Microbial processing of agricultural residues: from wheat bran to ethanol.

Currently ethanol production using crops (sugar cane and corn) as feedstock is well-established but the 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. However, the primary obstacle impeding the large-scale commercial production of ethanol from lignocellulosic substrates is the general absence of low-cost technology for overcoming the recalcitrance of these materials (Lynd et al., 2005). This study focused on ethanol production from wheat bran as a model of abundant and cheap agricultural by-product. With the aim of developing an efficient and economically feasible method to process bran into ethanol, two distinct research perspectives have been conducted. Firstly, the hydrolysis of wheat bran was efficiently achieved combining acid addition, milling and optimised dosages of commercial cellulolytic 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. Saccharomyces cerevisiae s1 exhibited high ethanol yield, while S. diastaticus proved effective for the secretion of extracellular glucoamylase resulting in additional ethanol production. Since wheat bran was mainly composed by starch (about 30% of the dry matter), a biotechnological approach started aiming at the development of an efficient amylolytic S. cerevisiae strain for the industrial Consolidated Bioprocessing of starchy materials into ethanol. Several fungal amylolytic genes were screened for their high expression into the laboratory strain S. cerevisiae Y294. The most proficient sequences were then selected to be integrated into natural yeasts previously selected for their fermentative traits and robustness suitable for the industrial bioethanol process (Favaro et al. 2012). The resulting recombinant strains exhibited high amylolytic activities both on soluble and raw starch. During anaerobic cultivation, the yeasts produced a 79% of the theoretical ethanol yield from soluble starch. On raw starch, the engineered yeasts exhibited improvable fermentative performance with an ethanol yield of 75% of the theoretical. Considering the preliminary fermentation studies, the recombinant strains could be promising for the Consolidated Bioprocessing of starchy substrates such as corn, wheat and several industrial residues (cereal brans, potato peels and spent brewers’ grains). 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. Moreover, the obtained engineered amylolytic yeasts will be useful for the direct fermentation of wheat bran, towards the one-step microbial processing of agricultural residues to bioethanol.