Direct transesterification of oleaginous yeast lipids into biodiesel: Development of vigorously stirred tank reactor and process optimization

Oleaginous yeast Yarrowia lipolytica is considered as an environmental friendly oil source due to its ability to convert industrial wastes into microbial oils and their fatty acid compositions similar to those of plant oils. After cultivation in 20 L bioreactor, the yeast cells with lipid content >30% were directly used in transesterification process. A 2-L vigorously stirred tank reactor (VSTR) with balloon-whisk impeller was designed for simultaneously disrupting the yeast cells and converting the yeast lipids into biodiesel in one reactor. Operating conditions for direct transesterification in VSTR were: the use of glass beads as cell disrupting agents at a bead:biomass ratio of 1:1 (w/w), methanol as lipid extracting solvent and feedstock at a methanol:biomass ratio of 4:1 (v/w), sulfuric acid as catalyst at a concentration of 2% (v/v), agitation speed at 1000 rpm and reaction temperature at 50 °C. Under these conditions, the highest biodiesel yields of 94.99% and 80.91% were obtained when using dried and wet yeast cells, respectively. The cell debris left after the reaction contained carbohydrate at 63.41% and protein at 15.76% indicating its potential use as nutrient sources for further fermentation process.