The Chemistry of Milk
Although you don’t need a chemistry degree to get through this, studying the chemistry of milk can get nerdy and technical. However, it is necessary if you want to get a beautifully textured milk and a perfect latte art. It is susceptible to changes in the cows’ dietary conditions, health, food intake and living region. All of these variables change the taste and makeup of milk.
Assuming all cows are raised in the same environment and eat the same food, there are three things in milk that directly influence the taste of coffee: lactose (milk sugar), fat and protein content.
Lactose created the sweetness in milk. Lactose is a combination of two sugar molecules, galactose and glucose, held together is suspension in our milk. Lactose doesn’t break down in water. It doesn’t taste sweet when cold. However, when it is heated the solubility breaks down and brings the sweetness, releasing the sugars into our cups.
Put whole milk and skim milk on blind tasting, skim milk has lighter body (more watery), shorter after taste. Whole milk has fuller body, thicker and heavier. Why? It’s because of the fat content. Whole milk contains 4% fat content. Cream is the higher-butterfat layer rises to the top of milk, and it’s even more tasty than milk.
Through mechanical extraction, manufacturers can alter fat content of milk to 2% (50% extracted), ½% (almost all fat extracted) and skim (where the fat has been removed, basically leaving water, proteins and sugars). Arguably whole or full cream milk will give the coffee a fuller body and taste, but still some believe in skim as believing fat is bad for health.
Now comes to milk frothing. Proteins are the third main concern for the barista. When you steam your milk, the proteins will stick, building a cage around each air molecule and cause it to ‘float’. Fat content, protein, and temperature influences each other.
As the fat content increases, the ability to make large volumes of froth and the stability of that froth, decreases. Therefore, it is easier to get more and denser froth with skim milk than it is with whole milk. BUT, as the fat content continues to rise above 4%, say from 5% onwards, froth volume and stability go up once again. You’ll see high volumes and stability in your froth in a cream with about 10% fat. That’s why table cream has 18% fat and whipping cream has 35%…volume and stability.