Fully integrated biotransformation of fumaric acid by permeabilized baker's yeast cells with in situ separation of L-malic acid using ultrafiltration, acidification and electrodialysis

Engineering strategies for producing medium-long chain dicarboxylic acids in oleaginous yeasts

Medium-long chain dicarboxylic acids (DCAs, C ≥ 6) are essential chemical raw materials, with wide applications in the chemical, pharmaceutical, material and food industries. However, the traditional chemical synthesis methods cause environmental pollution and are not in line with goals of sustainable development. With the development of synthetic biology, high-value-added DCAs can be biosynthesized from hydrophobic substrates (HSs) using suitable microorganisms. This review first summarizes the biosynthetic pathway of DCAs in oleaginous yeasts and then emphasizes the related engineering strategies for increasing the product yield, including promoter, enzyme, pathway, cell, fermentation, and downstream engineering. In addition, the challenges and development trends in the biosynthesis of DCAs are discussed, in light of the current progress, challenges, and trends in this field. Finally, guidelines for future research are proposed. Overall, this review systematically summarizes recent engineering strategies for DCAs production in oleaginous yeasts and offers valuable insights for future DCAs biosynthesis.

Valorization of a Pyrolytic Aqueous Condensate and Its Main Components for L-Malic Acid Production with Aspergillus oryzae DSM 1863

Pyrolytic aqueous condensate (PAC) might serve as a cost-effective substrate for microbial malic acid production, as it is an unused side stream of the fast pyrolysis of lignocellulosic biomass that contains acetol and acetate as potential carbon sources. In the present study, shake flask cultures were performed to evaluate the suitability of acetol and its combination with acetate as substrates for growth and L-malate production with the filamentous fungus Aspergillus oryzae. Acetol concentrations of up to 40 g/L were shown to be utilized for fungal growth. In combination with acetate, co-metabolization of both substrates for biomass and malate formation was observed, although the maximum tolerated acetol concentration decreased to 20 g/L. Furthermore, malate production on PAC detoxified by a combination of rotary evaporation, overliming and activated carbon treatment was studied. In shake flasks, cultivation using 100% PAC resulted in the production of 3.37 ± 0.61 g/L malate, which was considerably improved by pH adjustment up to 9.77 ± 0.55 g/L. A successful scale-up to 0.5-L bioreactors was conducted, achieving comparable yields and productivities to the shake flask cultures. Accordingly, fungal malate production using PAC was successfully demonstrated, paving the way for a bio-based production of the acid.

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

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.

Reactive crystallization: a review

Reactive crystallization is not new, but there has been recent growth in its use as a means of improving performance and sustainability of industrial processes.

Malic Acid Carbon Dots: From Super-resolution Live-Cell Imaging to Highly Efficient Separation

As-synthesized malic acid carbon dots are found to possess photoblinking properties that are outstanding and superior compared to those of conventional dyes. Considering their excellent biocompatibility, malic acid carbon dots are suitable for super-resolution fluorescence localization microscopy under a variety of conditions, as we demonstrate in fixed and live trout gill epithelial cells. In addition, during imaging experiments, the so-called "excitation wavelength-dependent" emission was not observed for individual as-made malic acid carbon dots, which motivated us to develop a time-saving and high-throughput separation technique to isolate malic acid carbon dots into fractions of different particle size distributions using C₁₈ reversed-phase silica gel column chromatography. This post-treatment allowed us to determine how particle size distribution influences the optical properties of malic acid carbon dot fractions, that is, optical band gap energies and photoluminescence behaviors.