Malic acid derived from fossil resources is currently applied in the food and beverage industries with a medium global production capacity. However, in the transition from a fossil-based to a bio-based economy, biotechnologically produced L-malic acid may become an important platform chemical with many new applications, especially in the field of biopolymers. In this review, currently used petrochemical production routes to DL-malic acid are outlined and insights into possible bio-based alternatives for microbial L-malic acid production are provided. Besides ecological reasons, the possibility to produce enantiopure L-malic acid by microbial fermentation is the biggest advantage over chemical synthesis. State-of-the-art and open challenges concerning production host engineering, substrate choice and downstream processing are addressed. With regard to production hosts, a literature overview is given covering the leading natural production strains of Aspergillus, Ustilago and Aureobasidium, as well as Escherichia coli as the most important engineered recombinant host. The utilization of renewable substrates as an alternative to glucose is emphasized in particular as a key aspect for a competitive bio-based production. Out of the alternative substrates discussed in this review, the industrial side-streams crude glycerol and molasses seem to be most promising for large-scale L-malic acid production.

As one of the most abundant renewable resources, rice straw is an attractive lignocellulosic material for animal feeding or for the production of biochemical. An appropriate pre-treatment technique is essential for converting rice straw to rich fodder or biofuel. Based on previous work, Coprinopsis cinerea can grow on rice straw medium and therefore it is useful for the treatment of rice straw. However, little is known regarding its degradation systems and nutrition values. In this study, we firstly found that C. cinerea could grow rapidly on rice straw without any additives by the production of a series of enzymes (laccase, cellulase, and xylanase) and that the microstructure and contents of rice straw changed significantly after being treated by C. cinerea. We propose that a possible underlying mechanism exists in the degradation. Moreover, C. cinerea has a high nutrition value (23.5% crude protein and 22.2% total amino acids). Hence, fermented rice straw with mycelium could be a good animal feedstuff resource instead of expensive forage. The direct usage of C. cinerea treatment is expected to be a practical, cost-effective, and environmental-friendly approach for enhancing the nutritive value and digestibility of rice straw.

Aminotransferases (ATs) received much attention as biocatalyst in the last decades and are utilized for the synthesis of chiral amines, amino alcohols and amino acids. More recently, enzymatic synthesis of enantiopure β‐amino acids is of increasing interest, for their application in peptidomimetics or other drug related compounds. For economic process management it is of major interest to acquire reusable biocatalysts. This is achievable by immobilization of the enzymes. A wide range of immobilization techniques for ATs exists. Two of the most common procedures are entrapment in sol‐gel or calcium‐alginate and alternatively covalent binding to chitosan support. For these traditional immobilization techniques, high amounts of purified enzyme solutions are required which is cost intensive. A major step toward faster and straightforward immobilization is the insertion of affinity tags, like a His‐tag, to the enzyme coding gene. Immobilization on non‐porous polyvinyl magnetic micro beads functionalized with IDA (iminodiacetic acid) leads to a fast and easy one‐step purification and immobilization of a β‐amino acid AT from Variovorax paradoxus. For loading of the functionalized beads, three divalent metal ions (Ni2+, Co2+ and Cu2+) were tested. The best activity was obtained with Ni2+ and almost no activity was detected after loading the beads with Cu2+. The immobilized AT was successfully tested to synthesize several enantiopure β‐amino acids via kinetic resolution. Further examination and characterization of this immobilized AT showed improved stability compared to the crude cell extract. The immobilized AT revealed enhanced pH and thermal stability compared to the crude cell extract. High activity was preserved up to 38 days of storage at 4°C. Moreover, the use of magnetic beads allows an easy and mild recycling of the immobilized enzyme after the reaction, leading to an efficient reuse of the immobilized enzyme, for at least seven cycles.