Ethanol production from green coconut fiber using a sequential steam explosion and alkaline pretreatment

The pretreatment step has a major role in ethanol production as it breaks the lignocellulosic material structure, which improves the digestibility in the enzymatic hydrolysis and increases the ethanol yield during fermentation. The purpose in this study was to pretreat green coconut fiber (GCF) with steam explosion (SE) to produce ethanol. Different severity factors (ranging between 2.18 and 4.24) were performed and the humidification process of the raw material before pretreatment was also evaluated. Considering that SE did not provide lignin removal nor significant improvement in enzymatic digestibility, a subsequent alkaline pretreatment was carried out to overcome these challenges. The alkaline pretreatment using NaOH increased delignification, either as a stand-alone or in the sequential pretreatment. The sequential SE/alkaline pretreatment provided higher values of hydrolysis yield (54.74%), total reducing sugars (29.17 ± 1.21 g/L), and glucose production (16.65 ± 0.82 g/L) when compared to the separated pretreatments. Thereafter, simultaneous saccharification and fermentation (SSF) and semi-simultaneous saccharification and fermentation (SSSF) strategies were evaluated using Saccharomyces cerevisiae PE2, Saccharomyces cerevisiae CAT-1, and Kluyveromyces marxianus ATCC 36,907 yeasts with different solid loadings (5, 10, and 15% w/v). The pre-saccharification step increased ethanol yield and ethanol concentration in all experiments. The highest ethanol concentration (24.88 ± 1.04 g/L) was achieved using SSSF with 15% (w/w) solid loading, while the ethanol obtained in SSF experiments was equal to 21.62 ± 0.87 g/L using the same solid loading. The sequential SE/alkaline pretreatment was a successful combination, improving sugars’ production and leading to a high ethanol concentration.