Maintenance and growth requirements in the metabolism of Debaryomyces hansenii performing xylose‐to‐xylitol bioconversion in corncob hemicellulose hydrolyzate

Abstract

In order to improve the biotechnological production of xylitol, the metabolism of Debaryomyces hansenii NRRL Y-7426 in corncob hemicellulose hydrolyzate has been investigated under different conditions, where either maintenance or growth requirements predominated. For this purpose, the experimental results of two sets of batch bioconversions carried out alternatively varying the starting xylose concentration in the hydrolyzate (65.6 < or = S(0) < or = 154.7 g L(-1)) or the initial biomass level (3.0 < or = X(0) < or = 54.6 g(DM) L(-1)) were used to fit a metabolic model consisting of carbon material and ATP balances based on five main activities, namely fermentative assimilation of pentoses, semi-aerobic pentose-to-pentitol bioconversion, biomass growth on pentoses, catabolic oxidation of pentoses, and acetic acid and NADH regeneration by the electron transport system. Such an approach allowed separately evaluating the main bioenergetic constants of this microbial system, that is, the specific rates of ATP and xylose consumption due to maintenance (m(ATP) = 21.0 mmol(ATP) C-mol(DM) (-1)h(-1); m(Xyl) = 6.5 C-mmol(Xyl) C-mol(DM) (-1)h(-1)) and the true yields of biomass on ATP (Y(ATP) (max) = 0.83 C-mol(DM) mol(ATP) (-1)) and on xylose (Y(Xyl) (max) = 0.93 C-mol(DM) C-mol(Xyl) (-1)). The results of this study highlighted that the system, at very high S(0) and X(0) values, dramatically increased its energy requirements for cell maintenance, owing to the occurrence of stressing conditions. In particular, for S(0) > 130 g L(-1), these activities required an ATP consumption of about 2.1 mol(ATP) L(-1), that is, a value about seven- to eightfold that observed at low substrate concentration. Such a condition led to an increase in the fraction of ATP addressed to cell maintenance from 47% to 81%. On the other hand, the very high percentage of ATP addressed to maintenance (> 96%) at very high cell concentration (X(0) > or = 25 g(DM) L(-1)) was likely due to the insufficient substrate to sustain the growth.

Corncob hydrolysate, an efficient substrate for Monascus pigment production through submerged fermentation

“…Corncob has been exploited as the fermentation substrate to produce ethanol , biohydrogen , xylitol , lactic acid , and cellulose , and recently, it has been proved as a novel substrate for solid‐state fermentation using Monascus sp. However, its feasibility as an alternative substrate for submerged fermentation has not yet been assessed.…”

Techno‐economic analysis of the valorization of brewers spent grains: production of xylitol and xylo‐oligosaccharides

“…The hydrolysate is separated from the cellulignin by a centrifuge at 80 °C to minimize risk of precipitating XOS 49 and the cellulignin is washed using a 2:1 ratio to minimize to recovery (>95%) of solubilized oligomers 50 . The hydrolysate is pumped to a pressurized stirred reactor for acid post‐hydrolysis at 125 °C using 0.5 wt% H 2 SO 4 for 165 min (the equivalent of combined severity factor of 1.1) to produce xylose by depolymerization of the XOS oligomers 51,52 . In preparation for fermentation, the hydrolysate is concentrated through evaporation (70 °C) to a xylose concentration of 70 g L −1 , where volatile components are removed 53 .…”

“…This is followed by neutralization of the hydrolysate pH to 6.5 through the addition of lime; the precipitated gypsum and other impurities are removed by filtration 54 . The hydrolysate is further purified by adsorption using activated carbon (3.5 wt%) to further reduce fermentation inhibitors 51,55 . The xylose‐rich solution is finally concentrated to 100 g L −1 by evaporation to further remove volatiles, a process that serves as sterilization of the hydrolysate for fermentation.…”

“…Fermentation of the neutralized xylose hydrolysate is carried out in a stirred‐batch fermenter under microaerobic conditions using Debaryomyces hansenii NRRL Y‐7426 , a nonpathogenic halotolerant yeast found in cheese and cured meat production 56 ; which as non‐GMO, is not a bio‐hazard to the environment 57 . Fermentation is conducted with a microbial loading of 12.5 g L −1 at pH 6.5 at 30 °C for 60 h (0.81 g xylitol g –1 xylose) that resulted in the yeast fully consuming glucose with minimal arabinose assimilation 51,58 . After fermentation is completed, the yeast, soluble salts and organic impurities need to be removed to improve the purity of the xylitol broth 59,60 before producing a pure crystalline xylitol product.…”

“…xylose by depolymerisation of the XOS oligomers (Vigo-ourense and Lagoas, 2004;Rivas et al, 2009). In preparation for fermentation, the hydrolysate is concentrated through evaporation (70 ºC) to a xylose concentration of 70 g/l, where volatile components are removed .…”