Hydrolysis and fermentation of sweetpotatoes for production of fermentable sugars and ethanol

► Enzymatic processing technologies for industrial sweetpotato conversion are studied. ► Up to 90% of starch in industrial sweetpotatoes can be converted with current enzymes. ► Processing of fresh and flour ISPs differed with benefits and challenges to each approach. ► Complete fermentation of ISP sugars by yeast is possible without nutrient supplementation. ► Industrial sweetpotatoes offer a viable alternative starch resource for biobased products. Liquefaction, saccharification, and fermentation of FTA-94 industrial sweetpotatoes (ISPs) were examined using α-amylase and glucoamylase for the production of ethanol. Starch degradation and sugars produced over time were examined for (1) α-amylase (Liquozyme SC) at different loading rates (0.045, 0.45, and 4.5%KNU-S/g dry ISP) during liquefaction; and (2) three glucoamylases (Spirizyme Fuel, Spirizyme Plus Tech, and Spirizyme Ultra) at different loading rates (0.5, 1.0, and 5.0AGU/g dry ISP) during saccharification. The majority of starch, 47.7 and 65.4% of dry matter, was converted during liquefaction of flour and fresh sweetpotato preparations, respectively, with the addition of 0.45KNU-S/g dry ISP of Liquozyme SC after 2h (66.4 and 80.1% initial starch in dry matter, respectively). The enzymes used during saccharification increased starch breakdown, but was more effective in conversion of short chain carbohydrates to fermentable sugars. The addition of 5.0AGU/g of Spirizyme Ultra after 48h produced 795.4 and 685.3mg glucose/g starch with flour and fresh preparations, respectively. Yeast fermentation on hydrolyzed starch was examined over time with and without the addition of salt nutrients. Yeast converted all fermentable sugar (e.g. glucose, fructose, maltose) and produced 62.6 and 33.6g ethanol/L of hydrolysate for flour (25% w/v, substrate loading) and fresh (12.5% w/v, substrate loading) ISP, respectively, after 48h without salt addition.