Advances in Fermentation and Separation Technologies

The productivity of the classical batch reactor for lactic acid fermentation is lower than desirable for economically viable large-scale production. A great deal of research has been carried out on continuous fermentation processes over the past 10—15 years that promises to raise productivity. The research on continuous processes up to 1995 has been reviewed (Litchfield 1996). 

Added productivity of lactic acid fermentations can be achieved by combining continuous systems with mechanisms that allow higher bacterial cell concentrationsResearch is concentrated on two mechanisms (1) membrane recycle bioreactors (MRBs) and (2) immobilized cell systems (ICSs). The MRB consists of a continuous stirred-tank reactor in a semiclosed loop with a hollow fiber, tubular, flat, or cross flow membrane unit that allows cell and lactic acid separation and recycle of cells back to the bioreactor. The results of a number of laboratory studies with various MRB systems demonstrate the effect of high cell concentrations on raising lactic acid productivity (Litchfield 1996). O Table 1.12 lists examples of published results employing various MRB systems. 

The volumetric productivity (g of lactic acid/l-h) is usually greater than ten-fold that of batch or continuous processes. Ceramic tubular membranes have been used which are both steam sterilizable and resistant to mechanical stress (Xavier et al. 1995). Fermentation production of lactic acid directly from starch was optimized in an MRB using I. amylovorus (O Table 1.12). No saccharification or preliquefaction of starch 

Organic Acid and Soivent Production Acetic, Lactic, Gluconic, Succinic, and Polyhydroxyalkanoic Acids 

Examples of lactic acid production in membrane recycle bioreactors 

---