Glycerol fermentation with Propionibacteria and optimisation of the production of propionic acid

The propionic fermentation has been intensively studied for propionic acid production from carbohydrate feedstocks. Despite various advanced fermentation processes (continuous fermentation, immobilized cells, membrane bioreactor, extractive fermentation....), poor performances of the propionic fermentation of carbohydrates, in terms of conversion yield to propionic acid and of the level of acetic acid, have limited its development. Glycerol is a substrate that can be fermented by propionic acid-producing micro-organ isms, in particular propionibacteria strains. This pathway leads various perspectives for the improvement of the propionic acid production. The fermentation of glycerol comparatively to glucose was carried out using two propionibacteria strains: P. acidipropionici and P. freudenreichii ssp shermanii. During batch fermentation of glycerol (20 g/l), the conversion yield reached for P. acidipropionici and P. freudenreichii ssp shermanii, 0.79 and 0.58 mol/mol respectively, were 49% and 45% higher than with glucose (20 g/l). Final concentration of acetic acid was twice lower from 20 g/l glycerol (2 g/l) than from 20 g/l glucose (4g/l) for both strains. Increasing the initial glycerol concentration resulted in a maximal concentration of propionic acid: 42 g/l, with P. acidipropionici. The molar ratio: propionic acid / acetic acid with P. acidipropionici, during batch (5.5) and continuous fermentations (88), suggested the hypothesis of the homopropionic behaviour of the glycerol fermentation. Nevertheless, n-propanol and succinic acid, the minor by-products, were formed in higher quantities from glycerol than glucose. The immobilization of P. acidipropionici with calcium alginate was carried out for glycerol fermentation. Sequencing batch runs have shown good growth and productivity of propionic acid of the immobilized biomass. The study of the continuous glycerol fermentation, carried out with immobilized cells in an expanded bed fermentor (0.8 l), has shown, for a dilution rate of 0.154 h(-1), a very high productivity of propionic acid (3 g/l.h.) associated to a high yield (0.78 mol/mol). Glycerol fermentation to propionic acid has given, at once, a high yield, a high propionic acid content, a low acetic acid level and a high productivity. It demonstrated fine perpectives of optimisation of propionic acid production by microbial route.