Abstract
l-Malic acid is a C4-dicarboxylic acid and a potential key building block for a bio-based economy. At present, malic acid is synthesized petrochemically and its major market is the food and beverages industry. In future, malic acid might also serve as a building block for biopolymers or even replace the commodity chemical maleic anhydride. For a sustainable production of l-malic acid from renewable resources, the microbial synthesis by the mold Aspergillus oryzae is one possible route. As CO2 fixation is involved in the biosynthesis, high yields are possible, and at the same time greenhouse gases can be reduced. In order to enhance the production potential of the wild-type strain Aspergillus oryzae DSM 1863, process characteristics were studied in shake flasks, comparing batch, fed-batch, and repeated-batch cultivations. In the batch process, a prolonged cultivation time led to malic acid consumption. Keeping carbon source concentration on a high level by pulsed feeding could prolong cell viability and cultivation time, however, did not result in significant higher product levels. In contrast, continuous malic acid production could be achieved over six exchange cycles and a total fermentation time of 19 days in repeated-batch cultivations. Up to 178 g/L l-malic acid was produced. The maximum productivity (0.90 ± 0.05 g/L/h) achieved in the repeated-batch cultivation had more than doubled than that achieved in the batch process and also the average productivity (0.42 ± 0.03 g/L/h for five exchange cycles and 16 days) was increased considerably. Further repeated-batch experiments confirmed a positive effect of regular calcium carbonate additions on pH stability and malic acid synthesis. Besides calcium carbonate, nitrogen supplementation proved to be essential for the prolonged malic acid production in repeated-batch. As prolonged malic acid production was only observed in cultivations with product removal, product inhibition seems to be the major limiting factor for malic acid production by the wild-type strain. This study provides a systematic comparison of different process strategies under consideration of major influencing factors and thereby delivers important insights into natural l-malic acid production.
Highlights
Over the past decades, concerns about climate change and oil depletion led to a growing interest in the idea of a bio-based economy, in which fuels, chemicals, and pharmaceuticals are produced from renewable resources via eco-friendly synthesis routes
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)
The L-malic acid production in batch cultivations can be divided into three phases (Batch A1-3 in Figures 1A–C): the fungal growth phase, the major production phase, and a final malic acid reduction phase
Summary
Concerns about climate change and oil depletion led to a growing interest in the idea of a bio-based economy, in which fuels, chemicals, and pharmaceuticals are produced from renewable resources via eco-friendly synthesis routes. As a dicarboxylic acid with an additional hydroxy group, malic acid has several established applications in the food and beverages industry, as a buffer and chelating agent, and in the pharmaceutical industry. It might be applied as a building block for homo- and heteropolymers, for biomedical applications, such as drug carriers, and as a component in low transition temperature mixtures for eco-friendly extraction purposes. Enantiopure L-malic acid (physiological form) can be exclusively produced enzymatically or by microbial synthesis Advantageous in this case is the classification as “nature identical” (NATRUE lable, https://www.natrue.org/), which results in a higher sales value and is needed for pharmaceutical applications. Homo- and heteropolymers derived from L-, D-, or DL-malic acid might have different characteristics since the racemate and both enantiomers vary in melting and boiling point (Fiume, 2001)
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