Simultaneous saccharification and fermentation of industrial sweetpotatoes for ethanol production and anthocyanins extraction

A simultaneous saccharification fermentation (SSF) system was studied for ethanol production in flour industrial sweetpotato (ISP) feedstocks (lines: white DM02-180 and purple NC-413) as an integrated cost saving process, and to examine the feasibility of extracting anthocyanins from flour purple ISPs under a simultaneous extraction and fermentation (SEF) system. Furthermore, a separate hydrolysis fermentation (SHF) configuration was carried out to establish a baseline in sugar consumption and ethanol production from the ISP lines. The thermotolerant ethanol producing yeast strain Kluyveromyces marxianus NCYC 851 and the mesophilic Saccharomyces cerevisiae Ethanol Red were evaluated, using commercial alpha amylases for hydrolysis of available ISP starch to sugars. Fermentation by S. cerevisiae during SHF had an ethanol yield of 0.32 g/g dry ISP, a 1.1-fold increase above that produced by K. marxianus. Subsequent studies showed that ethanol yield could be increased in a SSF system with a maximum ethanol yield of 0.39 g/g dry ISP achieved, a 15% increase compared with using a SHF system when S. cerevisiae was used. Simultaneous extraction and fermentation of flour purple-fleshed NC-413 ISPs was studied to evaluate the effect of pH on extraction of total monomeric anthocyanins (TMA) and ethanol production. On average, maximum ethanol yield ranged from 0.31 to 0.34 g/g dry ISP and TMA concentration ranged from 45 to 64 mg cyanidin-3-glu/100 g dry powder (10-22 mg/100 g fresh weight) with the greatest ethanol production coming from non-adjusted pH fermentations. The highest anthocyanin recovery, 64 mg cyanidin-3-glu/100 g dry powder was obtained at 35 °C and pH 4.5 using S. cerevisiae Ethanol Red. This study showed the feasibility of extracting anthocyanins and producing ethanol simultaneously in one unit operation without the need of purified solvents.