Electrochemical Evaluation of Sweet Sorghum Fermentable Sugar Bioenergy Feedstock

Abstract

Although sweet sorghum is a promising feedstock for bioenergy and biobased products, sweet sorghum-based biorefineries in the U.S. are still in the planning or pilot-scale stages. Accurate, rapid, and inexpensive metrology is known to streamline (bio)­refining operations and drive the return on investment. In this study, new cyclic voltammetry (CV)-based methods were developed to rapidly classify sweet sorghum fermentable sugar feedstocks for electroactive functionalities. In addition to providing industrial QA/QC protocols, developed methods could be used to screen for the pest-resistant cultivars containing redox-active antifeedants (e.g., flavonoids, alkaloids, and aconitic acid), enabling germplasm development for a sustainable feedstock supply chain. Developed CV methods were tested on five male (Atlas, Chinese, Dale, Isidomba, and N98) and three female (N109B, N110B, and N111B) inbred lines and their hybrids (23 cultivars total) planted in April, May, and June of 2015 in Georgia, and harvested at the hard-dough stage. The peak anodic potential (E pa in volts) of derivative CV (pH 5, 0.1 M KCl) overlapped with quercetin and tannic acid model reductants. Fluorescent porphyrin/chlorophyll-like condensed and recalcitrant aromatic structure is likely to be the primary electron-enriched (highest CV peak areas) secondary product, and showed significant (p < 0.05) cultivar and planting date dependencies.

Electroactivity of polyphenols in sweet sorghum (Sorghum bicolor (L.) Moench) cultivars

“…The CV and CDPV of diluted (2-fold by DDW) juice samples (buffered at pH 5 with 40 mM phosphate in 0.1 M KCl) were obtained using a WaveNow potentiostat with disposable screen-printed carbon electrodes (SPE with Ag/AgCl reference electrode; Pine Research Instrumentation, Durham, NC) of two different surface areas: 2 mm diameter disk, and 4×5 mm rectangle patterned carbon working electrode. The larger surface area of the 4x5 mm carbon SPE is expected to enhance the current signal relative to the 2 mm SPE used in our previous report [ 7 ]; however, the background current could increase as well. Both SPEs are compatible with aqueous solution, and are composed of polyethylene terephthalate plastic screen with Ag/AgCl reference electrode and carbon working/counter electrodes on conductive silver layer.…”

“…Sweet sorghum cultivars were planted in April, May, or June 2017, and harvested at the hard dough stage of grain fill, which occurred during July through November, depending on cultivar and planting date. Field experimental design and juice characterization procedures were described in detail previously [ 7 , 21 , 22 ], and are summarized in Section I of S1 File . Group membership and Plant Introduction (PI) number [ 23 ] of 24 sweet sorghum genotypes examined in this study are provided in S1 Table in S1 File .…”

“…As described in detail previously [ 7 , 21 ], distilled, deionized water (DDW) with a resistivity of 18 MΩ cm (APS Water Services, Van Nuys, CA) was used in all laboratory procedures. All chemical reagents were obtained from Sigma-Aldrich (Milwaukee, WI) with the highest purity available.…”

“…Our previous report used cyclic voltammetry (CV) to oxidize the stem juice of sweet sorghum; polyphenolic structure similar to quercetin was determined to be the primary redox-active moiety [ 7 ]. A sweet sorghum genotype called No.5 Gambela showed resistance to a recently emerged pest in the U.S.: sugarcane aphid [( Melanaphis sacchari (Zehntner)].…”

“…First, DPV was employed to increase the sensitivity of detection for the peak previously attributed to quercetin-like structures in sweet sorghum juice. Peak identification relied on the comparison of juice against electroactive structures in juice or bagasse of sweet sorghum [ 7 , 8 ]: tryptophan, tyrosine, trans-aconitic acid, quercetin, and riboflavin.…”