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 0.44 g/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 9 g/L), thus enhancing specific xylose uptake rates (qXylose) as well as ethanol yields. The total molar yield of fermentation by-products (glycerol, xylitol) was constant (0.36 ± 0.03 mol/mol xylose) at different qXylose. Compound distribution changed with glycerol and xylitol being chiefly formed at low and high qXylose, respectively.