With a cell concentration of 125 g dry biomass l-1 and a dilution rate of 0.1 h-1, Propionibacterium acidipropionici produces 30 g propionic acid l-1 from sugar with a productivity of 3 g l-1 h-1. The yield of propionic acid is approx. 0.36-0.45 g propionic acid g-1 sucrose and is independent of the dilution rate and cell concentration. Acetic acid is an unwanted by-product in the production of propionic acid. The concentration of acetic acid only increases slightly when the cell concentration is increased. A two-stage fermentation process was developed for the conversion of sugar or molasses of various types to propionic acid and vitamin B12. By fermentation of blackstrap molasses (from sugar beet and sugar cane) in the first fermentation stage 17.7 g propionic acid l-1 with a yield of 0.5 g propionic acid g-1 carbohydrate was produced with a dilution rate of 0.25 h-1. In the second stage 49 mg vitamin B12 1-1 was produced at a dilution rate of 0.03 h-1.

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The effects of yeast extract (0-10g/l), methanol (0-10%v/v), acetic acid (0-1.0g/l), furfural (0-0.5g/l), glucose (0-30g/l), inoculum age (15-70h) and product concentration (18-230g/l) on the xylose-xylitol conversion by Candida guilliermondii FTI 20037 were studied. The xylitol specific productivity increased about 35% at a yeast extract concentration of 1.0g/l, whereas glucose showed a strong inhibitory effect on the xylitol production and a stimulating effect on the growth of C. guilliermondii. Methanol, acetic acid and furfural under the employed concentrations did not show any positive effect neither on the growth or on the xylose-xylitol conversion by the yeast. The inoculum age showed a strong influence on xylitol formation and the best fermentative parameters were attained using a 40-h inoculum age. A xylitol concentration in the fermentation medium higher than 80g/l inhibited mark­edly the xylitol productivity by the yeast C. guilliermondii.

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Propionibacteria have been reported as being oxygen sensitive bacteria. Nevertheless, in this work, we intend to show that Propionibacterium shermanii CDB 10014 is able to grow even at high volumetric oxygen transfer coefficients (KLa). Similar final cell concentrations were achieved in experiments at different KLa values, with increasing cell yield from 0.08 g/g cells in anaerobic conditions to 0.22 g/g at KLa 61 h-1. The organic acid production pattern changes depending on the KLa. At KLa higher than 36 h-1, propionic acid was not produced and acetic acid was the main product. At KLa higher than 20h-1, lactic acid began to be produced. In further studies, no important differences in the growth yield were observed in the pH range 5.5-7.0, while the cell production was enhanced by tem peratures in the range 30-35 °C

Cell growth and organic acid production by Propionibacteria are dependent on the vitamin-nitrogen source in the culture medium. Final cell and propionic acid concentrations produced by Propionibacterium shermanii, using corn-steep liquor, were higher than those obtained utilizing yeast extracts. Since corn-steep liquor is much cheaper than yeast extract, the process becomes more attractive. By calculating the specific growth rates, it was observed that the critical propionic acid concentration, that prevents all growth (px= 0). is different depending on the vitamin-nitrogen source used and its concentration. For example. for 5.0 and 15.0 g/l Oxoid yeast extract, those critical propionic acid concentrations were 16.0 and 27.0 gn, respectively. Such propionic acid concentrations inhibit the cell growth, but not the formation of acid. The specific propionic acid production rate also indicates that the critical concentration for metabolic activity, when propionic acid is no longer produced (pp = 0). varies according to the vitamin-nitrogen source and its concentration in the medium. For 5.0 and 15.0 g/l Oxoid yeast extract, those concentrations were 22.1 and 30.1 g/l. respectively.Corresponding author

Production of propionic acid by Propionibacterium shermanii CDB 10014 was enhanced by a pH value of 6.5 and by temperatures in the range 35-37 °C. Depending on the type of yeast extract, succinic acid can be produced in higher proportions, with decreasing propionic acid yields. With respect to propionic acid production, Difco yeast extract has shown the best results when yeast extract preparations from other different suppliers were compared. To replace yeast extract by a cheaper vitamin-nitrogen source, corn-steep liquor was tested. A complete depletion of glucose was achieved, yielding a final propionic acid concentration of over 35 g/l. These results are even better than those obtained with Difco yeast extract and suggest the possibility of an economical process based on corn-steep liquor.

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