Impacts of kafirin allelic diversity, starch content, and protein digestibility on ethanol conversion efficiency in grain sorghum

Authors: CREMER, JULIA - University Of Queensland; LIU, LIMAN - Kansas State University; Bean, Scott; Ohm, Jae-Bom; Tilley, Michael - Mike; Wilson, Jeff; Kaufman, Rhett; Vu, Thanh Hien; GILDING, EDWARD - University Of Queensland; GODWIN, IAN - University Of Queensland; WANG, DONGHAI - Kansas State University

Submitted to: Cereal Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/3/2014
Publication Date: 5/1/2014
Citation: Cremer, J.E., Liu, L., Bean, S., Ohm, J., Tilley, M., Wilson, J.D., Kaufman, R.C., Vu, T.T., Gilding, E.K., Godwin, I., Wang, D. 2014. Impacts of kafirin allelic diversity, starch content, and protein digestibility on ethanol conversion efficiency in grain sorghum. Cereal Chemistry. 91(3):218-227.

Interpretive Summary: Grain sorghum is a highly water and nutrient efficient cereal grain that has potential to be an important feedstock for bio-fuels production. Starch content in grains is a key component of ethanol fermentation, but at least in sorghum, proteins also play a role in governing fermentation efficiency. To test the hypothesis that kafirin (prolamin) seed storage proteins specifically influence the efficiency of biofuel production in sorghum, ten diverse genetic lines with allelic variation in the beta-, gamma- and delta-kafirins, including three beta-kafirin null mutants, were tested for ethanol conversion and fermentation efficiency. Our selected lines showed wide variation in grain biochemical features such as total protein, starch and free amino acid (FAN) content. Starch content and protein digestibility were positively correlated to overall ethanol yield and an increased FAN content enhanced fermentation efficiency. Investigation of the impact of kafirin on ethanol production indicated that variation in the beta-kafirin allele is correlated to grain digestibility and FAN, with subsequent negative effects on ethanol yield. A large seeded variety, carrying a novel gamma-kafirin allele, was rich in FAN and exhibited excellent short term fermentation efficiency, but produced a low overall ethanol yield predominantly due to low starch content. RP-HPLC profiling of the alcohol-soluble kafirin protein fraction in the seed endosperm revealed a significant relationship between a specific protein peak and grain digestibility, providing further evidence for direct links between seed protein composition and end-uses such as digestibility and bioethanol production.

Technical Abstract: Seed protein and starch composition determines the efficiency of ethanol conversion in the production of grain-based biofuels. Sorghum, highly water- and nutrient-efficient, has the potential to replace fuel crops with greater irrigation and fertiliser requirements, such as maize. However, sorghum grain is generally less digestible due to extensive disulfide cross-linking among sulfur-rich storage proteins in the protein/starch matrix. Thus, the fine structure and composition of the seed endosperm directly impacts grain end use, including fermentation performance. To test the hypothesis that kafirin (prolamin) seed storage proteins specifically influence the efficiency of biofuel production in sorghum, ten diverse genetic lines with allelic variation in the beta-, gamma- and delta-kafirins, including three beta-kafirin null mutants, were tested for ethanol conversion and fermentation efficiency. Our selected lines showed wide variation in grain biochemical features such as total protein, starch and free amino acid (FAN) content. Starch content and protein digestibility were positively correlated to overall ethanol yield and an increased FAN content enhanced fermentation efficiency. Investigation of the impact of kafirin on ethanol production indicated that variation in the beta-kafirin allele is correlated to grain digestibility and FAN, with subsequent negative effects on ethanol yield. A large seeded variety, carrying a novel gamma-kafirin allele, was rich in FAN and exhibited excellent short term fermentation efficiency, but produced a low overall ethanol yield predominantly due to low starch content. RP-HPLC profiling of the alcohol-soluble kafirin protein fraction in the seed endosperm revealed a significant relationship between a specific protein peak and grain digestibility, providing further evidence for direct links between seed protein composition and end-uses such as digestibility and bioethanol production.