Lactase (or β-galactosidase) is the enzyme involved in the hydrolysis of lactose to galactose and glucose.

Lactase produced commercially can be extracted from yeast fungi such as Kluyveromyces fragilis. Its primary commercial use is to break down lactose in milk to make it suitable for people with lactose intolerance. 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 stay liquid and contribute to a smoother texture. Lactase is used in the conversion of whey into syrup.

Lactase has an optimum temperature of about 48 °C for its activity and an optimum pH of 6.5.

(E. coli β-galactosidase: PDB 1DPO (http://www.rcsb.org/pdb/cgi/explore.cgi?pid=242441034361517&page=0&pdbId=1DP0), EC 3.2.1.23)