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CACD – that stands for Citric Acid Cool Device. It wasn’t just the device that was ‘cool’, but also the whole competition and the Mittweida team. In mid-July, the Institute of Food and Bioprocess Engineering at TU Dresden hosted the 99€ Bioreactor Competition for the 10th time, welcoming 11 teams to the Saxon capital.
Making citrate on a budget of 99 euros
The aim of this year’s competition was to produce citrate using the yeast Yarrowia lipolytica in a homemade bioreactor – whilst staying within a budget of 99 euros. Citrate is closely related, in industrial terms, to citric acid, which was traditionally obtained from the processing of citrus fruits; today, its large-scale production is predominantly carried out by means of fermentation using the mould Aspergillus niger. However, this process has a number of drawbacks, such as an unsatisfactory yield following purification and the accumulation of significant quantities of solid and liquid waste; for this reason, the yeast Yarrowia lipolytica is increasingly becoming the focus of research, as it offers the potential to produce citrate more efficiently [1]. Citrate is a by-product of various energy-producing metabolic pathways, such as glycolysis and the citric acid cycle [2].
Cool kit calls for a creative team
Mittweida University of Applied Sciences was represented this year by Master’s students Anna Naruschat, Jenny L. Woide and Vanessa Bergler, as well as Bachelor’s students Emilia Korb, Josephine Speer, Julian Spies and Pia Lange, under the supervision of René Kretschmer from the Biotechnology Department.
A plan to build a loop reactor quickly took shape. Loop reactors are characterised by a continuously circulating flow, in which the reaction medium remains constantly in motion. Gas-driven loop reactors in particular, such as those used in CACD, are considered energy-efficient and have the advantage of being able to supply oxygen-dependent organisms, such as Yarrowia lipolytica, with a reliable and sustained supply of oxygen [3], [4].
Conventional trickle stones and other equipment from the aquarium trade were used to build the reactor. The planned pH control system, comprising a pH electrode and a pump, failed shortly before the competition began for technical reasons, whereupon the team responded with great ingenuity: A syringe containing diluted caustic soda was quickly glued to a shelf, and the flow rate was adjusted using a hose clamp so that the drip rate would be sufficient to stabilise the pH level between 4.5 and 5.5. The reactor itself was housed in a repurposed beer keg to prevent any potential water damage. This unusual ‘packaging’ ultimately generated a great deal of excitement, as the interior of the reactor remained a mystery to the competing teams throughout the entire 24-hour cultivation period.
With a yield of 0.616 g and a low cost of €66.86, HS Mittweida ultimately secured 5th place. The winners were the home team from TU Dresden, with an impressive yield of 2.88 g of citrate.
We offer our warmest congratulations on this success!
References
[1] Anna Żywicka et al, ‘Significant enhancement of citric acid production by Yarrowia lipolytica immobilised in a bacterial cellulose-based carrier’, *Journal of Biotechnology*, Volume 321, 2020, pp. 13–22, ISSN 0168-1656, doi.org/10.1016/j.jbiotec.2020.06.014.
[2] Książek, E., 2023. Citric Acid: Properties, Microbial Production, and Applications in Industries. Molecules 29, 22. doi.org/10.3390/molecules29010022
[3] Reschetilowski, W. (Ed.), 2020. Handbook of Chemical Reactors: Chemical Reaction Engineering: Theoretical and Practical Fundamentals, Chemical Reaction Apparatus in Theory and Practice, Springer Reference Natural Sciences. Springer Berlin Heidelberg, Berlin, Heidelberg. doi.org/10.1007/978-3-662-56434-9
[4] Behr, A., Agar, D.W., Vorholt, A.J., Jörissen, J., 2025. Introduction to Technical Chemistry. Springer Berlin Heidelberg, Berlin, Heidelberg. doi.org/10.1007/978-3-662-70148-5
