Bioethanol production from water-soluble and structural carbohydrates of normal and high sugary corn stovers harvested at three growth stages

In the current practice of generating ethanol from corn stover (CS), mostly cellulose is converted into ethanol, and water-soluble carbohydrates (WSCs) and/or hemicellulose fractions are often discarded. This incomplete conversion of fermentable components consequently lowers the final ethanol yield of CS. Normal corn stover (NCS) that is commonly used in the stover-based ethanol production, contains poor amounts of WSCs, and eventually, it is not attractive to conduct a separate extraction of WSCs from NCS. Alternatively, high sugary stover (HSS) could be a promising feedstock system over NCS for particularly its excellent sugar content. This study investigated the effects of converting both water-soluble and structural carbohydrates on ethanol production, by comparing the ethanol yields of HSS and NCS obtained from four high sugary and four normal corn genotypes, respectively. The corn genotypes were cultivated and stovers were harvested at three growth stages of plants, including dough (R4), dent (R5), and maturity (R6) stages. Despite the WSC contents of both types of stovers were found to decrease as the plants moved from R4 to R6 stage, quantities of WSCs in HSS were significantly higher at all stages than the respective WSC contents of NCS. At the R4 stage, the amounts of WSCs ranged from 19.1 to 29.2% in HSS, which varied in NCS between 10.2% and 13.6% at the same stage, and finally reached 10.2-14.9% in HSS and 4.1-5% in NCS at the R6 stage. WSCs of HSS produced a maximum of 14.9 g/L of ethanol at the R4 stage in contrast to only 5.5 g/L of ethanol produced by the WSCs of NCS. Irrespective of the growth stages, the net specific ethanol yields (SEY) of HSS and NCS varied from 149.7 to 204.8 g/kg and 139.6 to 170.1 g/kg, respectively, where stovers harvested at the R4 stage provided the maximum SEY.