Production of xylitol and carotenoids from switchgrass and Eucalyptus globulus hydrolysates obtained by intensified steam explosion pretreatment

This study investigated the fermentability of hemicellulosic hydrolysates obtained by intensified steam explosion pretreatment of switchgrass (Panicum virgatum) and Eucalyptus globulus. Xylitol and carotenoids were the high-value molecules produced by fermentation. The intensified pretreatment allowed to process a large amount of biomass per unit of time and resulted in hydrolysates containing high amount of sugars, among of which, a significant fraction was in the form of oligomers (eucalyptus hydrolysate: 25.0 g/L of oligomers and 50.6 g/L of monomeric sugars; switchgrass hydrolysate: 18.9 g/L of oligomers and 39.6 g/L of monomeric sugars). To be used as fermentation media, a post-hydrolysis step was applied to increase the content of monomeric sugars in the hydrolysates. Then, a detoxification process was carried out to reduce the concentration of inhibitors present. Two evolved yeasts were used for fermentation: Kluyveromyces marxianus for xylitol production, and Rhodosporidium toruloides for carotenoids production. Results revealed that the hydrolysates produced by intensified steam explosion of switchgrass and eucalyptus present good fermentability and can be used to produce valuable compounds such as xylitol, after detoxification. K. marxianus presented better tolerance to inhibitory compounds still present in the detoxified hydrolysates (acetic acid up to 3.94 g/L and phenolic compounds up to 2.28 g/L) than R. toruloides, which favored the production of xylitol. Finally, the intensified pretreatment was found to be a potential strategy to obtain hydrolysates with high concentration of sugars, reducing the need of concentration in a subsequent step. Moreover, the detoxification strategy applied in this study allowed to recover valuable compounds from the hydrolysates, offering extra value to a biorefinery. Altogether, the findings of this study contribute to the advancement of a technology for valorization of hemicellulosic hydrolysates.