Depending on whether you’re a health nut, your mouth might start salivating at the sound of the word butter. But after eating four buttermilk biscuits, I was more curious about what it was and how it worked.

Butter, like cheese and cream, comes from raw milk. So logically, it should have a similar composition of nutrients. Since images don't work on this blog, I will summarize. The fat content is 11g and the protein content is 0g per serving for standard Land O Lakes brand butter you can get at the store. In comparison, Great Value whole milk has 8g of fat and 7g of protein per serving.

Where did the protein go? Even if the serving sizes don't match up, the ratio of fat to protein is far different. So I googled the process of butter making, because it must be some sort of byproduct that is thrown away (or something like that).

First, I learned about the structure of whole milk. Whole milk is an emulsion of fat (and some other stuff) in water. An emulsion is a mixture of two liquids that can't normally be mixed together. A basic emulsion has two parts, a dispersed part and a continuous part. You can think of the dispersed part as being inside the continuous part -- like oil inside water. Water might disperse inside oil as well, if you have more oil than water.

Emulsions are also unstable, which means they require some input energy (like shaking) and will separate after a while. This doesn't happen to milk, however, because of emulsifiers. These are an addition to emulsions that help keep them from separating. Without getting too technical, emulsifiers have parts that are hydrophilic (water attaching) and hydrophobic (water fearing) and so they help keep the emulsion from separating. Specifically in milk, the main part of the emulsifier is composed of the 2 main proteins in milk: casein and whey protein. These proteins form a membrane around the droplets of fat in the milk and prevent it from separating.

When milk goes bad, it's because bacteria convert milk sugar (lactose) into lactic acid. This acid disrupts the protein membrane and cause the milk to curdle -- which is the emulsion separating! It also tastes bad because of all the bacteria and acid.

Next, butter is made from cream, not whole milk. This means we have to convert the whole milk to cream first, before we can use it. This process is called skimming, because it involves skimming the fatty milk off of the top of the whole milk mixture. In unhomogenized whole milk, the less dense fatty part of the milk floats to the top and can be skimmed off. We don't see this in supermarket milk, because homogenization makes the milk particles so small that the density is more uniform. As a side note, skimming happens industrially using separator machines that literally pull out the denser (and heavier) milk using centrifugal force.

Now that we have cream, we can disrupt the membranes by applying physical force, or churning. After churning the cream enough, we separate it into the butter and buttermilk.

To answer our original question, the protein from milk stays in the milk. Butter is made from cream, which doesn't have very much protein at all to start out with.

Sources: (Mostly Wikipedia, some other stuff)

1. https://en.wikipedia.org/wiki/Churning_(butter)
   
2. https://en.wikipedia.org/wiki/Emulsion
   
3. Milk Emulsions: Structure and Stability, from the MDPI journal on food.
   
4. https://en.wikipedia.org/wiki/Separator_(milk)
   
5. https://milklife.com/articles/nutrition/types-of-dairy-milk
   
6. https://en.wikipedia.org/wiki/Cream