is the consolation for a smaller dish.

Rough shrinkage, by food

Useful for planning rather than precise. Everything varies with cut, heat and time.

Roast meat

Around a fifth to a quarter of raw weight, more if cooked well done.

Mince browned in a pan

Around a quarter, and more if it was frozen.

Bacon

Roughly a third to a half, depending on how much fat renders.

Spinach and other leaves

Around a tenth of the raw volume. A whole bag becomes a side dish for two.

Mushrooms

Roughly half by weight, and far more by volume.

Onions cooked down slowly

A very large reduction — several kilos become a few jars of chutney.

Roast vegetables

A fifth to a third, depending on water content.

Fish

Less than meat, roughly a tenth to a fifth.

Reducing it where you can

• Do not overcook. Every extra degree costs more liquid. This is the largest single factor.
• Brine lean meat. Salt lets the muscle hold more water through the cook, so less is lost.
• Cook at a lower temperature. A gentler cook contracts the fibres less violently and loses less.
• Buy fattier cuts where dryness matters. They lose fat, which is less noticeable than losing water.
• Do not press or squeeze. Pressing a burger or a steak forces out the very thing you are trying to keep.
• Rest it. Some expelled liquid is reabsorbed as the fibres relax.
• Use the liquid. What ends up in the tin is concentrated and belongs in the sauce.

Planning quantities

1. Buy meat on the bone by a larger margin than boneless. Bone is weight that never becomes a portion.
2. For leafy greens, assume a bag serves far fewer people than it looks like it will. Two large handfuls per person is not excessive.
3. For mushrooms and onions in a dish where they should be noticeable, buy roughly twice what looks right.
4. For a roast, work from the finished weight you want and add a quarter.
5. For a braise, shrinkage matters less because the liquid is part of the dish.
6. Write down what a quantity actually served. It is the only reliable way to learn your own portions.

Common questions