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.
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.
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
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