Acetate as substrate for l-malic acid production with Aspergillus oryzae DSM 1863

Abstract

Background Microbial malic acid production is currently not able to compete economically with well-established chemical processes using fossil resources. The utilization of inexpensive biomass-based substrates containing acetate could decrease production costs and promote the development of microbial processes. Acetate is a by-product in lignocellulosic hydrolysates and fast pyrolysis products or can be synthesized by acetogens during syngas fermentation. For the fermentation of these substrates, a robust microorganism with a high tolerance for biomass-derived inhibitors is required. Aspergillus oryzae is a suitable candidate due to its high tolerance and broad substrate spectrum. To pave the path towards microbial malic acid production, the potential of acetate as a carbon source for A. oryzae is evaluated in this study. Results A broad acetate concentration range was tested both for growth and malic acid production with A. oryzae. Dry biomass concentration was highest for acetic acid concentrations of 40–55 g/L reaching values of about 1.1 g/L within 48 h. Morphological changes were observed depending on the acetate concentration, yielding a pellet-like morphology with low and a filamentous structure with high substrate concentrations. For malic acid production, 45 g/L acetic acid was ideal, resulting in a product concentration of 8.44 ± 0.42 g/L after 192 h. The addition of 5–15 g/L glucose to acetate medium proved beneficial by lowering the time point of maximum productivity and increasing malic acid yield. The side product spectrum of cultures with acetate, glucose, and cultures containing both substrates was compared, showing differences especially in the amount of oxalic, succinic, and citric acid produced. Furthermore, the presence of CaCO3, a pH regulator used for malate production with glucose, was found to be crucial also for malic acid production with acetate. Conclusions This study evaluates relevant aspects of malic acid production with A. oryzae using acetate as carbon source and demonstrates that it is a suitable substrate for biomass formation and acid synthesis. The insights provided here will be useful to further microbial malic acid production using renewable substrates.

Detoxification of a pyrolytic aqueous condensate from wheat straw for utilization as substrate in Aspergillus oryzae DSM 1863 cultivations

“…The concentration was converted into g/L assuming the density of water [ 2 ]. The only exception was acetate, for which, according to the results of Kövilein et al [ 42 ], no growth limit was determined so far b GC/MS analysis performed by Thünen Institute of Wood Research in Hamburg for PAC that was neither pH adjusted nor filtered…”

“…The concentration was converted into g/L assuming the density of water [ 2 ]. The only exception was acetate, for which, according to the results of Kövilein et al [ 42 ], no growth limit was determined so far…”

“…While publications dealing with microbial acetol conversion are scarce [ 12 ], many microorganisms are known to consume acetate as sole carbon source [ 10 , 39 – 41 ]. For instance, it has recently been shown that the filamentous fungus A. oryzae is able to utilize high acetate concentrations of up to 70 g/L for its growth and l -malic acid production [ 42 ]. Moreover, the fungus is characterized by a high-tolerance against pyrolysis oils and also the impact of various PAC components on its growth and production behavior has been reported [ 19 ].…”

Enhanced l-Malic Acid Production by Aspergillus oryzae DSM 1863 Using Repeated-Batch Cultivation

“…Previously, we reported that A. oryzae DSM 1863 is a promising natural l -malic acid producer with a wide substrate range ( Ochsenreither et al, 2014 ; Dörsam et al, 2017 ; Kövilein et al, 2021 ). For further process understanding and optimization of malic acid production, we compared the three fermentation strategies, batch, fed-batch, and repeated-batch, in parallel shake flask cultivations.…”

“…Moreover, the additional calcium and CO 2 supply via calcium carbonate might also have a positive effect on the malic acid production. In a study that investigated the role of calcium carbonate in the l -malic acid production with A. oryzae from acetate, the authors concluded that the formation of Ca-malate might have been the main function of calcium carbonate ( Kövilein et al, 2021 ), whereas its role as a CO 2 donor and buffering agent was assumed to be less important. However, for malic acid production on glucose, all functions of calcium carbonate could be beneficial.…”