Carboxylic Acid Production

Carboxylic acids are central compounds in cellular metabolism, and in the carbon cycle in nature. Carbon dioxide is captured from the atmosphere and enters living cells through the formation of carboxylic groups, and it is released from living cells by the decarboxylation of carboxylic acids. The aerobic extraction of free energy from sugars in cellular respiration hinges on the ingeniously designed tricarboxylic acid cycle involving a range of carboxylic acids, and the reactivity of the carboxylic group with aminoor hydroxyl-groups enables the formation of peptide and ester bonds in macromolecules. The functionality of the carboxylic group is, not surprisingly, also of huge importance in the industrial world for a wide range of applications. The loosely bound hydrogen provides weak acid functionality, much desired for food industry applications in preservatives and flavor compounds, and citric acid is one of our oldest and quantitatively largest industrial fermentation products. The presence of two carboxylic groups, or a combination of one carboxylic group and another functional group, make the compounds interesting building blocks for polymer production. Several carboxylic acids, including e.g., lactic, succinic, 3-hydroxypropionic and itaconic acids, have been recognized as suitable platform chemicals for a foreseen growing non-fossil based industry. Economic margins are, however, narrow when competing with petroleum-based products. Microbial production strains, fermentation technology and—not least—downstream processing, all need to be improved to enable a viable commercial production and speed up the transition towards non-fossil-based production of carboxylic acids. This special issue is devoted to the microbial production of carboxylic acids. Several reviews give updates on the production of some of the most interesting acids. Succinic acid is one of the products for which bio-based production has increased most rapidly in recent years. Nghiem et al. [1] give an overview of both the microbial production and the state of commercialization of this acid. Malic acid, a close neighbor to succinic acid in the tricarboxylic acid cycle, is treated by West [2], with a special focus on use of biofuel related co-products as substrates (e.g., corn stover, straw, and glycerol, but also syngas). Interestingly, one way to produce malic acid is via poly β-L-malic acid, which accumulates intracellularly in Aspergillus pullulans. 2-oxopropanoic acid is the proper (but rarely used) name for pyruvic acid. Many metabolites are at metabolic crossroads, but few are more centrally localized in terms of major pathways than pyruvate. Maleki and Eiteman [3] give a detailed account of the involved pathways, and review engineering and process strategies for pyruvate production. Propionic acid is a fermentation product which has received less attention than the previously mentioned acids. Gonzalez-Garcia et al. [4] cover this acid in their review, which includes an in-depth analysis of the energetics of various possible pathways towards propionic acid. A broader review by Murali et al. [5], covering the production of several acids, such as acetic, butyric, lactic, and propionic acid using microbial consortia, is also included in this issue. The choice of substrate is clearly crucial in bio-based production, primarily for economic reasons but also for environmental reasons. Couvreur et al. [6] report work on response surface optimization of growth media for Lactobacillus reuteri based on agro-industrial by-product streams, and the subsequent