Continuous fermentation of lactose for lactic acid production by simultaneous bioreaction and product separation

Lactic acid fermentation includes several reactions in association with the microorganism growth. Some factors that affect lactic acid fermentation are: microorganism, culture media, inoculum preparation, presence of oxygen, pH, and temperature. Batch experiments were performed with Lactobacillus bulgaricus. Based on experimental evidence of substrate and product inhibition, an unstructured Monod type kinetic model was developed to describe microorganism growth and lactic acid production. By using cell-recycle, a higher cell density and an enhancement in production was obtained. The batch kinetic model was modified to describe growth and lactic acid formation in continuous cultures. Lactic acid recovery was studied using downstream extraction, hollow fiber membrane extraction, and in a coupled fermentation membrane extractor system. The factors considered for extractive fermentation are: solvent, pH, temperature, and lactic acid accumulation in the broth. Aliquat 336, an anion exchange reagent, was selected as the extraction solvent using kerosene as diluent because of its high distribution coefficient (K$\sb{\rm D}$ = 1.90), and both undissociated (lactic acid) and dissociated (lactate) forms can be extracted at pH 5.6 $\pm$ 0.1 which corresponds to the optimum pH for the microorganism. The effect of solvent viscosity, diluent concentration, temperature, and lactic acid concentration on the distribution coefficient was investigated. As the temperature increased, solvent viscosity significantly decreased. Appropriate working conditions were obtained in the range from 20 to 40% Aliquat in kerosene in the organic phase at 42$\sp\circ$C. At these conditions the solvent exhibits low viscosity (7.8 to 29 cp) and high distribution coefficient (1.41 to 0.87) for initial aqueous lactic acid concentrations from 10 to 40 g/L. For low initial lactic acid concentrations (below 20 g/L), high distribution coefficients were obtained. Since Aliquat 336 is toxic, membrane extraction of lactic acid was conducted using a Hollow Fiber Membrane Separator System. The overall mass transfer coefficients ($8.52\times10\sp{-5}$ to $9.68\times10\sp{-5}$ cm/s) and the lactic acid concentration profiles in the organic and aqueous reservoirs were determined experimentally. Simulation showed that simultaneous bioreaction and product separation-concentration should lower the cost in obtaining a purified lactic acid.