Optimum growth conditions for the fermn. of nonconcd. whey permeate by K. fragilis NRRL 665 were defined. Use of 3.75 g yeast ext./L, a growth temp. of 38 Deg and a pH of 4.0 allowed a max. productivity of 5.23 g EtOH [64-17-5]/L-h in continuous culture with a yield 91% of theor. Complete batch fermn. of permeate with 100 g lactose [63-42-3]/L was possible with a max. specific growth rate of 0.276/h without any change in EtOH yield. Fermn. of concd. permeate resulted, however, in a general decrease of specific substrate consumption rate, demonstrated by the inability to completely convert an initial 90 or 150 g lactose/L in continuous culture, even at diln. rates as low as 0.05 and 0.08/h, resp. The decrease could be related to substrate inhibition, to an increase in osmotic pressure caused by lactose and salts, and to EtOH inhibition of both alc. and biomass yield. The decrease in specific productivity could be counterbalanced by use of high cell d. cultures, obtained by cell recycle of K. fragilis. Fermn. of a nonconcnd. permeate at a diln. rate of 1 vol./h resulted in a productivity of 22 g/L-h at 22 g EtOH/L. Cell recycle using flocculating K. lactis NCYC 571 was also tested. With this strain a productivity of 9.3 g/L-h at 45 g product/L was attained at a diln. rate of 0.2/h, with an initial lactose concn. of 95 g/L. [on SciFinder (R)]