Lactase (LCT), a part of the ÃŸ-galactosidase family of enzymes, is a glycoside hydrolase involved in the hydrolysis of the disaccharide lactose into constituent galactose and glucose monomers. In humans, lactase is present predominantly along the brush border membrane of the differentiated enterocytes lining the villi of the small intestine. Lactase is essential for digestive hydrolysis of lactose in milk. Deficiency of the enzyme causes lactose intolerance. The optimum temperature for lactase is about 48 Â°C (118.4 Â°F) for its activity and has an optimum pH of 6.5. Lactase produced commercially can be extracted both from yeasts such as Kluyveromyces fragilis and Kluyveromyces lactis and from fungi, such as Aspergillus niger and Aspergillus oryzae. Its primary commercial use is to break down lactose in milk to make it suitable for people with lactose intolerance. However, the U.S. Food and Drug Administration (FDA) has not formally evaluated the effectiveness of these products. Lactase is also used in the manufacture of ice cream. Because glucose and galactose are sweeter than lactose, lactase produces a more pleasant taste. Lactose also crystallises at the low temperatures of ice cream; however, its constituent products, glucose and galactose, remain dissolved and contribute to a smoother texture. Lactase is used in the conversion of whey into syrup. Lactase is also used to screen for blue white colonies into the MCS of various plasmid vectors in Escherichia coli or other bacteria. The reaction that lactase catalyzes is C12H22O11 + H2O ? C6H12O6 + C6H12O6 + heat.