Bioethanol production from cactus cladode biomass: considerations of harvesting time, dry matter concentrations, and enzymatic hydrolysis

Cactus pear biomass has the potential for bioethanol production in dry regions. However, its low solids concentration and pH variations may hinder the process of alcoholic fermentation. The objectives of this study were as follows: (1) to evaluate the effects of harvest time and season on biomass pH and dry matter concentration of cladodes of two cactus pear species commonly used as forage ( Nopalea cochenillifera and Opuntia stricta ); (2) to compare the hydrolysis of fresh and dried biomass (10% w v −1 solids); (3) to compare increasing periods of two temperatures (65 °C or 105 °C) to concentrate the biomass to 10% and 30% solids; and (4) to perform enzymatic hydrolysis and fermentation of biomass dried to 30% solids. Biomass pH ranged from 3.0 to 5.6, from early morning to late afternoon, with higher diurnal variation during the dry season, when their solids concentrations were higher than in the rainy season (12–16% × 7–10%). Hydrolyzed fresh and dried biomass had similar glucose, xylose, and galacturonic acid concentrations. Drying at 105 °C for 12 h was the best temperature and period to reach 30% of solids. The enzymatic hydrolysis of the biomass dried to 30% solids yielded 65.3 and 80.0 g of glucose L −1 . After fermentations (33 °C; 8 h; Saccharomyces cerevisiae ), ethanol was produced to 29.4 and 37.5 g L -1 from N. cochenillifera and O. stricta biomasses, respectively. Therefore, early morning during the dry season is the best moment to harvest the cladodes, whose biomass can be partially dried at 105 °C for 12 h to 30% solids load before being hydrolyzed and fermented for bioethanol production. This procedure reduces the time, energy, and inputs needed in the process.