Characterization of spent yeast streams from precision fermentations driven by genetically engineered yeasts: valorization potential for a circular bioeconomy

Currently, valuable biomolecules are sustainably produced through fermentation using genetically modified microorganisms. However, this industry also generates several waste streams, such as spent yeast (SY). To close the sustainability loop in bioproduction, it is essential to define valorization strategies for such waste streams. Hence, this study aimed to investigate the compositional differences among these novel SYs, intending to pave the way for devising highly effective and beneficial valorization strategies for these streams. In this work, SYs derived from the production of beer (SBY) and six biomolecules [β-farnesene (Fene), rebaudioside M (Reb M), manool (MN), patchouli (PAT), sandalwood (SDW), and sclareol (SCL)] through fermentations driven by engineered yeasts were characterized. The results showed that SY from Fene, MN, and PAT production have similar concentrations of protein (35.9–44% dw), minerals, and amino acids, being compatible with bulking and use as nitrogen sources. Those derived from Reb M production showed significantly higher levels of lipids (21 ± 2.6% dw) compared to all the other SYs (1.7–10.8% dw), whereas the ones derived from SCL production showed significantly higher levels of inorganic matter (45%) compared with those found in the other SYs (1.2–11.3% dw). The results showed that assessing the extent of compositional variation among different SYs derived from precision fermentations represents a primary step toward developing efficient valorization or biorefining strategies.