Bioprocessing of Industrial Sweetpotatoes for Biobased Products

Demand for petroleum-derived products has been growing over the years. To reduce fossil fuel dependence, renewable materials have been explored for alternative transportation fuels. In the US, the use of corn for biofuel production has implication on other markets that utilize corn for animal feed and competes with land needed for food production. Sweetpotato (Ipomoea batatas ) is an important starch-based crop grown worldwide with agronomic benefits including low requirements of fertilizer and drought tolerance. Sweetpotatoes bred for high starch content (>70 % in dry matter basis), referred to as industrial sweetpotatoes (ISPs), have the additional benefit of growth on marginal lands. The high starch content and yield per unit of land cultivated makes ISPs attractive alternative starch feedstocks for industrial applications. Conversion of sweetpotato to sugars has been achieved using thermotolerant enzymes during liquefaction and saccharification of the starch. Resulting sugars can be used to produce ethanol and/or other sugar-based value-added chemicals. The overall goal of this project was to evaluate the potential use of North Carolina ISPs (lines NC-413 and DM02-180) as an alternative starch feedstock for value-added products production and to further examine their needed conversion parameters.;The specific objectives of the project were to: 1) evaluate starch hydrolysis and glucose production rates in the ISPs lines as fresh and flour preparations and compare them with Covington, an important the table-stock sweetpotato; 2) determine process conditions and assess ethanol production potential through Separate Hydrolysis and Fermentation (SHF) and Simultaneous Saccharification and Fermentation (SSF) of flour ISPs preparations using the yeasts Kluyveromyces marxianus NCYC 851 and Ethanol Red (Saccharomyces cerevisiae). Additionally, for the purple-fleshed NC-413 ISP line, examine anthocyanin yield through Simultaneous Extraction and Fermentation (SEF) methods; and 3) determine the potential of Near Infrared Spectroscopy (NIRS) and develop models for prediction of major sweetpotato constituents (moisture, protein, fiber, and starch) during processing to provide a rapid analytical tool to replace standard lab methods.;Results from the saccharification studies showed that differences in chemical composition among the sweetpotato cultivars affected the starch digestibility and that ISPs had greater starch conversion and glucose production (>0.5 g/g dry ISP) than the table-stock Covington. Considering the most promising hydrolysis conditions found, the yeast K. marxianus was investigated for its ethanol production potential at higher temperatures and compared with S. cerevisiae in SHF and SSF. Fermentation by S. cerevisiae during SHF had a maximum 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 increased in a SSF system and that higher yeast concentrations significantly enhanced ethanol production. Maximum ethanol production at 0.39 g/g dry ISP was obtained in a SSF system using S. cerevisiae, 15% greater than that produced by K. marxianus under the same conditions.;A SEF of flour purple-fleshed ISP was conducted in order to evaluate the effect of pH on recovery of anthocyanins and ethanol production. Total monomeric anthocyanin (TMA) concentration was measured and it was found that non adjusted pH produced more ethanol than the fermentations with an adjusted pH of 4.5. It was possible to extract 64.4 mg cyanidin-3-glu/100 g dry powder (22.1 mg cyanidin-3-glu/100 g fresh weight) and produce 0.34 g ethanol /g dry ISP demonstrating that SEF can be used to reduce chemical and energy inputs for anthocyanin extraction. The combination of high dry matter and anthocyanin content in the purple-fleshed ISP line make it a suitable substrate for production of industrial colorants and ethanol.;This work also developed calibration models to predict the major constituents of biologically processed sweetpotatoes using NIRS. Our calibrated and validated models indicated that NIRS is a tool capable of rapidly predicting the composition of different constituents present in sweetpotato samples before and after exposure to conversion process conditions.