Fermentation of Sweet Sorghum Juice

Sweet sorghum (Sorghum bicolor (L.) Moench) is a tropical grass native to Africa, and was introduced into the U.S. in 1854. It was originally cultivated in the southern U.S. for small-scale syrup production, and was developed as a sugar source during World War II. We are testing the juice/sugars in sweet sorghum as a direct feedstock (not requiring starch hydrolysis) for ethanol production in Arizona. In previous work we found great variation for sugar profile (amounts of sucrose, glucose and fructose) among lines, and we developed a predictive model to estimate ethanol yield per area. Three questions were addressed in this study: (1) how does actual fermentation of juice compare to our predicted model; (2) how do sugar profiles change after fermentation (what sugars are left); and (3) how do the different sugar profiles affect final ethanol yield?

Juice samples were collected from a date-of-planting study that was harvested between August and December 2008. The resulting juice was fermented utilizing a bench-top procedure, and the sugars were analyzed (by HPLC with refractive index detection) before and after fermentation to determine changes in the sugar profile. Correlations were made between sugar profile and final ethanol yield.

Fermentations resulted in ethanol yields between 8 and 11% (about a 77% conversion rate). Most samples fermented 24 hours or longer had no detectable amounts of remaining fructose, glucose, or sucrose, and two samples had very small concentrations of fructose. This can be due to the fact that the strain of Saccharomyces cereviseae prefers glucose and, once that has been depleted, will also utilize sucrose and fructose. However, this is unlikely and we may be observing post-harvest changes in the sugar profile (one sample that had been stored frozen for four months showed a significant increase in sucrose concentration with a concomitant reduction of fructose and glucose). Most interesting was the occurrence of an unidentified peak around 3.8 minutes, found in all fermented samples.