Cultivar and maturity effects on the quality attributes and ethanol potential of sweet sorghum

Authors: Cole, Marsha; Eggleston, Gillian; Petrie, Eric; Uchimiya, Sophie; Dalley, Caleb

Submitted to: Biomass and Bioenergy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2016
Publication Date: 1/1/2017
Citation: Cole, M.R., Eggleston, G., Petrie, E., Uchimiya, S.M., Dalley, C. 2017. Cultivar and maturity effects on the quality attributes and ethanol potential of sweet sorghum. Biomass and Bioenergy. 96:183-192.
DOI: https://doi.org/10.1016/j.biombioe.2016.12.001

Interpretive Summary: Sweet sorghum is a promising feedstock crop for the manufacture of biofuels and bioproducts. Its large starch content may be an opportune source of untapped fermentable sugars to add economical value to sweet sorghum juices and processing by-products. In this study, four commercial cultivars, Dale, M81E, Theis, and Top 76-6 were grown in Louisiana (over 2 years) and studied at the milk, soft dough, hard dough, physiological, and post-physiological maturity stages to determine the value of sweet sorghum starch and the optimal cultivar, maturity, and environment ideal for processing and fermentation juice quality. Juice quality and physical crop attributes essential to processing and fermentation were strongly influenced by environmental and cultivar interactions; however, total sugars was specifically affected by cultivar and maturity interactions. Total sugars and total starch in extracted juice were greatest at hard dough maturity specifically in Top 76-6 and M81E, respectively. In consideration of this result, a theoretical added-value (~6-17%) could be obtained from starch if fermented in raw sweet sorghum juices and possibly more if the other processing by-products and seed-heads were recycled back into fermentations. In conclusion, the higher sugar and low starch content of Top 76-6 makes it most suitable for syrup manufacturing. Dale and M81E, however, are most attractive for fermentations only if their high starch content can be economically incorporated into the manufacturing and fermentation processes.

Technical Abstract: Sweet sorghum is a promising feedstock crop for the manufacture of biofuels and bioproducts. Its large starch content may be an opportune source of untapped fermentable sugars to add economical value to sweet sorghum juices and processing by-products. In this study, four commercial cultivars, Dale, M81E, Theis, and Top 76-6 were grown in Louisiana (over 2 years) and studied at the milk (Mi), soft dough (SD), hard dough (HD), physiological (PM), and post-physiological (PPM) maturity stages to determine the value of sweet sorghum starch and the optimal cultivar, maturity, and environment ideal for processing and fermentation juice quality. Juice quality and physical crop attributes essential to processing and fermentation were strongly influenced by environmental and cultivar interactions (p<0.0001); however, total sugars was specifically affected by cultivar and maturity interactions. Although a moderately-strong correlation between total sugars and juice Brix was found (R=0.767, p<0.0001, n=151), the use of Brix as an in-field harvesting indicator to predict stalk maturity is not recommended since it does not consider cultivar influences. Total sugars and total starch in extracted juice were greatest at HD maturity in Top 76-6 and M81E, respectively. Added-value (~6-17%) can be obtained from starch if fermented in raw sweet sorghum juices, and possibly more when including other processing by-products and seed-heads that can be recycled back into fermentations. Top 76-6 was the best cultivar for syrup manufacturing; Dale and M81E were most attractive for fermentations only if their high starch content can be economically incorporated into the manufacturing and fermentation processes.