Fermentative production of carboxylic acids from lignocellulose sugars

To date, malic acid has been used primarily in the food and beverage industry. It improves the shelf-life of baked products and provides the sour taste of jams and juices. But it also boasts considerable potential as a building block in the chemical industry. Together with succinic and fumaric acid, it belongs to the group of C4 dicarboxylic acids. C4 acids can be converted into 1,4-butandiol (BDO) – an important precursor for further conversion into a wide variety of chemicals, including plastics, polymers and resins; the possible applications for these chemicals range from golf balls to printing inks and cleaning agents.

Fermentative production of malic acid was developed through the collaboration of the Industrial Biotechnology working group at Fraunhofer IGB and the Biotechnological Processes group at Fraunhofer CBP. Fermentation was carried out with the fungus Aspergillus oryzae, which is designated as a harmless food additive according to the GRAS (generally recognized as safe) status of the US Food and Drug Administration (FDA). In addition to glucose, the strain can also utilize the C5 sugar xylose, which is the main component of hemicellulose and thus can be sourced from wood residues.

Initially, the process was optimized at the laboratory scale; it was then established in stirred reactors and finally successfully scaled up to the 1 m³ scale using the substrate xylose for the first time. Downstream processing could be demonstrated using crystallization. In doing so, several kilograms of malic acid were produced that are now available as a sample for application tests.

Xylonic acid as a replacement for gluconic acid

Gluconic acid is an important constituent of foodstuffs, construction materials and dyes [1]. The acid is produced from glucose, which is obtained from plants rich in starch and thus competes with the production of foodstuffs. An alternative to gluconic acid is xylonic acid: on the one hand, this has similar properties and, on the other hand, it can be obtained from plant components containing lignocellulose or from agricultural waste material. The aim was therefore to develop an efficient process for obtaining xylonic acid from xylose.

250 g/L xylonic acid through optimization

The fermentation-based conversion of xylose is conducted using whole cell catalysis (Gluconobacter sp.), with addition of oxygen as a second reactant. In contrast to competing solutions, fermentation with Gluconobacter sp. has the advantage of being a specific, sustainable and efficient conversion. To date, the team of Industrial Biotechnology Group has achieved a xylonic acid concentration of over 250 g/L through optimization – with a yield of over 90 percent. In the subsequent rudimentary purification process, xylonic acid was obtained at a purity of over 80 percent, which is adequate for technical applications.

Scale-up and sample quantities for application-related investigations

The scalability of the process has already been demonstrated at the Fraunhofer Center for Chemical-Biotechnological Processes CBP by the team in the Biotechnological Processes Group with the 100-liter fermentation, a scale-up to 300 liters is planned. We are already making smaller quantities available for investigations for specific applications. For example, xylonic acid can be tested as a substitute for gluconic acid as a curing retardant for concrete or chelating agent.

Literature

[1] Toivari, M.H., Y. Nygard, M. Penttila, L. Ruohonen, and M.G. Wiebe, Microbial D-xylonate production. Applied Microbiology and Biotechnology, 2012. 96(1): p. 1-8.