Casein (from the Latin word for cheese) is a family of proteins found in various proportions in milk, which also provides carbohydrates, amino acids and minerals, particularly phosphorus and calcium. As well as being a major component of cheese, it is popular as a dietary supplement.
Micelle is a chemical term (derived from the Latin word for granular) used to describe a structure that allows very large molecules such as casein to remain suspended in aqueous solution. Casein molecules are formed in this way so that the protein core and component minerals can remain suspended in the water which makes up the rest of the milk, and when ingested, take much longer to break down in the stomach acids than simpler chemical structures.
This component of casein is usually produced by microfiltration, which separates it from the whey protein that remains, and concentrates the micellar nucleus. The milk proteins can then form a mass or bolus in the stomach, which allows for a sustained slow release (measurable in hours) into the blood stream of the essential amino acids. For this reason, many athletes and bodybuilders use micellar casein supplements before bedtime, so muscle mass is not lost overnight.