Cooking science

Fat rendering

Crisp skin is not made by high heat. It is made by getting the fat out from under the skin first — and that takes time at a moderate temperature, which is the opposite of what most people do.

What rendering is

Animal fat is held in cells within a layer of connective tissue. Heat melts the fat and breaks down the tissue holding it, so the fat runs out as a liquid, leaving behind the solid material that browns and crisps.

Skin sits on top of that fat layer. As long as the fat is still there, the skin is sitting on a wet, insulating cushion and cannot get hot enough or dry enough to crisp. Turn the heat up and you brown the outside of the skin while the fat beneath it is still liquid and soft, which gives you skin that is dark and flabby at once — a very common result with roast chicken.

So the sequence is: render, then crisp. Moderate heat for a long time to get the fat out, then high heat briefly to colour and crisp what is left.

How to render properly

• Start cold or moderate. Chicken and duck skin go into a cold or barely-warm pan, skin side down, and come up slowly. The fat has time to melt before the skin sets.
• Score the fat, not the meat. Cuts through a thick fat layer give the melted fat a route out and increase the surface exposed to heat.
• Dry the skin thoroughly. Uncovered in the fridge overnight is the best version — it is a dry brine and a drying step at once.
• Salt the skin and leave it. Salt draws moisture out of the surface.
• Pour off the rendered fat as it collects. Skin sitting in a pool of its own fat is frying, which is fine, but it will not dry.
• Press it flat for the first minute, so the whole surface is in contact.
• Finish hot. A blast of high heat or a grill at the end is what turns rendered skin into crackling.

By food

Chicken skin

Skin-side down in a cold pan, medium heat, ten minutes or more without moving. Most of the cooking happens on the skin side.

Duck breast

Score the fat in a crosshatch, start in a cold dry pan, render for eight to twelve minutes pouring off fat, then a minute on the flesh side. The rendered fat is the best roasting fat there is.

Pork belly and crackling

Dry the skin for hours or overnight, salt it, then a long moderate roast followed by a very hot blast. Any moisture and it will be leathery rather than crisp.

Bacon and lardons

Start in a cold pan. A hot pan seizes the outside before the fat has rendered, and you get chewy bacon.

Beef fat and suet

Rendered slowly for dripping. Excellent for roast potatoes.

Fatty fish skin

Different — there is little fat to render, so this is a hot, fast pan and a dry surface.

What the rendered fat is for

It is one of the more valuable by-products in a kitchen and most of it goes down the sink.

• Roast potatoes. Duck, goose and beef fat all give a better crust than oil.
• Frying eggs and bread, in bacon fat.
• Confit — meat cooked slowly submerged in its own rendered fat.
• Enriching a stew or a sauce, a spoonful at a time.
• Storing. Strain it through a fine sieve into a jar and keep it cold. It keeps for weeks.
• Not down the drain, where it solidifies and blocks pipes. Cool it, jar it, and bin what you will not use.

Common questions