Milk vs cream
The same materials in opposite proportions — and past a certain fat level the behaviour changes in kind, not just in degree.
The short answer
Cream is milk with most of the water removed and the fat left behind, so substituting one for the other changes the fat and the liquid together and in opposite directions. Below roughly the double-cream fat level, cream is a richer milk and behaves like one. At and above it, something changes in kind: there is enough fat for partly crystallised globules to build a network around air bubbles, and the cream whips. That is why the fat percentage on the pot decides what a cream can do, and why substituting a pouring cream into something that must be whipped fails completely rather than partially. Using cream where milk was specified enriches heavily and reduces the liquid; using milk where cream was specified removes the richness and, if whipping was the point, removes the mechanism.
Where they differ
| Dimension | First | Second |
|---|---|---|
| Fat | Low, and it varies by type. | High enough to build a fat network. |
| Water | About seven-eighths. | Much less; a substantial part is emulsified with fat. |
| Whipping | Cannot. There is not enough fat. | Can, when cold — and fails when warm, because the crystals have melted. |
| As a liquid | A straightforward liquid phase. | A poor one. Treating cream as liquid gives a far richer result than intended. |
| Effect on crumb | Softens modestly. | Shortens and enriches substantially. |
Related
Compares
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One side of the comparison, so it is reachable from the record a reader is more likely to land on first.