Enhancing microbial oils production from food waste-derived short-chain fatty acids via gas-permeable membrane nitrogen recovery

Short-chain fatty acids-rich fermentation liquids from food waste acidogenic fermentation contain high nitrogen concentrations that hinder lipid accumulation in yeast. Thus, achieving high carbon/nitrogen (C/N) ratio remains a key challenge for the valorization of fermentation liquors for microbial oils production. This work provides the first evidence that nitrogen recovery through a gas-permeable membrane (GPM) contactor effectively increases the C/N of food-waste fermentation liquids, resulting in a significantly enhanced lipid accumulation. GPM was able to recover 97 % of ammoniacal nitrogen in fermentation liquids, doubling the C/N and increasing lipid content by 2-3-fold in shake flask cultivations. Scaling up the process to a lab-scale bioreactor further increased lipid accumulation, reaching up to 30 % w/w lipids (4-fold increase) due to more controlled operational conditions. The study also evaluated the ability of Yarrowia lipolytica to metabolize organic and inorganic nitrogen sources, which is important to understand how the C/N influences microbial oils production. Although Y. lipolytica showed a preference for inorganic nitrogen (i.e. ammonium sulfate), bovine serum albumin was also consumed, though at approximately half the consumption rate. Overall, these findings demonstrate that nitrogen recovery via GPM is an effective strategy to increase the C/N ratio of fermentation liquids enabling its implementation in food waste valorization strategies and promote lipid accumulation in yeast.