Single Cell Oil: a potential and innovative industrial platform for the production of new generation biodiesel and bioproducts starting from biomass

Single cell oil (SCO), consisting mainly of triacylglycerols (TAG), represents an innovative and versatile platform to produce a wide range of added-value bioproducts, such as biodiesel, bioplastics and biosurfactants. SCO is produced via fermentation by oleaginous yeasts that are microorganisms able to accumulate up to 70% of their dry cell weight as TAG. The key strategy necessary to make economically sustainable this process is the use of second generation (2G) sugars, obtained from the hydrolysis of cellulose and hemicellulose of residual or dedicated lignocellulosic biomasses. Among lignocellulosic crops, Arundo donax L. was defined as a promising grass in the biorefinery field. The present study was focused on two main goals: the first one was the investigation of sustainable homogeneous and heterogeneous catalysts for the biomass hydrolysis in order to produce glucose and xylose, by using microwaves as heating system; the second one was the fermentation of lignocellulosic hydrolysate into SCO by oleaginous yeast Lypomyces starkeyi. From an industrial point of view, the innovative aspect of this study was the fermentation of hydrolysates without any detoxification steps. The quantitative conversion of hemicellulose into xylose was performed by optimizing both homogeneous and heterogeneous catalysis. The cellulosic fraction remained in the solid residue recovered at the end of the first step was then hydrolyzed by testing dilute H2SO4 or the enzymatic mixture Ctec2, obtaining an overall yield for both approaches of about 40-50%, expressed as g glucose/g glucan in the starting biomass. The biological conversion of 2G sugars into oil reached the yield of 15 wt% respect to consumed sugars, very close to the theoretical one (20 wt%) reported in literature for wild-type strains of oleaginous yeasts.