Fuel ethanol production from commercial grain sorghum cultivars with different tannin content

• Study of commercial sorghum cultivars for ethanol production.
• Effect of tannin content on starch hydrolysis and fermentation into ethanol.
• The addition of a protease did not improve the starch hydrolysis efficiency.
• Tannin removal improved the ethanol production from commercial sorghum cultivars.

The two-step enzymatic starch hydrolysis of three commercial sorghum grain cultivars (Flash 10 Plus, 8419 and Flash 1) containing different tannin content (<0.2%, 1.1% and 2.0% respectively) was evaluated, in order to determine the most promising process for the production of ethanol. High viscosities were obtained for suspensions of the three cultivars (6440–8071 cP). The hydrolysis efficiency values were two and three times greater in the cultivars with higher tannin content (8419 and Flash 1, respectively) when a gelatinization step previous to starch hydrolysis was performed, reaching similar values for the three cultivars (87%–88%); for the low-tannins cultivar a lower improvement was observed. The addition of a protease, pepsin, which could disrupt the interaction between the protein and starch, did not improve the starch hydrolysis efficiency. Simultaneous saccharification and ethanol fermentation (SSF) assays were carried out. The removal of tannins improved the performance of the SSF, suggesting that this treatment may have eliminated inhibitors and also contributed to a partial depolymerization or loss of crystallinity of starch. For a 1:3 solid–liquid ratio, an ethanol concentration of 107 g/L at 68 h, a fermentation efficiency of 100% and an industrial yield of 390 L/t of sorghum (dry basis) were obtained.