Recovery and upcycling of residual lactic acid and ammonium from biowaste into yeast single cell protein

The yeast Saccharomyces cerevisiae is a potential candidate for converting waste resources into food-grade single cell protein (SCP). Food waste fermentation broth rich in lactic acid was tested as an alternative substrate source for S. cerevisiae in this study. An electrodialysis (ED) system was developed to recover lactate, acetate and ammonium ions (La^-, Ac^- and NH₄⁺) from the broth. First, we evaluated the system resilience under various volatile fatty acids (VFAs) and pH levels. Initial acetate and lactate concentrations did not affect La^- and Ac^- recovery efficiencies but affected their ion fluxes and competitive ion transport. The migration of La^- was delayed by increasing the initial acetate concentration, while Ac^- migration was not affected by lactate concentrations. Increasing the broth pH reduced competitive H⁺ ions and thus improved the NH₄⁺ recovery efficiency. Further, broth recirculation and pH control were examined to identify potential challenges associated with continuous flow in real-life applications. Recirculation caused a greater loss of recovered Ac^- and NH₄⁺ over the extended period. The pH control decreased VFAs recovery efficiencies because VFAs could not fully dissociate in the dilute, and more VFAs were lost in the concentrate. Finally, the recovery from lactate-rich fermentation broth was used as yeast substrate and contributed to a robust SCP yield of 0.62 g/g-C. Manufacturing one kilogram of SCP costs €6-10 in electricity and €3-6 in raw material. This study provides a better understanding of competitive ion transport during ED and uncovers potential biowaste resources for sustainable SCP production.