Bioconversion of Deproteinized Cheese Whey to Metabolites by Understudied <i>Cryptococcus</i>-Related Yeasts: Characterization and Properties of Extracted Polysaccharides

Microbial bioconversion of agro-industrial by-products into high-value-added metabolites such as polysaccharides or lipids serves a dual purpose: mitigating environmental pollution through waste reduction and supporting the development of novel bioproducts. In this study, a non-conventional, poorly studied <i>Cryptococcus albidus</i> strain was initially assessed for its ability to grow on semi-defined media containing lactose, glycerol, or glucose under three distinct nitrogen availability conditions at C/N equal to 20, 80, and 160 mol/mol in shake flask cultures. The goal was to evaluate biomass production and synthesis of valuable metabolites under these conditions. <i>C</i>. <i>albidus</i> demonstrated robust growth on all commercial carbon sources, particularly under nitrogen-rich conditions, producing more than 25.0 g/L of microbial biomass with a high intracellular polysaccharide content (>45%, <i>w</i>/<i>w</i>). Additionally, mannitol production was detected in cultures with glycerol and glucose (9.1 and 13.1 g/L, respectively), especially after nitrogen depletion. Subsequently, <i>C</i>. <i>albidus</i> and a <i>Cutaneotrichosporon curvatus</i> strain were batch-cultivated using pretreated secondary cheese whey (SCW) as a carbon-rich waste substrate. When cultivated on SCW, both yeast strains partially metabolized lactose and produced polysaccharide-rich biomass, dominated by β-glucans (>29% of total biomass), compounds known for their functional and bioactive properties. The cellular polysaccharides (cPS extracted from <i>C</i>. <i>albidus</i> exhibited cytotoxic effects against cancer cells, suggesting their potential use as biological response modifiers. In contrast, the cPS from <i>C</i>. <i>curvatus</i> did not affect cell viability, indicating their promise as ingredients for applications in the food, feed, pharmaceutical, or cosmetic sectors.