Saccharification and Fermentation of Waste Sweet Potato for Bioethanol Production

We have investigated the relative performance of the two enzymes, α‐amylase and amyloglucosidase, and two microbial strains, Zymomonas mobilis AX101 and 8b, for saccharification and fermentation of sweet potato. Modified Michaelis–Menten and Monod‐type mathematical models were developed and verified with experimental results for saccharification using amyloglucosidase and fermentation using Z. mobilis 8b, respectively. The results showed that 91.5% of the starch and sucrose were converted to glucose and fructose using amyloglucosidase at pH 3.5 and 55C. The Z. mobilis 8b was able to convert more than 90% of the total sugars into ethanol within 18 h with 87.2% of the theoretical yield and 49.07 g/L final concentration of ethanol. A mass balance and cost analysis show that commercial production of ethanol from sweet potato is limited by the feedstock cost. Practical Applications This study shows that sweet potatoes that are unmarketable, small in size, bruised, cut or damaged otherwise from the harvesting process, supplied by a local farmer in the Central Texas area, can be used to produce bioethanol. Normally, farmers do not collect the damaged sweet potatoes because there is no demand for those in the market. This is supported by the fact that 20% of the total sweet potatoes cultivated are left in the ground due to damage while harvesting with plows. If collected and used for ethanol production, local farms can earn additional revenue.