Mixed glycerol and orange peel-based substrate for fed-batch microbial biodiesel production

The aqueous extraction of orange peel waste (OPW), the byproduct of the juice extraction process generated annually in massive amounts (21 Mton), yields a carbohydrate-rich liquid fraction, termed orange peel extract (OPE). Several studies highlight that the combination of glycerol, a biodiesel byproduct, with carbohydrate mixtures might boost microbial lipid production. This study performed first a shaken flask screening of 15 oleaginous yeast strains based on their growth and lipid-producing abilities on OPE- and glycerol-based media. This screening enabled the selection of R. toruloides NRRL 1091 for the assessment of the process transfer in a stirred tank reactor (STR). This assessment relied, in particular, on either single- and double-stage feeding fed-batch (SSF-FB and DSF-FB, respectively) processes where OPE served as the primary medium and nitrogen-containing glycerol-OPE mixtures as the feeding one. The continuous supply mode at low dilution rates (0.02 and 0.01 h^-1 for SSF-FB and DSF-FB, respectively) starting from the end of the exponential growth of the initial batch phase enabled the temporal extension of biomass and lipid production. The SSF-FB and DSF-FB processes attained high biomass and lipid volumetric productions (LVP) and ensured significant lipid accumulation on a dry cell basis (Y_L/X). The SSF-FB process led to LVP of 20.6 g L^-1 after 104 h with volumetric productivity (r _L) of 0.20 g L^-1 h^-1 and Y_L/X of 0.80; the DSF-FB process yielded LVP, r _L and Y_L/X values equal to 15.92 g L^-1, 0.11 g L^-1 h^-1 and 0.65, respectively. The fatty acid profiles of lipids from both fed-batch processes were not significantly different and resembled that of Jatropha oil, a vastly used feedstock for biodiesel production. These results suggest that OPE constitutes an excellent basis for the fed-batch production of R. toruloides lipids, and this process might afford a further option in OPW-based biorefinery.