From yeast screening for suitability as single cell protein to fed-batch cultures.

Purpose: Fed-batch cultures have rarely been used in single cell protein (SCP) research. This work evaluated multiple yeast species for suitability as SCP cultivated using glucose- and sucrose-based substrate and performed in-depth studies of fed-batch SCP cultivation kinetics for selected yeasts, including determination of specific crude nitrogen-to-protein conversion factors.

Methods: SCP was cultivated using fully synthetic media in flask batch or bioreactor fed-batch cultures. Crude nitrogen and nucleic acid content were determined using the Dumas method and fluorescence assay kits, respectively.

Results: C. utilis compared favorably to other yeasts in flask batch cultures in terms of process yield (0.52 ± 0.01 g_x g_s^-1) and crude nitrogen content (10.0 ± 0.5 and 9.9 ± 0.5%_CDW for glucose and sucrose, respectively). This is the first time biomass composition data was reported for SCP cultivated in fed-batch mode. C. utilis crude nitrogen content was consistent across the tested conditions (protein content stabilized around 50%_CDW in fed-batch), while that of the benchmark yeast S. cerevisiae was higher in batch cultures and at the beginning of fed-batch relative to the end (protein content decreased over time and stabilized around 43%_CDW). Total nucleic acid content of the yeasts was similar (6.8%_CDW and 6.3%_CDW, for C. utilis and S. cerevisiae, respectively), with crude nitrogen-to-protein conversion factors of 4.97 and 5.80.

Conclusion: This study demonstrated the suitability of C. utilis as SCP, notably the robustness of its crude nitrogen content (as an indicator of protein content) across batch and fed-batch conditions, compared to that of the benchmark yeast S. cerevisiae.