Batch and continuous 2-stage cultures were conducted to det. the effect of yeast ext. (YE) on the homolactic fermn. of whey permeate by L. helveticus. Supplementation with YE had a significant effect on lactic acid concn., volumetric productivity, and substrate conversion, but not on lactic acid yield. Volumetric productivity in the 1st stage increased from 2 to 9 g/L-h by increasing the YE concn. from 1.5 to 25 g/L. At the same time conversion improved from 22% to 93% at a diln. rate of 0.2/h. The 2nd stage demonstrated the effect of YE at a lower diln. rate (0.14/h). A high system conversion (97%) and a high final lactic acid concn. (40 g/L) were achieved with 10 g/L YE. [on SciFinder (R)]