Sour cream
Cultured cream — acid, fat and liquid together — which is why it tenderises more aggressively than yoghurt and enriches where yoghurt lightens.
Also called soured cream (UK); creme fraiche (INTERNATIONAL) — A close relative with a higher fat content and a milder acidity; not identical, and it splits less readily when heated.; what does sour cream do in baking (INTERNATIONAL)
Sour cream is cream fermented by bacteria that convert its lactose to lactic acid. It brings three things at once, and confusing which one is wanted is the commonest reason a substitution disappoints.
The ACID reacts with bicarbonate of soda to generate carbon dioxide, and it interferes with gluten development, which tenderises. This is the part yoghurt and buttermilk also supply.
The FAT is the part they do not. Sour cream is a cultured cream rather than a cultured milk, so it carries substantially more fat than yoghurt or buttermilk, and that fat shortens the crumb and enriches the bake. A cake made with sour cream and the same cake made with yoghurt are not the same cake, even though both are acidic dairy of similar thickness.
The LIQUID is real and thick. It contributes to the liquid phase, and it contributes less per gram than milk does because fat and protein occupy some of the volume.
Substituting yoghurt for sour cream keeps the chemistry and loses the richness. Substituting cream keeps the richness and loses the leavening reaction, which matters enormously in a formula built around bicarbonate of soda and not at all in one built around baking powder.
What changes if it is left out
Depends entirely on which of its three jobs the formula was using. Where bicarbonate of soda is the leavener, removing the acid removes the rise. Where the fat was providing richness, the bake becomes leaner and firmer. Where it was simply the liquid, milk substitutes straightforwardly.
Follows from the three functions recorded on this ingredient, each of which carries its own edge in the graph.
RecordedFollows from evidence recorded elsewhere on BakeHQ
Related
What it does
Hydration
Thick, and it contributes to the liquid phase. Less per gram than milk, because fat and protein occupy some of the volume — and how much is present is exactly what BakeHQ cannot know.
pH modification
Lactic acid from the culture reacts with bicarbonate of soda. This is the function it shares with buttermilk and yoghurt and the reason the three are so often swapped.
Gluten inhibition
THE FUNCTION THAT DISTINGUISHES IT from yoghurt and buttermilk. Sour cream is a cultured CREAM, so it carries substantially more fat, and that fat shortens the crumb.
Moisture retention
Fat slows the perceived firming of a crumb, which is much of why sour-cream cakes are reputed to keep well.
Can be replaced by
Natural yoghurt— for the acid, and not for the fat
Keeps the chemistry and loses the richness. Yoghurt is a cultured milk and sour cream a cultured cream, so the acid transfers and the shortening does not — a cake made each way is tenderer with the sour cream.