Vera Novy

Co-fermentation of hexose and pentose sugars in a spent sulfite liquor matrix with genetically modified Saccharomyces cerevisiae

Spent sulfite liquor (SSL) is a by-product of pulp and paper manufacturing and is a promising substrate for second-generation bioethanol production. The Saccharomyces cerevisiae strain IBB10B05 presented herein for SSL fermentation was enabled to xylose utilization by metabolic pathway engineering and laboratory evolution. Two SSLs from different process stages and with variable dry matter content were analyzed; SSL-Thin (14%) and SSL-S2 (30%). Hexose and pentose fermentation by strain IBB10B05 was efficient in 70% SSL matrix without any pretreatment. Ethanol yields varied between 0.31 and 0.44g/g total sugar, depending on substrate and process conditions used. Control of pH at 7.0 effectively reduced the inhibition by the acetic acid contained in the SSLs (up to 9g/L), thus enhancing specific xylose uptake rates (q(Xylose)) as well as ethanol yields. The total molar yield of fermentation by-products (glycerol, xylitol) was constant (0.36±0.03mol/mol xylose) at different q(Xylose). Compound distribution changed with glycerol and xylitol being chiefly formed at low and high q(Xylose), respectively.

Toward "homolactic" fermentation of glucose and xylose by engineered Saccharomyces cerevisiae harboring a kinetically efficient l-lactate dehydrogenase within pdc1-pdc5 deletion background.

l‐Lactic acid is an important platform chemical and its production from the lignocellulosic sugars glucose and xylose is, therefore, of high interest. Tolerance to low pH and a generally high robustness make Saccharomyces cerevisiae a promising host for l‐lactic acid fermentation but strain development for effective utilization of both sugars is an unsolved problem. The herein used S. cerevisiae strain IBB10B05 incorporates a NADH‐dependent pathway for oxidoreductive xylose assimilation within CEN.PK113‐7D background and was additionally evolved for accelerated xylose‐to‐ethanol fermentation. Selecting the Plasmodium falciparum l‐lactate dehydrogenase (pfLDH) for its high kinetic efficiency, strain IBB14LA1 was derived from IBB10B05 by placing the pfldh gene at the pdc1 locus under control of the pdc1 promotor. Strain IBB14LA1_5 additionally had the pdc5 gene disrupted. With both strains, continued l‐lactic acid formation from glucose or xylose, each at 50 g/L, necessitated stabilization of pH. Using calcium carbonate (11 g/L), anaerobic shaken bottle fermentations at pH ≥ 5 resulted in l‐lactic acid yields (YLA) of 0.67 g/g glucose and 0.80 g/g xylose for strain IBB14LA1_5. Only little xylitol was formed (≤0.08 g/g) and no ethanol. In pH stabilized aerobic conversions of glucose, strain IBB14LA1_5 further showed excellent l‐lactic acid productivities (1.8 g/L/h) without losses in YLA (0.69 g/g glucose). In strain IBB14LA1, the YLA was lower (≤0.18 g/g glucose; ≤0.27 g/g xylose) due to ethanol as well as xylitol formation. Therefore, this study shows that a S. cerevisiae strain originally optimized for xylose‐to‐ethanol fermentation was useful to implement l‐lactic acid production from glucose and xylose; and with the metabolic engineering strategy applied, advance toward homolactic fermentation of both sugars was made. Biotechnol. Bioeng. 2017;114: 163–171.