Potential of Dioscorea spp. for Bioethanol Production Using Separate Hydrolysis and Fermentation Method

Thailand is globally recognized for its rich biodiversity, a characteristic that extends to the diverse species of indigenous yams within the Dioscorea spp. genus. With the energy crisis, the selection of agricultural materials for ethanol production to replace the use of economic crops is therefore necessary. This research focuses on six specific yam varieties: Amorphophallus konjac, Colocasia esculenta, Dioscorea bulbifera, Dioscorea alata, Dioscorea hispida, and Dioscorea esculenta. The primary objective is to investigate their bioethanol productivity through the implementation of a separate hydrolysis and fermentation process, with the broader aim of contributing to the potential preservation of these invaluable indigenous yam species. Initiating the study, the experimental samples underwent hydrolysis by alpha-amylase enzyme (1% w/winitial starch) for 180 min, followed by glucoamylase enzyme (1% w/winitial starch) for an additional 72 h. Post the initial hydrolysis, the detection of small molecule sugars by high-performance liquid chromatography, revealed a range of 14.49 ± 1.10 to 28.89 ± 0.03 gtotal sugar/L, with maltose emerging as the predominant sugar compound. Subsequent to the second hydrolysis, it was observed that maltose and starch residues across all six samples underwent successful digestion. The peak glucose productivity was attained at the 12-h mark post-glucoamylase hydrolysis. Among the diverse yam varieties under examination, Dioscorea hispida exhibited the highest glucose production, showcasing a hydrolysis efficiency of 62.53 ± 2.08%. Following the fermentation process with S. cerevisiae, the optimal fermentation time was identified as 72 h, at which point all available glucose in the hydrolysate was fully utilized. The bioethanol productivity spanned from 32.31 ± 3.40 to 43.66 ± 0.02 g/L, with Dioscorea hispida demonstrating the highest ethanol productivity at 0.61 ± 0.00 g/L/h, and a yield of 0.42 ± 0.01 gEtOH/gGlucose. These findings underscore the potential of Dioscorea hispida as a promising contributor to bioethanol production.