How Alternative Yeasts Turn Cheese Whey into Valuable Bioproducts?

In the bioeconomy arena, scientists are turning waste into valuable resources. A recent study tested two little‑known yeasts – Cutaneotrichosporon curvatus and Papiliotrema laurentii – to see if they could grow on cheap carbon sources like lactose, glycerol and glucose. In shake‑flask experiments with different nitrogen‑to‑carbon ratios, both microbes produced over 20 g/L of biomass, with polysaccharides making up more than 40 % of the dry weight. When C. curvatus was grown on glucose under nitrogen‑limited conditions, it secreted up to 13 g/L of mannitol, a low‑calorie sweetener. Both yeasts also synthesized lipids rich in oleic acid, a promising feedstock for second‑generation biodiesel. The most striking part was the use of secondary cheese whey (SCW), a dairy by‑product usually treated as waste. In a fed‑batch bioreactor, P. laurentii turned SCW into 25.6 g/L of exopolysaccharides, 37.9 g/L of biomass and 4.6 g/L of lipids. These findings illustrate how unconventional yeasts can support a circular economy by converting waste streams into high‑value bioproducts.

The unconventional yeasts studied belong to two phylogenetically distant genera. Cutaneotrichosporon curvatus, a member of the Tremellomycetes class, is known for its ability to metabolize complex sugars and accumulate intracellular polysaccharides. Papiliotrema laurentii, on the other hand, is a Basidiomycota with a flexible metabolism toward reduced carbon sources such as glycerol. In the screening tests, researchers varied the N:C ratio (20, 80, 160 mol/mol) to assess the effect of nitrogen limitation on secondary metabolite production. Under nitrogen‑excess conditions, cell growth was maximized, with biomass exceeding 20 g/L and polysaccharide content above 40 % w/w, indicating a strong capacity for extracellular glucan synthesis. Mannitol production by C. curvatus was favored by a redox‑balanced pathway in the presence of glucose, where the fructose‑6‑phosphate reduction pathway to mannitol is activated. The lipids accumulated by both yeasts show a fatty‑acid composition dominated by oleic acid (≥40 % w/w), typical of long‑chain triglycerides suitable for biodiesel transesterification. The use of secondary cheese whey (SCW) as substrate required a thermal pretreatment to denature proteins and reduce viscosity, making lactose and soluble proteins available. In fed‑batch fermentation, P. laurentii displayed rapid growth kinetics, with a lactose consumption rate of about 0.8 g/L·h, and continuous secretion of exopolysaccharides (EPS) of branched glucan type, with an average molecular weight of 1.2 MDa. NMR analysis of the EPS revealed a predominance of β‑1,3 and β‑1,6 linkages, conferring the biopolymer interesting viscosity and thermal‑stability properties for food and pharmaceutical applications. Moreover, lipid production in SCW reached 4.6 g/L, with a yield of 0.12 g lipids/g substrate, demonstrating process efficiency. These results pave the way for integrated bioprocess plants where dairy waste can be valorized into a product chain: biomass for protein, EPS for additives, mannitol for sweeteners, and lipids for bio‑fuels. Process scalability will require optimization of operational parameters (pH, aeration, feeding regime) and the adoption of product recovery strategies via precipitation, ultrafiltration, or green‑solvent extraction. In summary, the deployment of C. curvatus and P. laurentii offers a promising solution for converting agro‑food residues into bio‑economic resources, with positive environmental and economic impacts.