Estimation methods and parameter assessment for ethanol yields from total soluble solids of sweet sorghum

• Ethanol production from measured lab conversion of juice from sweet sorghum was compared with estimates from five published equations.
• Two of the five estimation equations were similar to measured ethanol yields.
• Three of the published equations for estimates of ethanol production are over-estimated.
• Multiple juice traits entered into models affecting ethanol conversion from multiple regression analysis.

Estimation methods and evaluation of ethanol yield from sweet sorghum (Sorghum bicolor (L.) Moench) based on agronomic production traits and juice characteristics is important for developing parents and inbred lines of sweet sorghum that can be used by the bio-ethanol industry. The objectives of this study were to compare published indirect methods for the calculation of ethanol yields from sweet sorghum and test them against direct ethanol production in laboratory, as well as to determine the relationships among total soluble sugar and juice traits with ethanol concentrations over time. Four sorghum varieties (KKU40, Theis, BJ248 and SPV1411) were compared for juice characters and ethanol yield in a randomized complete block design with four replications. Agronomic and juice traits of sweet sorghum were recorded during flowering and at harvest. Juice of sweet sorghum was fermented by yeast (Saccharomyces cerevisiae) to obtain ethanol yields in the laboratory, which were then compared with ethanol yields calculated based upon five calculation methods from the literature. Ethanol yield estimates calculated from published methods were generally higher than laboratory values. However, estimates based upon Somani and Taylor (2003) and on Smith et al. (1987) when multiplying theoretical yields by 80% were not significantly different from laboratory results. Though ethanol yield are strongly correlated with sugar yields, juice traits influenced the rate of fermentation of sugars over time. For example, glucose, fructose and nitrogen content in the juice had a positive effect on ethanol concentration after 12 h of fermentation while multiple juice traits were significantly associated with ethanol concentrations after 24, 36 and 48 h of fermentation.