Rezaul Karim: Friday, June 1, was World Milk Day. If you’re only learning this now and you’re disappointed at missing the opportunity to celebrate your love of all things milk, good news: there’s still time to learn about its chemistry! Chemistry can help answer a number of questions about milk, including why it’s white, and why some people can’t drink it. The graphic above covers the basics, and we’ll look at some aspects in a little more depth below. For the sake of convenience we’ll be focusing on cow milk, though the general principles are applicable to the milk of other animals too!
Milk’s simple appearance belies its complex chemical nature. It’s mostly made up of water, with smaller amounts of fat, protein, minerals, and other compounds. Fats and water don’t usually mix, but in milk the fat and water form an emulsion, for reasons that we’ll explain shortly. As well as being an emulsion of fat in water, milk is also a suspension of a multitude of different proteins in water.
To pick apart this complex chemical mixture, let’s start with the fats. Triglycerides make up the fats in milk. These are molecules with a glycerol backbone and three fatty acid chains attached. The chemical identity of the fatty acid chains can vary; the most common fatty acids in milk are palmitic, oleic, stearic, and myristic acids. The variations in the amounts of these acids are a consequence of what cows eat. For example, the green grass of summer leads to a higher oleic acid content. This variation can affect how hard the fat is when isolated from the milk.
The tiny droplets of fat in milk are individually only about three or four micrometres wide. The membrane surrounding these droplets contains a variety of different compounds. These include proteins, which help the droplets form an emulsion with the water in milk. The fat has a lower density than water, so if the milk is left to stand it floats to the top, and can be skimmed off. This gives skimmed milk. Proteins are another key component in milk – and one that gives milk its white appearance. Proteins are long, chain-like molecules formed from smaller amino acid building blocks. There are hundreds of types of proteins in milk, of which casein is the main type. In milk, proteins cluster together to form structures called micelles. These clusters grow from small clusters of calcium phosphate, which help hold them together. There are a number of different models of these micelles, with the exact structure still being subject to scrutiny.
It’s the protein micelles which give milk its white appearance. The micelles are on average about 150 nanometers in diameter, and this very small size means they are able to scatter light that hits them. The overall effect of this scattering by the huge number of micelles in milk is that it looks white. Further compounds of interest are found dissolved in the water that makes up the bulk of milk. These include minerals and vitamins (with some fat-soluble vitamins found in fat droplets). It also includes lactose.
Lactose is a sugar found only in milk and dairy products. In our bodies, the enzyme lactase breaks it down into two smaller sugars, galactose and glucose. However, some people don’t produce enough lactase for this enzymatic breakdown to happen – these people are lactose intolerant. In their digestive systems, lactose passes through unchanged, and eventually reaches the colon. There, bacteria break it down, producing gas which can cause discomfort.
Bacteria are also to blame for the sour taste of milk when it goes off. The fermentation of lactose produces lactic acid, which causes the sour taste. It also increases the acidity of the milk; the knock-on effect of this is that proteins in the milk clump together, forming lumps. While this is an undesirable effect in your milk bottle, it’s an effect used intentionally in the production of cheese. There’s lots of interesting chemistry in cheesemaking, too – but that’s for another graphic!