D-Lactic acid production from Cistus ladanifer residues: Co-fermentation of pentoses and hexoses by Escherichia coli JU15

In this study, glucan-rich solids, and xylose-rich hydrolysates obtained from Cistus ladanifer distillery residues (CLR) were used for D-lactic acid (D-LA) production by the D-lactogenic Escherichia coli strain JU15. Firstly, hemicellulosic hydrolysates obtained by the autohydrolysis process were submitted to dilute sulfuric acid catalysed post-hydrolysis. The influence of operational conditions on oligosaccharides hydrolysis was assessed by the combined severity parameter (CS) in the range of 1.1-2.3. The optimum post-hydrolysis conditions were found for CS of 1.6 (300 mM H2SO4, 15 min, 121 degrees C). Subsequent detoxification procedures on post hydrolysed liquors were carried out, where 9.1% (w/v) powdered activated charcoal enabled a full removal of furfural, 5-hydroxymethylfurfural (HMF), and phenolic compounds together with a reduction of acetic acid (37%), and formic acid (27%). Diverse fermentation modes using detoxified and non-detoxified hydrolysates, as well using previously NaOH delignified glucan-rich solids alone (SHF or SSF) or together with pentoses liquors (SSCF) (5% loading) were performed. For all the tested conditions, both hemicellulose-and cellulose-derived sugars can be efficiently used as the carbon source to produce D-lactic acid by E. coli JU15 with a D-LA yield always surpassing 92 g(D-LA)/100 g sugars.