Bioethanol from wheat bran using mild pre-treatments and efficient fermenting yeasts

Current ethanol production processes using crops (sugar cane and corn) as feedstock are well-established but their energy and substrate costs are high and the need to develop more feasible technologies is evident. Lignocellulose is the most promising raw material considering its great availability and limited value, but the large-scale commercial production of ethanol from lignocellulosic substrates has still to be implemented. Currently, wheat is used as feedstock and the wheat starch is converted into ethanol in many European plants. However, wheat bran, produced in large quantities as residue of the milling process, is not yet used for bioethanol production. Since the market for wheat bran is restricted in terms of suitable applications, its alternative valorisation may be desirable. This study focused on ethanol production using wheat bran as a model of abundant agricultural by-product. A new method for the hydrolysis of wheat bran was defined combining acid addition, milling and optimised dosages of commercial enzymes. Each treatment was evaluated in terms of total sugar yield and inhibitory by-product release. The maximum total sugar amount was obtained when limited concentrations of acid were added to milled bran at the pre-treatment step. The unfiltered hydrolysates were then fermented by highly fermentative yeasts. S. cerevisiae s1 exhibited high ethanol yield, while S. diastaticus proved effective for the secretion of extracellular glucoamylase resulting in additional ethanol production. The results of this work showed the great potential of wheat bran as a low-cost feedstock for bioethanol. This starchy-cellulosic substrate was efficiently converted into ethanol using mild treatments and proficient fermenting yeasts. Such strains, having promising ethanol yields, will be evaluated for their ability to ferment other unfiltered hydrolysates of agricultural wastes. In addition, these natural yeasts could be genetically modified to obtain recombinant strains producing efficient hydrolytic enzymes for the one-step conversion of lignocellulose into ethanol.