Malic acid production by Aspergillus oryzae : the effect of alkaline-earth carbonate buffer identity

Abstract

Malic acid is a specialty chemical that is currently mainly used in the food and beverage industry (market value of $182 million) but has a potential market value of $3.5 billion if used to produce maleic anhydride. The results from the study indicated that the production of malic acid by A. oryzae requires the presence of the alkaline earth metals calcium or magnesium in significant quantities. It was observed that replacing an amount of CaCO3 (240 g.l-1 CaCO3), significantly over that required for pH buffering (21 g.l-1 CaCO3), with an equivalent amount of MgCO3 (192 g.l-1 MgCO3 based on CO32+) results in similar malic acid yields and final malic acid titers. In contrast, a marked reduction in glucose consumption and malic acid production rates were observed. These observations are likely due to an evolutionary response to calcareous soils. These soils tend to immobilize minerals in solid precipitates resulting in nutrient depletion, while the production of malic acid solubilizes these minerals making them bioavailable. The higher rates observed for the calcium vs magnesium runs were likely a result of the stimulatory effect of Ca2+ on the ATP generating pathways as well as several regulatory responses within the fungal physiology. In addition, it was found that A. oryzae was capable of assimilating malic acid from the environment, therefore, minimizing the loss of valuable carbon due to malic acid excretion. This study provides invaluable information required for economically viable malic acid production by A. oryzae which could markedly reduce reliance on the petrochemical industry.