Production of Bioethanol from Wild Cassava Manihot glaziovii through Various Combinations of Hydrolysis and Fermentation in Stirred Tank Bioreactors

Aim of the Study: The aim of this study was to evaluate three ethanol fermentation approaches namely (I) separate hydrolysis and common fermentation (II) separate hydrolysis and fermentation and (III) simultaneous saccharification and fermentation in stirred tank reactors using inedible wild cassava as feedstock.

Study Design: Tubers of wild cassava (Manihot glaziovii) were obtained from two districts in Tanzania. Fermentation of hydrolysate and partially liquefied cassava flour was performed in stirred tank reactors.

Methodology: Feedstock composition analysis for structural carbohydrate was performed using acid hydrolysis and high pressure liquid chromatography technique. Analysis of total nitrogen was done by Kjeldahl acid digestion technique, total cyanide was determined using linamarase loaded picrate paper whereas macro-and micronutrients were analysed by inductively coupled plasma atomic emission spectrometry. Thermostable α-amylase and glucoamylase were used to partially hydrolyze the cassava flour to fermentable sugars prior to yeast fermentation. The hydrolysis (liquefaction) was performed at 90°C, 1h followed by saccharification using glucoamylase at 60°C, 2h for approaches I and II. For approach III, liquefaction was performed at 90°C, 1h followed by direct saccharification and fermentation. Fermentation of hydrolysate and partially liquefied starch from wild cassava was done in stirred tank reactors at 30±2°C using baker’s yeast.

Place and Duration of Study: Department of Biotechnology, Lund University from January to June 2014.

Results: The wild cassava (M. glaziovii) tubers possessed comparable physical dimensions to the domesticated cassava, however they displayed higher average flesh proportion (76 to 79%) compared to the domesticated cassava (74%). Compositional analysis disclosed that the wild cassava possessed interesting properties for bioethanol production such as dry matter of up to 89% w/w, degradable carbohydrate up to 90% (dry weight basis), total kjeldahal nitrogen 0.8-1.6% w/w and satisfactory concentration of macro-and micronutrients. Amongst the three fermentation approaches, high ethanol titre of 10-11% (v/v) at high conversion efficiency of 97.6% was achieved for separate hydrolysis and fermentation and simultaneous saccharification and fermentation, whereas low ethanol titre (4.2% v/v) at efficiency of 39% was achieved for separate hydrolysis and common fermentation. Volumetric productivities for the three approaches; ‘separate hydrolysis and common fermentation’, ‘separate hydrolysis and fermentation’, and ‘simultaneous saccharification and fermentation’ were 2.0, 5.5 and 6.5 respectively.

Conclusion: The results obtained in the present study demonstrated that wild cassava has a high starch content, contain balanced nutrients required for efficient bioethanol production and that simultaneous saccharification and fermentation is the best approach for bioconversion of the wild cassava to bioethanol using stirred tank reactors.