Eli5: why do so many baking recipes require milk and butter are they not essentially the same thing chemically?
Not necessarily. Butter is made from the fat content in raw milk. But the milk that you drink regularly, the milk you buy in stores, has had the majority of the fat removed from it. The “full fat” milk you buy in the store has still only a small percentage of that original milk fat remaining.
But likewise, many recipes are also concerned about the moisture level of the ingredients, and their consistency. When cooking, you will get different results depending on how moist your ingredients are, or how much fat you are including it in the recipe.
No, they aren’t. Butter is only one of several components in milk.
Butter is mostly (about 80%) fat. The rest is a small amount of protein and water.
Whole milk, on the other hand, is only about 3.5% fat. Like most drinks, it’s mostly water, but it also contains several different types of protein, minerals, and lactose (a sugar).
To make butter, milk is left to stand until a layer of cream forms on the top; this cream is mostly fat, with a little bit of some of the proteins. The remaining proteins and nearly all of the sugar stay dissolved in the liquid, so butter contains a lot less protein than milk (or than cheese made from milk) and very little sugar.
Because the majority of recipes often require some source of fat (butter, margarine, oil, lard), and butter is pretty much pure dairy fat. There’s just not enough fat in milk alone to accommodate for most recipes.
Butter is concentrated dairy fat with most of the other elements such as water and milk solids removed. Milk is mostly water with a network of proteins and fats which include what could be extracted for butter. Often milk is used to add water and a light mix of proteins and such, while butter is strictly about adding fat.