Every day, we eat scientific innovations, and not just when snarf powdered cheese flavor. Our food is the result of remarkable discoveries by long-forgotten scientists. And one of their most important innovations resulted in the chemical wonder known as buttermilk.
Arguably, the stuff most people buy in the store isn't buttermilk. That argument, granted, is based on procedure more than chemical composition, but the procedure is important. The procedure can tell us how people came up with buttermilk in the first place.
You must have heard the phrase, "The cream rises to the top." When fresh milk sits out for a while, that's what happens. The fattier, richer part of the milk is less dense than the rest, and rises up to float on top. Farmers skim off the cream, and eat it separately. At some point, perhaps while transporting a container of cream from one place to another, someone must have noticed that jostling the cream around a bit beats air into it and makes it fluffy and firm.
Once people began jostling the cream around continuously, they would do so for longer and longer periods of time, trying to get firmer, richer cream. Inside the cream, something interesting is happening. The constant knocking of the globules of fat against each other cause the thin membranes around the fat globules to begin to split. Eventually, the globules of fat, no longer separated by their burst membranes, group together and make butter. The liquid leftover - dense, low in fat, just slightly sour, filled with rich particles of stray butter - is buttermilk.
The slight sourness of the milk is due to the fact that buttermilk has spent some quality time with lactic acid bacteria. These bacteria live in milk. Fresh milk generally already contains a few strains of these. As the cream sits, collected over a period of days, the bacteria multiply.
Their food is the lactose, a type of sugar, that's found in milk. They tear the lactose apart. Lactose is a complicated molecule that starts out with twelve carbon atoms and twenty-two hydrogen atoms. When a bacteria is done with it, it's down to three carbon atoms and six hydrogen atoms. Lactic acid is a common end-product of the tearing apart of sugars. It's even produced inside the human body. The cells in your body make lactic acid from stored sugars to give you energy, and leftover acid is why your muscles sting after too much exertion. The acid is doing damage to your body. It also does slight damage to the cream and to the buttermilk. The proteins in the buttermilk, including the protein casings recently shed by the butter globules, thicken and stick together. Too much acid curdles the milk, but just a little makes it thicker and creamier while at the same time making it taste just a bit tart. (A more extreme version of this is what happens in yogurt.)
Today most manufacturers make buttermilk without the long wait time or the churning of the butter. They add lactic acid bacteria to regular milk and wait. Store bought buttermilk, then, is usually as high in fat as regular milk and doesn't have butter flakes in it. It only has that slight thickness and tartness from the lactic acid.
Cream skimming and butter shaking aren't scientific genius, but it's important to note that with these very simple techniques, early people managed to alter the chemical composition of milk. That had long-term consequences.
First of all, buttermilk makes an important source of nutrition available to a whole new section of the population. Lactose isn't easily digestible. Most animals, including certain humans, lose the ability to digest it as they age. Premature infants often don't have the ability to digest lactose. When bacteria break lactose down to smaller components, they are pre-digesting it for people. People with mild lactose intolerance can take in buttermilk, and pre-formula medical guides recommend feeding premature babies buttermilk to keep them healthy.
The acid in buttermilk also rips into gluten, a plant protein. Although it only softens it slightly, it, again, makes a food that was, for many, indigestible, into a potential source of nutrition. The production of buttermilk wasn't just about taste. It gave early people a way of manufacturing a very gentle acid that allowed them to digest a lot of their food more easily.
Which isn't to say that we shouldn't applaud those who only managed to use buttermilk to make food taste better. Anyone who bakes knows that buttermilk makes cakes smoother because of its pre-digested gluten. Its slight acid makes the milk proteins in creamy dressings cling together, leaving the dressing thicker and creamier. And buttermilk pancakes fluff up because the slight acid in the buttermilk reacts with baking soda to make the mixture fizz. We still use buttermilk chemistry, even though very few of us can buy real buttermilk anymore.