Understanding the Role of pH Regulation and Neutralizing Agents in Organic Acid Production and Growth of Aspergillus oryzae.

Abstract

With growing interest in the valorization of renewable resources, the microbial production of organic acids using Aspergillus oryzae has gained attention. However, process parameters such as pH and neutralizing agents remain insufficiently understood. We investigated the effect of pH and different neutralizers on the production of malic, succinic, fumaric, pyruvic and citric acid and fungal growth using offline sampling and online monitoring of respiratory activity. Neutralizers included NaOH, Na₂CO₃, KOH, Mg(OH)₂, Ca(OH)₂, and CaCO₃ and were compared to the conventional use of excess CaCO₃. Using Na₂CO₃, malic acid reached 33.18 g L^-1 with a yield of 0.54 g g^-1 from glucose and a productivity of 0.14 g L^-1 h^-1. KOH enabled the highest citric acid concentration of 9.12 g L^-1 with 0.18 g g^-1 and 0.04 g L^-1 h^-1. At controlled pH with NaOH, pH 7.00 resulted in 39.14 g L^-1 malic acid with 0.60 g g^-1 and 0.17 g L^-1 h^-1. Citric acid peaked at pH 5.50 with 20.18 g L^-1, 0.36 g g^-1 and 0.09 g L^-1 h^-1. Under dynamic pH conditions, acidification suppressed the production of most acids, while citric acid was produced exclusively at low pH. Off-gas analysis at controlled pH revealed increased respiratory activity under acidic conditions, indicating active pH homeostasis. Furthermore, we detected a nutrient limitation via respiration monitoring in a medium widely used for decades, uncovering untapped optimization potential in previously published studies. These findings highlight the importance of pH and neutralizer selection for improving microbial organic acid production.