Upcycling waste organic acids and nitrogen into single cell protein via brewer's yeast

The food crisis due to the global population boom requires agriculture revolutions and a transition to sustainable, affordable sources of food proteins. Single cell protein (SCP) derived from microorganisms such as brewer's yeast is one of the best possible options to meet the protein challenges. Utilization of carbon and nitrogen recovered from waste streams, instead of sugar-rich substrate, for yeast-driving SCP production could improve the overall sustainability from both economic and environmental points of view. The lack of a comprehensive understanding of the process such as the interactions between different organic acids/ions and SCP production by yeast (e.g., Saccharomyces cerevisiae) has placed a major impediment to the valorization effort. This study tackles this challenge by exploring the impact of various organic acids, nitrogen and ionic species on SCP production by S. cerevisiae. It was found that the SCP production increased with acetate concentrations (2.6–20 g-COD/L). A maximal protein concentration of 0.75 g/L was harvested using 20 g-COD/L acetate. Intermittent acid addition for pH neutralization further enhanced SCP production by 67% at 0.94 g/L. Ammonia was the best inorganic nitrogen source for S. cerevisiae among nitrate and nitrite. S. cerevisiae was not able to utilize lactate, propionate and butyrate alone for growth, but it can catabolize lactate along with acetate. The addition of oleate and linoleate resulted in a significant increase in the protein content of biomass by 1.8- and 1.6-fold, respectively. S. cerevisiae using acetate required eight times fewer K⁺ than Na⁺. Typical anions including Cl⁻, NO₃⁻ and SO₄²⁻ (20–500 mM) were not found harmful for SCP synthesis. The results help to fill the knowledge gap in yeast-driving waste-to-protein conversion process, bring high impacts and positive implications for waste valorization and future feed and food supply.