Evaluation of bioethanol production from juice and bagasse of some sweet sorghum varieties

Abstract

Sweet sorghum, with sugar-rich stalks and water-use efficiency, has a very good potential as an alternative feedstock for ethanol and also non-competing with human feed on land. The present study evaluates the exploitation of juice and bagasse of five varieties of sweet sorghum for bioethanol production which can further improve the energy yield of the crop. The sweet sorghum varieties, GK-coba, Mn-1054, Ramada, Mn-4508 and SS-301, were analyzed for their productivity, and sugar and fiber contents. All varieties significantly differed in yield of stripped stalk, juice and bagasse. The sugar-rich juice and the fiber-rich bagasse, resulting from squeezing the striped stalks, were used for bioethanol production by two microorganisms; Saccharomyces cerevisiae ATCC 7754 and Zymomonas mobilis ATCC 29191. Stalks of varieties GK-coba, Mn-4508 and SS-301 contained high sugar contents and thus were utilized for bioethanol production directly from juice. Stalks of varieties Mn-1054, Ramada and SS-301 had higher content of fibers, so their bagasses were used for bioethanol production. Bagasse was pretreated and hydrolyzed thermo-chemically with 2% (v/v) sulfuric acid (98%) at 120°C for 60min and filtered and the sugar-rich filtrate was neutralized and supplemented with nutrients for bioethanol production. Fermentation of sweet sorghum sugars or acid-hydrolyzed neutralized bagasse into bioethanol was conducted by Sacch. cerevisiae, Z. mobilis or mixed-culture of both organisms at 1:1 ratio. The highest bioethanol production was obtained from juice and bagasse of variety SS-301, by the mixed-culture treatment. From the juice, bioethanol concentration was 50.26mLL−1, whereas from bagasse, bioethanol concentration was 10.5mLL−1. Finally, it could be estimated that 160mL of bioethanol can be produced out of each 1kg of variety SS-301, when using both juice and bagasse.