Two consecutive step process for ethanol and microbial oil production from sweet sorghum juice

• Ethanol and microbial oil consecutive production from sweet sorghum juice.
• Ethanol is produced first.
• Oleaginous yeast growth employing the residual carbon and nitrogen left.
• Glucose had the highest uptake rate.
• Fructose was slowly metabolized and a substantial amount remained.

The juice extracted from sweet sorghum stalks has been previously explored to produce ethanol and also to grow oleaginous yeasts and algae with the objective of producing microbial oil. In this paper we propose a different process route in order to produce ethanol and microbial oil in two consecutive fermentation steps. Ethanol is produced first followed by the growth of oleaginous yeast employing the residual carbon and nitrogen left from the first step. Two yeasts were compared for ethanol production. Trichosporon oleaginosus was cultivated for lipid production. The yeast selection for the first step was the most important factor for achieving a high ethanol yield and the effect of inorganic nitrogen addition was not significant. The remaining sugars consisted of a mixture of sucrose and fructose and no residual glucose was detected in any of the runs. In the second step T. oleaginosus DSM 11815 grew in the pooled juices remaining from the first step for 168 h and produced biomass with 28% lipid content. Glucose showed the highest uptake rate, sucrose was utilized until low glucose values prevailed, and fructose was slowly metabolized and a substantial amount remained. Although the two step process has flexibility in choosing the proper microorganism for each step, it is necessary to look for a rapid fructose uptake strain for both fermentations.

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