Dairy and Heat: Why Cream Splits and Butter Burns

Butter is not pure fat

Butter is about 80% milk fat plus 16-18% water and milk solids. That is why it sizzles in a pan: the sound is water boiling off.

The milk solids are the problem. Once the water has gone, they start to burn at around 150C. Clarified butter — ghee — has both removed and takes 250C. Indian cooking uses ghee for a temperature reason, not a traditional one.

• Gentle heat and butter flavour: butter.
• High heat needed: ghee, or oil with a knob of butter.
• Brown butter: you are deliberately browning those solids, and the next stage is burnt.

With cream, the fat percentage decides

Whether cream splits when boiled is largely set by its fat content. 35% and above survives boiling; 30% and below splits easily. That is usually the difference between what is sold as whipping cream and as cooking cream.

Three things split it:

• Boiling. Proteins clump and the fat separates out.
• Acid. Add lemon, tomato or wine and boil, and it goes much faster.
• Salt plus prolonged heat, both of which help proteins coagulate.

So cream pasta never boils after the cream goes in, and in any acidic sauce the cream goes in off the heat, last.

Yoghurt needs all three conditions

Yoghurt is already acidic and high in protein, making it the most fragile dairy of all. Three rules, all of them:

• Room temperature. Cold yoghurt into a hot pan sets on contact.
• Off the heat, with the pan given a moment to cool.
• In stages, stirring. Poured in at once, the local temperature cooks it.

This is why rogan josh nails all three into its steps. Once it floats as curds, no amount of stirring recovers it (emulsions and pan sauces).

Milk is vulnerable to acid and salt

Add tomato or lemon to a milk-based sauce and it curdles. Reordering usually fixes it: reduce the acid first, add the milk last and gently.

Evaporated milk is the exception — already heat-concentrated, its proteins are stabilised and it takes boiling well. That is part of why poo pad pong curry uses it rather than coconut milk.

Cheeses melt in different ways

• Young, high-moisture cheese (mozzarella) stretches, because the casein structure is still loose.
• Long-aged hard cheese (parmigiano) does not stretch; it melts in — but releases its fat readily.
• Mid-aged cheese like cheddar goes oily and clumpy if the heat is too high.

The fix for a clumping cheese sauce is lower heat plus a little starch, which gets between the casein and stops it knitting. That is exactly what the cornstarch and white wine in cheese fondue are doing — starch preventing clumps, the wine's acid keeping the casein loose.

In short

Dairy breaks for one of three reasons almost every time: too hot, acid present, or added too early. Moving it later and turning the heat down solves most of it (heat control).