Cultivating oleaginous yeast on spent mushroom substrate and logging residues hydrolysate : an assessment of yeast growth on side product substrates from the food, agriculture, and forestry industry

Abstract: There is an interest in finding alternatives to vegetable oils because of negative environmental impact. Many green sectors generate an abundant amount of side products that are inedible and consist in large of lignocellulose rich materials. Oleaginous yeast can ferment inedible substrates and accumulate a high amount of lipids that have a similar fatty acid profile to that of vegetable oils. Various screening methods exist to assess microbial growth with the aid of colour changing redox indicators. Two yeast strains, Rhodotorula toruloides CBS14 and Rhodotorula babjevae DVPG8058, were screened in a microtiter plate (MTP) system (Applikon Micro-Flask) with a tetrazolium salt as indicator (TTC). The strains were also cultivated on spent mushroom substrate hydrolysate (SMSH) and logging residues hydrolysate (LRH) as substrates. The TTC indicator likely caused inhibition at 1.0 g/l. It was visible at lower concentrations (0.1 and 0.05 g/l) but permanently stained the MTPs. The SMSH had the most positive impact on yeast growth while the cellulose fraction (CF) and hemicellulose fraction (HF) of the LRH showed less overall growth and inhibitory effects. The HF caused high inhibition in both yeast strains, but they could grow at 30% dilution. The CF had similar concentrations of glucose, xylose, and acetic acid as the SMSH but showed less growth. The SMSH could need less pre-treatment than LRH and possibly not need to be supplemented with nutrients because of mushroom residues. The Applikon-Micro Flask system proved sufficient for screening of yeasts on several substrates.