Butterfat is the fatty component of milk, composed of a mixture of saturated and unsaturated fatty acids. The amount of fat in cow milk varies, depending on a number of factors including the time of year, the cow's diet, and the cow's age. Regulations defining various dairy products and the differences between them often use butterfat as a standardizing measure because it is easy to measure and dairy products can be adjusted to ensure they have uniform fat contents.
In fresh milk, butterfat takes the form of small globules of fat that can be seen under magnification. If the milk is allowed to stand, the fat slowly rises to the top. Historically, people would let milk stand after milking and then skim it to remove the fat for churning into butter and cream products. Today, milk is more commonly spun in centrifuges to separate out the fatty components, allowing the milk to be kept in chilled and sterile conditions at all times.
In homogenized milk, the butterfat is broken up so it will not filter out, but instead remains suspended in the fluids of the milk, in a solution known as a colloid. This is done with milk and cream to prevent the butterfat from separating, although it is sometimes possible to purchase specialty products that have not been homogenized and will separate if allowed to stand.
Whole milk usually has around 3.5% butterfat. Whipping cream can have between 30 and 35%, and heavy cream has at least 36%. Butter is almost entirely fat, as it is made by separating out the fat and then whipping it to encourage it to coalesce into a mass. Butters usually have a minimum of 80% fat. Dairy products are tested to confirm that they are being labeled appropriately and their fat content may be adjusted to meet a desired sales standard.
As many dairy consumers have noticed, fat content has a profound impact on mouthfeel. Whole milk tends to taste more rich and creamy than nonfat and skim milk products, while whipping cream, if tasted straight, can feel almost heavy and greasy because it has so much fat. Ice creams are impacted by the amount of butterfat present. If it is too low, the ice cream tastes icy and watery, while high fat contents result in greasy, sticky ice cream products. Companies that make ice cream commercially have very strict standards for the milk they will accept for production to ensure their butterfat content remains consistent so consumers do not complain about flavor variations.