Brewer's spent grains-based biorefineries: A critical review

Barley and wheat were known to be domesticated, since the earliest ages when man took up agriculture. And the art of preparing fermented drinks from these grains was well documented throughout the ancient civilizations. Brewer's spent grain (BS grain) is the by-product obtained as a retentate during the brewing process. In contrast to its traditional application, as the cattle feed, the recent advances in the biorefinery industry have paved the way to repurpose it as a sustainable raw material for the production of much-valued bioenergy and an array of other products. BS-grain's biomass composition, availability, and eco-friendly nature make it an interesting biorefinery raw material. Nevertheless, there are very few industrial-scale biorefinery applications have been reported so far. The high moisture content of BS-grain demands an additional drying process to improve its shelf life, an additional pretreatment process is required to reduce its recalcitrance, and the transportation cost, are some of the economically challenging aspects, that need to be addressed before considering BS-grain as the raw material for biorefinery industry. Therefore, this review mainly focuses on BS-grain-based biorefineries, prospective applications such as biofuels, challenges, and the technological improvements achieved so far.