Sustainable Bioethanol Production and Phenolic Compounds from Avocado Stone Biomass Based on Microwave Pretreatment

The transition towards sustainable biofuels requires innovative strategies to maximize the utilization of agroindustrial biomass. Accordingly, the aim of this study was to evaluate avocado stone biomass as a renewable substrate for producing glucose and bioethanol, and to characterize potential co-products from the pretreatment stream, including avocado phenolic compounds. It was found that the whole avocado stone and the seed contained 41.7% and 42.8% of starch, respectively, accounting for more than 78% of the glucans. Using microwave-diluted acid pretreatment and multi-response optimization, a direct conversion of ~90% of glucans to glucose was achieved from avocado stone biomass at 1% <i>w</i>/<i>v</i> sulfuric acid, 140 °C, and 5 min. It also enabled minimizing inhibitor presence and reducing energy requirements. Then, the glucose-rich hydrolyzate was efficiently fermented into bioethanol (~24 g/L in 12 h) using <i>Saccharomyces cerevisiae</i>, without needing detoxification or enzyme addition. Additionally, the process yielded a lignin-rich solid fraction with an enhanced higher heating value (about 1.4 times) compared to the original biomass and an extract with phenolic compounds like caffeoylquinic acids and hydroxytyrosol, which enhances the valorization potential of this underutilized biomass. The overall balance can be 240 kg/t of bioethanol, along with 2.5 kg/t of phenolic compounds and 376 kg/t of lignin-rich solid. Finally, this work exemplified, in a real-world scenario, how we can fully leverage these often-overlooked, non-edible sources of starch to achieve the green transition and circularity.