Fermentation of cheese whey was conducted in a stirred 2.0-liter bench top fermentor with B. longum bacteria. The membrane unit (SEPA CF II, Osmonics, Minneapolis, MN) was coupled with 5.0L and 1.5 L fermentors to maintain constant pH and temperature. Nanofiltration membranes DS-5DK and DS-DL were used to separate the lactic acid from lactose and cells. The nanofiltration process was carried out at 200, 300, and 400 PSI. The nanofiltration permeate mainly consisting of lactic acid and water was concentrated with reverse osmosis membrane DS 11 AG. Reverse osmosis was run at 600 and 700 PSI in order to separate the lactic acid from water. A vacuum evaporation unit was used to further concentrate the lactic acid to produce pure lactic acid.
Nearly 100% lactose retention was obtained with the DS-5DK membrane while 60% lactic acid was recovered in the permeate. An increase in pressure resulted in a higher permeate flux and higher lactose retention, but low lactic acid recovery. The DS-11AG reverse osmosis membrane retained 90% lactic acid. As a result, lactic acid was concentrated with the reverse osmosis membrane.  Nearly 100% lactic acid was obtained when further concentrated with vacuum evaporation at 100°C and 450 mm Hg.