Fermentation of dried distillers’ grains and solubles (DDGS) hydrolysates to solvents and value-added products by solventogenic clostridia

Pretreatment and hydrolysis of lignocellulosic biomass using either dilute acid, liquid hot water (LHW), or ammonium fiber expansion (AFEX) results in a complex mixture of sugars such as hexoses (glucose, galactose, mannose), and pentoses (xylose, arabinose). A detailed description of the utilization of representative mixed sugar streams (pentoses and hexoses) and their sugar preferences by the solventogenic clostridia (Clostridium beijerinckii BA101, C. acetobutylicum 260, C. acetobutylicum 824, Clostridium saccharobutylicum 262, and C. butylicum 592) is presented. In these experiments, all the sugars were utilized concurrently throughout the fermentation, although the rate of sugar utilization was sugar specific. For all clostridia tested, the rate of glucose utilization was higher than for the other sugars in the mixture. In addition, the availability of excess fermentable sugars in the bioreactor is necessary for both the onset and the maintenance of solvent production otherwise the fermentation will become acidogenic leading to premature termination of the fermentation process. During an investigation on the effect of some of the known lignocellulosic hydrolysate inhibitors on the growth and ABE production by clostridia, ferulic and p-coumaric acids were found to be potent inhibitors of growth and ABE production. Interestingly, furfural and HMF were not inhibitory to the solventogenic clostridia; rather they had a stimulatory effect on growth and ABE production at concentrations up to 2.0 g/L.