Utilization of apple pomace as a novel substrate for citric acid production by Yarrowia lipolytica

Apple pomace (AP), a major byproduct of apple juice and cider production, poses significant environmental challenges owing to its high organic load, rapid biodegradability, and disposal difficulties. However, its sustainable valorization can mitigate environmental impacts and contribute to circular economic approaches. This study was designed to address the lack of research on the use of AP as a substrate for CA production by Yarrowia lipolytica in the existing literature. For this purpose, AP was pretreated to increase the concentration of fermentable sugars, and fermentation media containing different concentrations of AP were prepared. Y. lipolytica NRRL Y-1094 was selected to produce CA through submerged fermentation. The process conditions were optimized using response surface methodology (RSM), considering the following factors: concentration of AP (50, 75, and 100 g/L), initial pH level (4.5, 5.5, and 6.5), and fermentation time (4, 6, and 8 days). The optimum conditions were determined to be 100 g/L AP, pH 6.5, and 8 days, resulting in a CA concentration of 38.96 ± 1.65 g/L. Under these conditions, the concentration of isocitric acid (ICA), a by-product of the process, was found to be 10.07 ± 0.07 g/L. Furthermore, a remarkably high biomass of 31.49 ± 1.98 g/L was achieved. These results indicated that AP is a viable alternative to commercial sugar sources for CA production using Y. lipolytica.