MEMBRANE-BASED PROCESS FOR THE RECOVERY OF LACTIC ACID BY FERMENTATION OF CARBOHYDRATE SUBSTRATES CONTAINING FERMENTABLE SUGARS

摘要

MEMBRANE-BASED PROCESS FOR THE RECOVERY OF LACTIC ACID BY FERMENTATION OF CARBOHYDRATE SUBSTRATES CONTAINING FERMENTABLE SUGARS OF THE DISCLSOURE A substantially continuous membrane separation process has been discovered in which the known benefits of a cellrecycle fermentor are unexpectedly enhanced because pH is controlled from 4.0 to 5.5 with a nitrogen-containing base, most preferably ammonium hydroxide. The ammonium lactate formed is not precipitated. This allows continuous removal of lactic acid so as to maintain a concentration of lactic acid about 12%, preferably about 6% by wt in the broth, optionally supplemented with thin stillage or steep water, and optionally buffered with protease. When combined with recycle of UF retentate, operating this cell-recycle fermentor allows the build-up of a cell population in the range from above 5 x 109 CFU/ml to 5 x 1010 CFU/ml, which population is much denser than one which can be produced with the specified pH control in a batch fermentor, after self-termination of the fermentation. This super-dense population of cells, in turn, not only accelerates production of lactic acid but also minimizes contamination by foreign cells; and since the UF retentate is recycled, a relatively large portion of the broth in the fermentor may be withdrawn as a fermentor effluent stream, sufficient to produce a specified high fluid velocity when the effluent stream is flowed over UF membranes used in the UF modules. The high fluid velocity, preferably from 5-10 m/sec, at. relatively flow pressure less than 1140 kPa (150 psig), produces a scrubbing action over the membrane surfaces which are therefore not fouled over long periods of operation. In a batch fermentation with pH control, when the fermentation is self-terminated the total lactic acid present is about 10% by wt (measured by HPLC), though free always 4%, and the live cell population is lower than 5 x 109 CFU/ml because many cells die as pH drops.