We have identified the most relevant properties of a commercial recombinant lactase from Bifidobacterium bifidum (RBBL, Saphera 2600 L) for milk hydrolysis, with a yeast neutral lactase (GODO-YNL2) being used for comparison. Both products were characterized according to their lactase and invertase activities, protein profiles by sodium-dodecyl-sulfate-polyacrylamidegel electrophoresis, and Kms for lactose. RBBL exhibited properties that permitted milk hydrolysis over a broader range of conditions than YNL: apparently, milk's ionic composition was not an activity-limiting property, an optimum temperature range between 45 and 50 °C and a considerable activity at pH 4.5, which would permit lactose hydrolysis in acidic dairy products. Like YNL, the bacterial lactase retains a considerable activity under refrigeration (3-7 °C). To describe the lactose-hydrolysis time course, an empirical model was used in which the glucose obtained was expressed as a function of the standard activity per unit volume and the reaction time. This model proved adequate to describe the glucose-formation kinetics at 7 °C and 45 °C, up to a 95% hydrolysis and to facilitate calculation of the volumen-based enzyme dosage to obtain a certain degree of hydrolysis -a fundamental consideration in evaluating the costs of applying the enzyme