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Continuous fermentation studies with xylos-utilizing recombinant Zymomonas mobilis

Continuous fermentation studies with xylos-utilizing recombinant Zymomonas mobilis

Applied Biochemistry and Biotechnology 84-86: 295-310

ISSN/ISBN: 0273-2289

PMID: 10849797

DOI: 10.1385/abab:84-86:1-9:295

This study examined the continuous cofermentation performance characteristics of a dilute-acid "prehydrolysate-adapted" recombinant Zymomonas 39676:pZB4L and builds on the pH-stat batch fermentations with this recombinant that we reported on last year. Substitution of yeast extract by 1% (w/v) corn steep liquor (CSL) (50% solids) and Mg (2 mM) did not alter the cofermentation performance. Using declared assumptions, the cost of using CSL and Mg was estimated to be 12.5 cents/gal of ethanol with a possibility of 50% cost reduction using fourfold less CSL with 0.1% diammonium phosphate. Because of competition for a common sugar transporter that exhibits a higher affinity for glucose, utilization of glucose was complete whereas xylose was always present in the chemostat effluent. The ethanol yield, based on sugar used, was 94% of theoretical maximum. Altering the sugar ratio of the synthetic dilute acid hardwood prehydrolysate did not appear to significantly change the pattern of xylose utilization. Using a criterion of 80% sugar utilization for determining the maximum dilution rate (D(max)), changing the composition of the feed from 4% xylose to 3%, and simultaneously increasing the glucose from 0.8 to 1.8% shifted D(max) from 0.07 to 0.08/h. With equal amounts of both sugars (2.5%), D(max) was 0.07/h. By comparison to a similar investigation with rec Zm CP4:pZB5 with a 4% equal mixture of xylose and glucose, we observed that at pH 5.0, the D(max) was 0.064/h and shifted to 0.084/h at pH 5.75. At a level of 0.4% (w/v) acetic acid in the CSL-based medium with 3% xylose and 1.8% glucose at pH 5.75, the D(max) for the adapted recombinant shifted from 0.08 to 0.048/h, and the corresponding maximum volumetric ethanol productivity decreased 45%, from 1.52 to 0.84 g/(L.h). Under these conditions of continuous culture, linear regression of a Pirt plot of the specific rate of sugar utilization vs D showed that 4 g/L of acetic acid did not affect the maximum growth yield (0.030 g dry cell mass/g sugar), but did increase the maintenance coefficient twofold, from 0.46 to 1.0 g of sugar/(g of cell(.)h).

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Accession: 003693049

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