Butanol production from laccase-pretreated brewer's spent grain

Background: Beer is the most popular alcoholic beverage worldwide. In the manufacture of beer, various by-productsand residues are generated, and the most abundant (85% of total by-products) are spent grains. Thanks to itshigh (hemi)cellulose content (about 50% w/w dry weight), this secondary raw material is attractive for the productionof second-generation biofuels as butanol through fermentation processes.Results: This study reports the ability of two laccase preparations from Pleurotus ostreatus to delignify and detoxifymilled brewer’s spent grains (BSG). Up to 94% of phenols reduction was achieved. Moreover, thanks to the mild conditionsof enzymatic pretreatment, the formation of other inhibitory compounds was avoided allowing to apply thesequential enzymatic pretreatment and hydrolysis process (no filtration and washing steps between the two phases).As expected, the high detoxification and delignification yields achieved by laccase pretreatment resulted in greatsaccharification. As a fact, no loss of carbohydrates was observed thanks to the novel sequential strategy, and thusthe totality of polysaccharides was hydrolysed into fermentable sugars. The enzymatic hydrolysate was fermentedto acetone-butanol-ethanol (ABE) by Clostridium acetobutilycum obtaining about 12.6 g/L ABE and 7.83 g/L butanolwithin 190 h.Conclusions: The applied sequential pretreatment and hydrolysis process resulted to be very effective for the milledBSG, allowing reduction of inhibitory compounds and lignin content with a consequent efficient saccharification. C.acetobutilycum was able to ferment the BSG hydrolysate with ABE yields similar to those obtained by using syntheticmedia. The proposed strategy reduces the amount of wastewater and the cost of the overall process. Based on thereported results, the potential production of butanol from the fermentation of BSG hydrolysate can be envisaged.