Bioconversion of cassava stem to ethanol: oxalic acid pretreatment and co-culture fermentation

ABSTRACT

The aim of this study is to maximize the bioethanol production from cassava stem using statistical design of experiments. Therefore, organic acid catalysis was employed as an alternative pretreatment method to mineral acid catalysis. The effects of solid-to-liquid ratio (ratio between mass of cassava stem and volume of oxalic acid), reaction time and oxalic acid concentration were studied on the concentration of reducing sugars at 121°C and 15 psi. An optimum yield of 1.564 g/L reducing sugars was obtained at 0.08 g/mL solid-to-liquid ratio, 2.97% (w/v) oxalic acid concentration and 27 min reaction time with 5 g of cassava stem. Cassava stem was characterized using Fourier-transform infrared (FTIR) spectra and scanning electron microscopy (SEM) images before and after pretreatment. The residue was hydrolyzed with 0.5 mL of cellulase (16.8 FPU/g), and 1.889 g/L of glucose was obtained, after pretreatment. Finally, the fermentation was performed by co-culturing Kluyveromyces marixianus with Klebsiella oxytoca, Fusarium oxysporum, Zymomonas mobilis and Saccharomyces cerevisiae. A maximum ethanol concentration and yield of 1.622 g/L and 0.32 g ethanol/g cassava stem, respectively, were obtained for K. marixianus–K. oxytoca co-culture. Thus, cassava stem is proved to be an interesting substrate for ethanol production.

KEYWORDS:

• Cassava stem
• oxalic acid pretreatment
• enzymatic hydrolysis
• co-culture fermentation
• bioethanol

Acknowledgements

We would like to express our heartfelt thanks to the Management of Kongu Engineering College, Perundurai, and Kumaraguru College of Technology, Coimbatore, for providing the necessary facilities. We wish to sincerely thank the central research facility, Karunya University, Coimbatore, for providing SEM facilities, and the department of chemistry, PSG College of Arts & Science, Coimbatore, for providing FTIR facilities for this research.

Disclosure statement

No potential conflict of interest was reported by the authors.