There are hundreds of varieties of yeasts; the types commonly used in the production of Korean traditional wines are specially selected yeast. The small cakes and packets of yeast used in alcohol beverage-processing contain billions of individual yeast cells, each about 0.003 inch (0.075 mm) in diameter. Yeast is used to cause fermentation and leavening. The fungi feed on sugars, producing alcohol (ethanol) and carbon dioxide. One yeast cell can ferment approximately its own weight of glucose per hour. Yeast is 50 percent protein and is a rich source of vitamin B1, B2, niacin, and folic acid. Brewer ' s yeast is sometimes eaten as a vitamin supplement. In commercial production, selected strains of yeast are fed a solution of molasses, mineral salts, and ammonia. When growth ceases, the yeast is separated from the nutrient solution, washed, and packaged. Yeast is sold in compressed cakes containing starch or in a dry granular form mixed with cornmeal.