Improving lactic acid yield of hemicellulose from garden garbage through pretreatment of a high solid loading coupled with semi-hydrolysis using low enzyme loading

[Display omitted] •High solid loading of GG (1:7) reduced 40.8% byproduct from hemicellulose.•Carbon catabolite repression (CCR) reduced xylose conversion rate to 48.2%.•Semi-hydrolysis relieved CCR and restored xylose conversion rate to 85.7%.•Lactic acid yield of hemicellulose achieved to 0.49 g/g from GG hydrolysate.•High glucose induced ptsH and ccpA up-regulated and xylose genes down-regulated. Byproduct (acetate and ethanol) generation and carbon catabolite repression are two critical impediments to lactic acid production from the hemicellulose of lignocellulosic biomass. To reduce byproduct generations, acid pretreatment with high solid loading (solid–liquid ratio 1:7) of garden garbage was conducted. The byproduct yield was only 0.30 g/g during in the subsequent lactic acid fermentation from acid pretreatment liquid and 40.8% lower than that of low solid loading (0.48 g/g). Furthermore, semi-hydrolysis with low enzyme loading (10 FPU/g garden garbage cellulase) was conducted to regulate and reduce glucose concentration in the hydrolysate, thereby relieving carbon catabolite repression. During the lactic acid fermentation process, the xylose conversion rate was restored from 48.2% (glucose-oriented hydrolysis) to 85.7%, eventually achieving a 0.49 g/g lactic acid yield of hemicellulose. Additionally, RNA-seq revealed that semi-hydrolysis with low enzyme loading down-regulated the expression of ptsH and ccpA, thereby relieving carbon catabolite repression.