Single-stage extraction of whole sorghum extractives and hemicelluloses followed by their conversion to ethanol

Abstract

Maximizing the extracted carbohydrates from lignocellulosic biomass represents one critical step towards a wide implementation of industrial-scale second-generation ethanol production. To this purpose, a novel steam-based treatment strategy targeting the whole sweet sorghum biomass was developed. This extraction was followed by a non-enzymatic hydrolysis of the hemicellulose fraction and ethanol fermentation. By doing so, both the free carbohydrates present in the plant stems as well as the hemicelluloses trapped inside the various structural components were recovered and converted. A statistical design of experiments (DoE) approach was developed to investigate the most suitable steam-treatment severity to maximize the recovery of these carbohydrates while simultaneously minimizing the production of fermentation inhibitors. The severity of the treatment was estimated from the combined effects of temperature (150 °C – 210 °C) and cooking time (1–7 min). The optimal treatment conditions identified in this work (3 min retention time at 180 °C) successfully allowed recovering 30% of monomeric carbohydrates (based on sorghum dry weight). Three of the most efficient steam-explosion severities were investigated further in order to evaluate their impact on the subsequent ethanol fermentation using Saccharomyces cerevisiae yeast. A maximum ethanol yield of above 95% was achieved using the mixed carbohydrate streams recovered using the optimal steam-based extraction conditions. Thus, the present work thoroughly describes for the first time and to the best of our knowledge, a single-stage recovery of sorghum extractives and hemicelluloses, followed by their efficient fermentation to ethanol.