Immobilized fungal fermentation for lactic acid production

Lactic acid is gaining importance because of its potential as a source for biodegradable polymers. Current biological processes use lactic acid bacteria for the fermentation. These have the disadvantage of requiring expensive organic nitrogen sources and producing a racemic mixture. The fungal species, Rhizopus oryzae, being studied in our laboratory produces a very pure form of L(+) lactic acid and can grow on cheap inorganic nitrogen sources like urea. There were very few reports on lactic acid production by the fungus in over 40 years and so experiments were conducted to study the physiology, growth and lactic acid production by the fungal species. Fungi tend to attach to surfaces and grow in large aggregates in a stirred fermenter which makes the studies difficult. Immobilized growth form seemed like a natural choice for the organism. Hence a rotating biological contactor (RBC), was chosen to perform the fermentation. This device consists of a set of vertical disks made of a polymer material, rotating, half-submerged in the medium. The RBC and some disks are modified to allow aseptic access so that cell growth can be studied by measuring the dry weight of the species. The fungal species produced lactic acid even when it is not growing and so it was possible to operate the fermentor in a non-growth mode. The non-growth mode of operation resulted in greater product yields and improved process operation. Product inhibition was observed. In order to maintain a high productivity in the fermentor product has to removed as it is formed. Product removal can be attained by attaching a separation system to the fermentor. A polyvinylpyridine (PVP) resin column has been recommended because of the capacity of the resin to adsorb "free" lactic acid selectively. Having confirmed the potential of the fungal system as a viable process for making lactic acid suitable for the manufacture of biodegradable polymers, a brief insight into the overall process operation was provided by modeling the coupled fermentation-separation system which helped in predicting the effect of the product concentration on the productivity and column size. A brief analysis of the substrate diffusion in the biofilm was also done to get an insight into the film thickness requirements of the immobilized reactor.