Reader Nora asks:
You say that fat is what thickens a fermented dairy product like sour cream, but yogurt is even thicker and has almost no fat. Can you explain that?
I can. (I think). But first I should clarify that fat isn’t the only thing that thickens sour cream. Both yogurt and sour cream are thickened by coagulated milk proteins. Bacterially-generated acid creates this effect, changing the polarity of the bonding sites on the long and languid protein molecules, so that instead of being repelled by one another they become attracted. The result is a net that catches other molecules in it like so many fish. Since the mixture can’t flow as easily, it thickens.
There’s a difference, however, in the thickening powers of various lactic acid bacteria around the world. The by-products of the lactic acid bacteria that live in places like Afghanistan create very dense protein networks. Those made by bacteria that live in places like Kentucky don’t, which means that if I want firm yogurt instead of semi-liquid clabber (which is what you get when you ferment milk with bacteria that are native to America), I need to import microbes to do the job.
The interesting thing about yogurt is that when you add more fat to it (say, by using whole milk instead of skim milk) it actually gets thinner. This is because fat molecules get in the way of the protein net as it tries to form, interfering with its strength.
But that’s milk. Cream has a whole lot more fat than milk does, from five to eight times more depending on the type of cow that gave it. Since fat molecules and water molecules don’t really get along, the fat molecules form groups, then blobs. Combine those big blobs with even a weak, Kentucky-made protein network and the result is very little flow and thus thickening, albeit of a slightly different kind.
Think of yogurt thickening as a very fine-mesh net catching lots of very little fish, and sour cream thickening as a very broad-mesh net catching lots of very big fish. Make sense? Thought not. Dang.