Conversion of Industrial Sweetpotatoes for the Production of Ethanol

Abstract

Starch is a renewable resource which is currently being used to produce ethanol from corn. Although corn starch to ethanol is a mature conversion process, corn production is not feasible for every region of the United States. Sweet potatoes (Ipomoea batatas, Morning Glory Family) are a low-impact starch crop grown primarily in the southeast region of the U.S. and offer a viable, alternative starchy raw material that can be converted to useful sugar feedstocks needed for production of ethanol and other value added products. The process of converting sweetpotato starch into ethanol can be described in three basic steps: liquefaction using α-amylase or some other liquefying agent to gelatinize available starch, saccharification using at least one saccharifying enzyme to convert gelatinized starch into fermentable sugars, and fermentation of the sugars into ethanol. The overall goal of this project was to effectively generate the information necessary to define an environmentally friendly process for conversion of industrial sweetpotato varieties into simple sugars and ethanol. Specific objectives included: 1) Examining liquefaction, saccharification, and fermentation of FTA-94 industrial sweetpotatoes (ISP) using α-amylase and glucoamylase for the production of ethanol, and 2) Examining the enzymatic hydrolysis and fermentation of industrial sweetpotatoes with the addition of pullulanase for ethanol production. The significance of enzyme loading rate, incubation time, and temperature on changes in starch content, soluble sugar concentrations and ethanol yield (when appropriate) during each conversion step was measured. One α-amylase (Liquozyme SC) and three glucoamylases (Spirizyme Fuel, Spirizyme Plus Tech, and Spirizyme Ultra) were tested during liquefaction and saccharification at 85 and 65˚C, respectively, over time. Results showed that the majority of the available starch, 47.7 and 65.4%, was converted during liquefaction of flour and fresh sweetpotato preparations, respectively, with the addition of 0.45 KNU-S/g dry ISP of Liquozyme SC after 2 hours of incubation (66.4 and 80.1% initial starch contents). Saccharification was generally able to increase the breakdown of starch, but it main purpose was to convert the susceptible starch to fermentable sugars. The addition of 5.0 AGU/g of Spirizyme Ultra after 48 hours of incubation was able to produce 862.2 and 743.9 mg of simple sugars/g of starch with flour and fresh preparations, respectively. Fermentation with Ethanol Red Yeast of these appropriate loadings tested ethanol generation over time with and without the addition of salt niturients and was able to generate 62.6 and 33.6 g/L of ethanol for flour (25% w/v substrate loading) and fresh (12.5% w/v) ISP, respectively, after 48 hours of fermentation without salts. The addition of pullulanase during 48 hours of saccharification with 5.0 AGU/g Spirizyme Ultra at 45, 55, and 65ºC showed no consistent increase in the change in starch content. Adding 0.5 NPUN/g of Promozyme during saccharification at 55ºC slightly increased the amount of recovered glucose, producing 828.7 and 869.7 mg/g starch of glucose only for flour and fresh ISP. Fermentation of these treatment combinations yielded ethanol values of 310.7 and 333.3 mg/g dry ISP for flour and fresh preparations. Results of these experiments show that sweetpotatoes offer a viable alternative starch source for ethanol production. If implemented in the southeastern region of the United States, it wouild be possible to generate at least 700 gallon of ethanol per acre of industrial sweetpotatoes.