Valorization of Miscanthus × giganteus hydrolysates into heme-enriched yeast biomass and extract using xylose-fermenting Saccharomyces cerevisiae

Heme-enriched microbial products represent value-added bioproducts that can be produced through sustainable fermentation processes. Current microbial heme production platforms rely mainly on refined sugars and downstream purification, limiting cost-effectiveness. Here, we developed a lignocellulosic hydrolysate-based bioprocess for producing heme-enriched yeast extract using Saccharomyces cerevisiae. The xylose-fermenting S. cerevisiae SR8 accumulated approximately twofold higher specific intracellular heme on xylose than on glucose, reaching 0.25 ± 0.02 mg/g dry cell weight (DCW) compared with 0.13 ± 0.01 mg/g DCW, consistent with reduced Crabtree repression and enhanced respiratory metabolism. During fed-batch cultivation with Miscanthus × giganteus hydrolysates, residual glucose control suppressed ethanol overflow and promoted biomass formation, resulting in 17.5 ± 0.4 g/L DCW, a specific heme content of 0.20 ± 0.01 mg/g DCW, and a heme titer of 3.4 ± 0.1 mg/L. This performance exceeded that of a mock hydrolysate with identical sugar composition. Autolysis of the resulting biomass produced a lyophilized yeast extract containing 36 ± 2 µg heme/g, exceeding the heme content of the commercial yeast extract tested and supported comparable microbial growth. These results indicate that downstream heme recovery during yeast extract production remains a key bottleneck for improving the overall process efficiency and product heme density. Among the downstream extraction methods tested, high-pressure homogenization most effectively enhanced heme release. Overall, these results demonstrate lignocellulosic hydrolysate valorization into heme-enriched yeast extract and identify downstream recovery as a key target for process optimization.