Batch fermentation kinetics of sugar beet molasses by Zymomonas mobilis

Abstract

A new osmotolerant mutant strain of Zymomonas mobilis was successfully used for ethanol production from beet molasses. Addition of magnesium sulfate to hydrolyzed molasses allowed repeated growth without the need of yeast extract addition. The kinetics and yields parameters of fermentation on media with different molasses concentrations were calculated. The anabolic parameters (specific growth rate, mu, and biomass yield, Y(X/S)) were inhibited at elevated molasses concentrations while the catabolic parameters (specific ethanol productivity, q(p), and ethanol yield, Y(p/s)) were not significantly affected. In addition to ethanol and substrate inhibition, osmotic pressure effects can explain the observed results.

Batch fermentation kinetics of ethanol production by Zymomonas mobilis on cellulose hydrolysate

“…However, based on these data, it cannot be determined whether one or both mechanism were active. A very similar conclusion has been drawn with ceils grown on media containing molasses (Park and Baratti, 1991a). Therefore, the growth kinetics on industrial media such as cellulose hydrolysate or molasses are quite complex and difficult to analyse.…”

“…The pellets were washed twice with distilled water and used for dry cell weight determination after incubation at 105°C until constam weight (usually 48 h). Ethanol was determined by gas chromatography (Park and Baratti, 1991a). Glucose concentration was determined by enzymatic method using the glucose oxidase.…”

“…Glucose concentration was determined by enzymatic method using the glucose oxidase. Osmolality data were collected using an osmometer (X[ogel 6300) standardized with a 300 milliosmol kg-1 NaCI solution (9.463 g NaCI/1000 g distilled water) (Park and Baratti, 1991a). Kinetics and yield parameters describing the batch fermentation were determined using a polynomial fit as described by Ai't -Abdelkader and .…”

“…More surprising was the observed good growth of strain SBE15. This mutant showed higher performances for ethanol fermentations than the wild type strain on substrates such as molasses (Park and Baratti, 1991a), sugar based substrates (Park and Baratti, 1991b) and media with added KC1 (Park and Baratti, 1993) as expected from its phenotype of osmotolerance. However, these properties were found at 30°C at the usual growth temperature for Z. mobilis strains.…”

“…Therefore, a great interest has been devoted to cheap carbon sourcesfrom renewable agricultural resources such as inulin (Allais etal., 1987;Favela-Torres etal., 1986), molasses (Park and Baratti, 1991aand b, 1992 or starch (Fein et al, 1984;FavelaTorres and Baratti, 1988). In addition to these glycosidic substrates, lignocdlulosic materials from forestry, agriculture and wastes represent excellent alternative feedstocks for the production of bulk chemicals such as ethanol by fermentation (Lynd el a/., 1991).…”