Rhodotorula toruloides for carotenoid production using waste hardwood biomass

This work explores the potential of underutilized urban pruning residues from hardwood as feedstocks for bioprocesses based on the carotenogenic yeast Rhodotorula toruloides, investigating the correlation between biomass composition and carotenoids production. Enzymatic hydrolysates derived from woods and barks of sessile oak and mulberry tree were used as substrates for microbial fermentation. The results demonstrated superior titers, yields, and productivity for β-carotene and torulene compared to previously reported processes. While mulberry tree bark hydrolysate yielded the highest total sugars (16.4 g/L), sessile oak bark hydrolysate achieved the highest β-carotene production (362.7 mg/L) and yield on dry cell weight (163.4 mg/g) after 30 h of fermentation. Woody biomasses are known to contain significant amounts of extractive inhibitory compounds. Surprisingly, when these extractives were removed to promote growth, a significant drop in carotenoids titers were observed, bringing them in line with published values obtained from biomasses lacking such components. These data suggest that stress-inducing compounds present in the extractive fractions are crucial for promoting high productivity and yield, when compared with the use of biomasses lacking such components. This research highlights the untapped potential of urban woody residues, which thanks to the presence of triggering components can be considered as advantageous feedstock for microbial carotenoids production.