Cooking science

Fat crystallisation on cooling

Fats do not have a single melting point. They are mixtures of different molecules that solidify at different temperatures and into different crystal forms, and which form you get depends on how fast you cooled it. That one fact explains tempered chocolate, grainy ganache, spreadable butter and flaky pastry.

Fat is a mixture, not a substance

Butter, cocoa butter, lard and oil are all mixtures of many different triglycerides. Each has its own melting point, so as a fat cools some of its components solidify while others are still liquid. This is why butter straight from the fridge is hard, butter at room temperature is spreadable and butter in a warm kitchen is greasy — at every temperature a different proportion of it is solid.

It is also why fat has a melting range rather than a melting point, and why a recipe that says "soft butter" is describing a state rather than a temperature. Cocoa butter is the exception that proves the rule: it is unusually uniform, which is why chocolate melts so sharply and so close to body temperature.

The same fat, different crystals

A fat can solidify into several different crystal arrangements from exactly the same molecules. The arrangements differ in how tightly the molecules pack, and that changes the melting point, the hardness, the gloss and the mouthfeel of the finished solid.

Loosely packed forms appear quickly and are unstable; tightly packed forms take longer to form and are stable once they do. Left alone, a fat will slowly rearrange from an unstable form into a stable one — and because the stable form is denser and melts higher, that rearrangement changes the food. Chocolate that has bloomed grey has done exactly this.

Controlling which form you get is what tempering chocolate is. Cool it, encourage the stable crystals to seed, then warm it just enough to melt the unstable ones back out. The result snaps, shines and melts cleanly. Untempered chocolate is soft, dull and streaky, from the same ingredients.

Where it shows up in cooking

Chocolate

Tempering. Stable crystals give snap, gloss and a clean melt; unstable ones give a soft, dull bar that blooms.

Ganache

Cooling too fast, or agitating at the wrong temperature, gives large crystals and a grainy set. Cool it slowly and stir it as little as possible.

Butter for pastry

You want it solid enough to stay in flakes, plastic enough not to shatter. That is a narrow window and it is why pastry is made cold and worked fast.

Fudge and toffee

The butterfat crystallises alongside the sugar. Fierce boiling separates the fat out and gives a greasy result.

Chilled sauces and dressings

A sauce made with butter or oil can look split from the fridge simply because the fat has crystallised. It usually comes back on warming.

Ice cream

Fat crystals partially coalesce around the air bubbles and are a large part of why ice cream feels rich rather than icy.

Practical consequences

• Cool fat-rich things slowly where you want a smooth set. Fast cooling gives many small crystals of the wrong type; slow cooling gives fewer, better-ordered ones.
• Do not stir a setting ganache or a cooling fudge. Agitation while crystals are forming makes them grow larger, which is what "grainy" means.
• Grey or white streaks on chocolate are not mould. They are fat that has migrated to the surface and recrystallised. It is harmless and the chocolate is fine for cooking, though it will not temper well again without being re-melted.
• Butter that has melted and re-set is not the same butter. Its crystal structure has gone, which is why melted-and-cooled butter behaves differently in a creamed cake.
• Warmth is a tool. A ganache that has set too hard, a stiff buttercream, a solid coconut oil — all respond to gentle warming and a little patience.

Common questions