Reader Sandra wants to know why cream needs to be cold before you whip it. Excellent question and one I’m only too pleased to answer. Heavy cream is pretty amazing stuff: a liquid that can be turned (sort of) into a solid by agitating it. The geek in me, which is really most of me, says: neato.
Imagine liquid cream as a mixture of mostly water (about 60%) and little balloon-like fat blobs (38%) with a few stringy protein molecules, short-chain sugar molecules and minerals mixed in. Beat that and you introduce air bubbles into the mix, but you also do something still more interesting: you open a hole in some of those fat blobs, which are actually small quantities of flowing fat molecules enclosed in little protein-and-phospholipid (emulsifier) bags. The result? Some of those lipids inside get exposed to the watery medium outside.
And they don’t like that. For lipids have sections along their length that hate water (i.e. are hydrophobic). However they don’t mind air all that much, so the exposed lipid sides of the fat globules start collecting around the bubbles, where their water-hating parts can be exposed to air instead. As more fat blobs congregate around the bubbles, they encase the bubble in fat, preventing it from either popping or combining with a neighboring bubble. If the cream happens to be cool, so much the better as the fats insude the globules start forming crystals which give the bubble coating more rigidity. Does that make sense?
The volume of the cream increases as the agitation continues, however if it goes on for too long the bubble coatings get too thick, start congealing with one another, and well…butter is the result. Thanks for the question!