Jeff Jones, age 12, of Biloxi, Miss., for his question:

WHAT IS CASEIN?

Casein, pronounced kay seen, is the chief protein in milk. It is also the main ingredient in cheese.

Casein separates as curd when milk sours, or when acid is added. It also separates from sweet milk when the enzyme rennin is added.

Casein contains carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Pure casein is a tastless, odorless, white solid. Cow milk contains about three percent casein.

Commercially produced casein is made from skim milk. The curd is washed, dried and ground. It is pale yellow and has a pleasant odor. It is widely used in medicines, cosmetics and as a sizing or coating for paper.

Casein is also used in waterproof glues, casein paints and certain plastic articles.