There’s a definite difference between cheese and milk. It's clear in the tastes, textures, state of matter, and smells. However, when it comes to that iffy area in-between milk and cheese it get’s a little muddy. What exactly is the difference between sour cream and crème fraîche? How is yogurt different from kefir? Why is the buttermilk purchased in stores rarely ever true buttermilk?
I think having a firm understanding of your soured dairy not only makes you a smarter consumer, but a more accomplished cook and a more refined appreciator of milk. It gives you an appreciation for the people who produce it. It also makes you a smarty pants who can brag at parties when a waiter passes around the canapés.
First, let’s define souring. Souring occurs when a bacterial culture is added to cream. The bacteria then begin to convert the lactose -- a specific sugar found in milk -- into lactic acid. This lactic acid sours and thickens the cream by breaking down existing bundled proteins (called casein) and turning them into a patchwork that hold together pockets of fat and water. Imagine it this way: the bacteria take a few skeins of yarn and knit them into a net to hold the milk together as a loosely solid product.
Most buttermilk you see in stores is actually cultured buttermilk, and therefore not true buttermilk. True buttermilk is the low-fat leftovers of milk that remains after it’s been churned into butter. This milk would often sour quickly and become thick -- the flavors would be less sour, less sweet, and with a more complex flavor.
Cultured buttermilk is the low-fat leftover of milk that’s been run through a cream centrifuge (a device used by companies to quickly get cream out of milk to make butter). This low-fat milk is then given a heat treatment to help it thicken and the resulting product is then fermented. The result is a milk that’s sweeter and tangier.
Neither true nor cultured buttermilk is better than the other. It’s just a matter of taste.
Sour cream has about 20% milk fat. Due to its high protein levels it doesn’t cook well and curdles easily when heat is applied. Many commercial sour creams have thickening agents added to them such as gelatin as a naturally sour cream is often somewhat runny. Sour cream with thickeners added can still be advertised as natural and organic (those tricky marketing bastards).
Crème fraîche is like sour cream, but it has a 30% milk fat content. Due to a higher fat content and lower protein content it can be cooked without breaking apart of curdling. However, it is less sour than sour cream. The name is also much fancier than that of the lowbrow sour cream.
Yogurt goes through two primary stages of production: heating and fermentation. The heating process requires milk be heated to at least 185F/85C for 30 minutes or at 195F/90C for 10 minutes. This causes the various proteins and fats to bond together in a complex chain as opposed to blobs here and there floating in a liquid.
The fermentation stage is where bacterial cultures are added (most often Lactobacillus bulgaricus). The milk begins to thicken even more and sour intensely. Due to the protein matrix developed this ingredient does not separate when allowed to sit.
This is a cultured milk product that is often consumed as a beverage. To make kefir, kefir grains are added to milk that is then agitated often and placed in an acid-proof container in a dark place to ferment. The grains are cauliflower-looking crystals of yeast and bacteria trapped in a lattice of proteins and sugars. Kefir undergoes complex fermentation that releases carbon dioxide and can on occasion contribute a bit of fizziness.