Cooking science

Fat melting

Every fat melts over a range rather than at a point, and where that range sits decides almost everything about how it behaves — whether it can be rubbed into flour, whether it sets on a cold plate, and whether it feels waxy in the mouth.

Shape decides the melting point

Fat molecules with straight chains — saturated fats — pack together neatly and need more energy to separate, so they melt at higher temperatures and are solid at room temperature. Butter, lard, suet and coconut oil are all in this group.

Fat molecules with kinks in the chain — unsaturated fats — cannot pack tightly, so they melt lower and are liquid at room temperature. Olive, rapeseed and sunflower oils are all here.

Most natural fats are mixtures, which is why they soften over a range rather than melting sharply. Butter is spreadable at room temperature and pourable when warm because different fractions melt at different points.

Why melting point decides the job

Pastry

Needs a fat that stays solid while being rubbed in and melts in the oven, leaving pockets. Butter, lard and suet work; oil does not.

Suet

The highest melting point of the kitchen fats. Stays solid long enough for the flour around it to set, which is why suet puddings have their texture.

Coconut oil

Melts near room temperature, which is why it is solid in winter and liquid in summer, and why it sets hard on a cold plate.

Butter

Softens over a wide range, which is why it is spreadable at some temperatures and not others.

Chocolate

Cocoa butter melts just below body temperature. That is why chocolate snaps at room temperature and melts in the mouth.

Mouthfeel

A fat that is still partly solid at body temperature coats the mouth and reads as waxy. One that melts fully does not.

Practical consequences

• Keep pastry fat cold. The whole method depends on the fat still being solid when it goes into the oven.
• Do not substitute oil for butter in a rubbed-in pastry. There is nothing to leave a pocket.
• Half butter, half lard in shortcrust gives flake from the lard and flavour from the butter.
• A dressing made with a high-melting fat will set on a cold salad. Use oil.
• Duck fat coats potatoes better than beef dripping because it is softer and spreads more readily on a cold surface.
• Chill a fatty stew and lift the set fat off — the fat solidifies and the liquid does not.
• Warm the plates for anything with a lot of animal fat, or it congeals as you eat.

Choosing a fat by what you need it to do

The melting range is usually the deciding factor and almost nobody thinks about it directly.

1. Ask whether the fat needs to be solid at any point in the process. Pastry, suet puddings and a chocolate coating all say yes.
2. If it does, pick a fat that is solid at the temperature it will be handled at — and keep it that way. Warm butter in pastry has already failed.
3. If the fat only has to carry heat or flavour, the melting point barely matters and you can choose on taste and smoke tolerance instead.
4. For anything that will be eaten cold, check what the fat does at fridge temperature. A dressing made with a hard fat sets into flecks.
5. For anything rich and hot, warm the plates. A fat that congeals halfway through the meal ruins it.
6. Where a recipe specifies a particular solid fat, treat that as structural information rather than as a preference.

Common questions