Effect of alkaline/hydrogen peroxide pretreatment on date palm fibers: induced chemical and structural changes and assessment of ethanol production capacity via Pichia anomala and Pichia stipitis

The thermochemical/oxidative pretreatment of date palm fibers (DPF) was investigated aiming to enhance bioethanol production from the pretreated biomass. The chemical reagents used were sodium hydroxide and/or hydrogen peroxide in dilute aquatic solutions, to which DPF was subjected at a two-step or one-step process. The effect of the pretreatment was evaluated by estimating the direct removal efficiency of lignin and the solubilization of holocellose, as well as by assessing the induced structural and morphological changes via IR spectroscopy and SEM imaging. Based on the results, a two-step combined pretreatment with NaOH applied first was selected as the most efficient and its effect on the enzymatic digestibility of DPF using commercial cellulolytic enzymes was further studied. Subsequently, batch experiments with the whole pretreated DPF slurries and after separating the solids from the hydrolysate were conducted through simultaneous saccharification and fermentation (SSF) and also separate hydrolysis and fermentation (SHF), using the non-conventional yeasts Pichia anomala (Wicherhamomyces anomalus, strain X19) and Pichia stipitis, either separately or in co-culture. The co-culture yielded to higher ethanol for both slurries and solids, whereas the results obtained from SSF and SHF experiments did not seem to differentiate significantly. The maximum ethanol yield obtained was approximately 150 g/kg dry DPF biomass, indicating that DPF can be a promising feedstock for lignocellulosic ethanol production. Graphical abstract: [Figure not available: see fulltext.]