Sweet potato having a low temperature-gelatinizing starch as a promising feedstock for bioethanol production

A newly developed sweet potato “Kyushu No. 159” (K159) was used as a feedstock for ethanol production in comparison with an ordinary type “Daichinoyume” (DCY). Gelatinization temperature of isolated K159 starch (49.5–66.2 °C) was approximately 20 °C lower than that of DCY starch (70.9–85.4 °C). The lower gelatinization temperature of K159 starch would be attributed to its higher proportion of short amylopectin branch-chains. Sweet potato mashes containing > 0.3 kg dm−3 total solids were prepared using enzymatic viscosity reduction, and utilized for liquefaction and simultaneous saccharification and fermentation (SSF). Ethanol productivity as a function of liquefaction temperature (50, 60, 70, 80, 90, and 100 °C) was investigated. The results showed that liquefaction at 60 °C was sufficient to efficiently accelerate SSF process of K159, and ethanol yields of 15.1–15.4 % (v/v) (88.8–90.6% of theoretical yield) were obtained during 48–72 h. In contrast, DCY mash required higher liquefaction temperature (90 °C) to achieve the similar ethanol yield. An estimate of environmental impacts in liquefaction process showed that K159 would provide approximately 50% reduction in energy consumption and CO2 emission compared with DCY. Accordingly, the specialized sweet potato “Kyushu No. 159” would offer an energy-saving alternative for fuel-ethanol production.