To clarify butter, you must separate the fat from the nonfat ingredients. The more completely you remove the nonfats, the more suitable your clarified butter will be for frying and storing. Many otherwise intelligently written cookbooks detail unnecessarily burdensome procedures for doing this. There are less complicated methods. Moreover, it eliminates the risk of scorching even one molecule of butter, a threat posed by the frequently recommended technique of melting it in a pan over a flame.
When butter melts, its emulsion breaks down. The butter then begins to separate into three distinct strata: a thin, whitish upper layer of foam; a thick, yellow middle layer of fat (your clarified butter); and a medium-thin, whitish bottom layer of water infused with carbohydrates and, especially, proteins, casein being the most important.
The protein-carbohydrate water solution that makes up the bottom layer contains no fat because the fat, having a lower specific density than the bottom layer, has followed the immutable law of nature by rising. (Or you could say with equal logic that the protein-carbohydrate water solution has sunk).
The foamy upper layer is often erroneously referred to as an impurity-based scum. This layer is principally made up of water, proteins, and carbohydrates and is thus similar to the bottom layer. It is prevented from dropping through the fat layer because the trapped air in the frothy structure literally keeps these particular nonfats floating on top of the fat layer. By the time the bubbles burst, the fat layer will have solidified, preventing the denser upper layer from settling to its natural level.
The air bubbles form chiefly because, as the butter melts, bacteria attack some of the carbohydrate lactose, fermenting that milk sugar and thereby producing carbon dioxide gas (as well as some alcohol). Another source of the gas in the bubbles is the air content of the butter. (Most American unsalted butters are about 4 percent air by volume).
When your melted, de-emulsified butter is refrigerated, the fat layer solidifies. Refrigerator temperature is not cold enough to firm the watery bottom layer.
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