Abstract

Ethanol production by co-culture of ragi tapai and Saccharomyces cerevisiae from unhydrolyzed cassava starch without addition of enzymes was conducted in a 2 L batch fermentor. Taguchi’s method with orthogonal array of L8 was applied in design of experiment (DOE) and the results were analyzed using MINITAB v14 software. Seven factors: Nitrogen-phosphorus-potassium (NPK), urea, fermentation temperature, ragi tapai concentration, S. cerevisiae concentration, agitation and co-culturing time were varied at two levels for each factor. The significant factors for the production of ethanol were determined by setting S/N ratio to “larger-is-better” for high yield ethanol and “smaller-is-better” for low yield of ethanol byproducts. The optimum values obtained for each factor were similar to each other. Both have optimum factors of: Urea at 0.8 w/w%, dry ragi tapai and S. cerevisiaeconcentrations each at 10 w/w%, co-culturing time at 3 h gap, NPK at 0.09 w/w% and agitation speed at 100 rpm. The fermentation temperature for high ethanol yield was 35°C, whereas for the byproducts was 30°C. From the validation experiment conducted at 30°C in 10 L fermentor, the ethanol concentration obtained was 68.00 g/L, while all byproducts concentrations were below 9.00 g/L at the end of the fermentation.

Key words: Co-culture fermentation, ethanol, Taguchi’s experimental design, ragi tapai, Saccharomyces cerevisiae