The percentage on a carton of cream decides what it can be asked to do

Cream is butterfat suspended in milk

Every cream is the same two components in different proportions: milk fat in globules, and the watery milk phase carrying protein, sugar and minerals around them. Move the fat percentage and the behaviour changes in steps rather than smoothly, because several distinct physical thresholds sit within the range that dairies sell.

Naming conventions differ by country and the numbers on the carton are the only reliable guide. Broadly, single or light creams sit around eighteen to twenty percent, whipping creams around thirty to thirty-six, double or heavy creams around thirty-six to forty-eight, and clotted cream considerably higher. Butter, at the end of the sequence, is roughly eighty percent fat with the phases inverted.

Whipping needs enough fat to build a scaffold

Whipped cream isn’t a protein foam like meringue. It is a foam stabilised by fat: beating partially crystallised fat globules causes them to collide, their membranes to rupture, and the exposed fat to stick them together into chains that surround the air bubbles. Partial coalescence is the term, and it needs enough globules close enough together to find one another.

Below roughly thirty percent fat there are simply not enough of them, and cream will foam briefly and collapse regardless of technique. This is why light cream cannot be whipped and why no amount of persistence makes it happen. It is also why cream whips only when cold: the fat must be partly solid for the globules to hold their shape and link. At room temperature they merge into a greasy mass, which is the point at which whipped cream turns to butter.

Heat is where the low percentages fail

Fat coats and protects the milk proteins, so a high-fat cream can be boiled, reduced and combined with acid far more safely than a low-fat one. Single cream added to a hot acidic sauce or brought to a rolling boil is likely to separate, because the proteins are exposed and coagulate. Double cream in the same situation usually survives.

Reduction concentrates the fat as water boils off, so a cream that begins near the safe threshold moves further away from trouble as it cooks, while one that begins below it tends to break before it gets there. That is why a sauce made by reducing double cream thickens obediently and the same procedure with single cream produces something grainy. The starting percentage decides which of the two happens.

This is why recipes that specify a cream by percentage mean it, particularly for caramel sauces, reduced fillings and anything containing citrus or wine. The substitution people make most often, using a lighter cream and compensating with a thickener, addresses the viscosity and does nothing about the stability, which is the part that actually fails.

Ganache ratios are fat ratios in disguise

A ganache is an emulsion of the water in the cream with the fat from both the cream and the chocolate, held together by the emulsifiers present in each. Its final firmness depends on the total fat and the total water, so switching to a cream with a different percentage moves both at once and changes the set. A ganache made with a thirty percent cream is looser than the same weights made with a forty.

It is also why a ganache intended to be whipped is made with a higher proportion of cream than one intended to be piped firm. The first needs enough water phase and enough soft fat to hold air; the second needs enough cocoa solids and cocoa butter to hold a shape at room temperature. Same two ingredients, opposite ends of the ratio.

Splitting has the same origin. Too much fat for the available water, or too little emulsifier to hold them, and the emulsion breaks into oil and solids. The standard rescue, whisking in a small amount of cold milk or cream, works because it supplies the water phase that was in short supply, which is the opposite of what instinct suggests.

Cultured and concentrated creams are separate animals

Crème fraîche and sour cream have been fermented, so their proteins are already partly aggregated by acid and they behave differently again. They resist splitting when heated better than an unfermented cream of the same fat content, because the coagulation that would cause the split has largely already happened in a controlled way. That makes crème fraîche unusually useful in warm preparations.

Mascarpone and clotted cream go the other way, having been concentrated rather than fermented, and they carry so much fat that they behave almost as soft butters. Neither whips in the ordinary sense and both loosen if worked, since there is not enough water phase to maintain a foam. Knowing which of these three families a carton belongs to explains most of what it will do before you open it.