Production of uncommon carotenoids and lipids by red yeasts utilizing agri-food residues and waste cooking oil

In recent years, the biotechnological production of carotenoids and lipids by yeasts emerges as a valuable strategy at the industrial level, also fitting the circular economy pillars when agri-food waste can be used as the main components of the culture media. In this study, bioprocesses employing red yeasts were developed using cost-effective agri-industrial residues, such as soy okara (a soybean industry byproduct). This low-cost substrate was investigated as a source of carbohydrates and essential nutrients, with its enzymatic pre-treatment optimized to create a balanced and efficient fermentation medium. The screening of a collection of red oleaginous yeasts identified Rhodotorula paludigena CBS 6565 and Rhodotorula diobovata CBS 324 as promising strains capable of efficiently producing both lipids and uncommon carotenoids. These two strains were cultivated in a lignocellulose hydrolysate-based medium supplemented with urea, validating the promising results of the screening. Urea, a cost-effective nitrogen source, was found to enhance carotenoid production compared to ammonium sulfate. Finally, soy okara was used as the fermentation medium for Rhodotorula paludigena CBS 6565. Soy okara underwent optimized enzymatic hydrolysis to maximize fermentable sugar release, while the addition of waste cooking oil and syrup from candied fruit processing significantly boosted carotenoid production, reaching 262.4 mg/L in 90 h. Among these, β-carotene and torularhodin contributed 140 mg/L and 72.5 mg/L, respectively. Furthermore, the yeast cells accumulated lipids, constituting 56% of their dry weight, with a final concentration of 18 g/L. Overall, this study underscores the synergy between agri-food waste valorization and the sustainable production of yeast biomass enriched in carotenoids and lipids, offering a versatile and high-value resource for various industrial applications. KEY POINTS: • β-carotene, torulene, and torularhodin levels are highly strain-dependent in yeasts • R. paludigena CBS 6565 achieved high carotenoid-lipid co-production yields • Fed-batch with multiple agri-food residues enabled a sustainable bioprocess design.