The half-life (∼ 2 h) of commercial yeast lactase (Kluyveromyces marxianus) in milk at 45 °C was 20 times greater than in milk salts and 50 times greater than in phosphate buffer. K was a more effective stabilizer than Na, both in milk salts and buffer. Stability was markedly reduced by the absence of divalent cations. Lactose and caseinate both stabilized the enzyme about 5-fold in milk salts, but together they stabilized it almost 50-fold. At equimolar concentrations, galactose was almost as effective as lactose as a stabilizer, but glucose was much less effective and sucrose had no effect. Stability in milk was independent of enzyme concentration, but varied with concentration of milk solids, reaching a maximum at about 25% solids. In milk, enzyme denaturation was more sensitive to changes in temperature in the range 42–53 °C than in the range 30–40 °C. At a concentration of 10 O-nitrophenyl-β-d-galactopyranoside units/ml it took 19 min at 45 °C to achieve 80% hydrolysis of lactose in milk, compared to 87 min at 30 °C.

The stability of lactase from Streptococcus thermophilus at 55 °C increased 7-fold, 2-fold and 1·5-fold in the presence of lactose, galactose and glucose respectively; maltose had no effect. Total stability over an 8 h period was more than 10-fold better in milk and sweet whey than in lactose solution, owing to the stabilizing influence of the milk proteins and the milk salts. Ovalbumin and reduced glutathione provided some extra stability but were not as effective as the milk components. In the absence of lactose the enzyme was less stable in milk and was not protected at all by sweet whey constituents. None of the milk protein fractions was as effective in the absence of lactose as when it was present. Enhanced thermostability of the enzyme in milk and sweet whey is due to contributions by all major milk components, but binding of lactose to the enzyme is the major factor controlling the extent of stabilization by other components.