EFFECTS OF PH ON ACETIC ACID PRODUCTION FROM LACTOSE BY AN ANAEROBIC MIXED CULTURE FERMENTATION (MIXED CULTURE)

Acetic acid was successfully produced from lactose or whey permeate by mixed culture fermentation of Clostridium formicoaceticum (ATCC 27076) and Streptococcus lactis (strain ML1) at pH 7.0 - 7.6, 35(DEGREES)C. The yield of acetic acid from lactose was about 95%. The final acetic acid concentration in the fermentation broth was higher than 30 g/L, although the fermentation rate was very slow at acetic acid concentration higher than 20 g/L (0.33 M). The pH range for C. formicoaceticum grown on lactic acid was between 6.6 and 9.6 with an optimum around 7.6, at 35(DEGREES)C. The acetic acid yield was about 98 (+OR-) 0.8%. The specific growth rate and lag phase period were dependent on the medium pH. Hydrogen ion was a noncompetitive growth inhibitor at pH below 7.6 (K(,H) = 7.749 (+OR-) 0.043 x 10(\u27-8) g/L, (mu)(,o) = 0.220 (+OR-) 0.002 hr(\u27-1)). Acetic acid was also a noncompetitive growth inhibitor. This inhibition was largely dependent on pH and mainly resulted from the undissociated acid (K(,HA) = 45.67 mg/L). Cell growth stopped at either a pH below 6.23 or a concentration of undissociated acetic acid above 0.358 g/L (5.97 mM). S. lactis ML1 performed homolactic fermentation at pH above 7.0, at 35(DEGREES)C. The specific growth rate at pH 7.0 to 7.6 was not dependent on pH but was 15% lower at pH 7.9. However, the cell yield decreased from 0.18 to 0.075 g cell/g lactose and product yield increased from 89% to 98% as pH increased from 7.0 to 7.9. Acetic acid was a noncompetitive inhibitor (K(,TA) = 12.247 (+OR-) 0.893 g/L and (mu) = 0.466 (+OR-) 0.116 hr(\u27-1)), but did not change the homolactic fermentation pattern. Kinetic models previously developed by Yang (1984) were modified to include the effects of pH and acetic acid on the single culture fermentations and were successfully used to simulate the mixed culture fermentation