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Deutsches Technikmuseum - August

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The Websites of the donation Stiftung Deutsches Technikmuseum Berlin at a glance:

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Exhibit of the Month August 2017

Photo: The rubber stopper has a stem with a horizontal handle on the end. The lower rubber is brownish while the upper part is greyish. The stem has the same colour hue up to shortly before the handle. The rest of the stem and handle are darkly stained.

This rubber stopper was once used to seal fermentation tanks for citric acid production. © SDTB / Photo: C. Kirchner

Fermentation Tank Stopper (Boehringer Ingelheim)

"Acidifier citric acid" is very often found in the list of ingredients on food packaging. If you are thinking that citric fruits must be the source of this additive, which has the desig-nation E-330, you are mistaken.

Citric acid was indeed initially extracted from citric fruits, but starting in 1923 the transition was made to biotechnological fermentation in industrial-scale batches.

The starting material for this is sugar, or, to be more precise, sugar-containing solutions. Molasses, a by-product of sugar production, was mainly used in the process. The conversion of sweet sugar to citric acid is brought about by the fungus Aspergillus niger, which is used extensively in biotechnology. We routinely see the associated black mould on spoiled fruits and vegetables.

Photo: The sporophore can be seen by using a graphic scale of 100 mi-crometre. A long stem ends in a round bulge that is studded with many long structures, on the ends of which the round spores are budding.

Microscopic image of a sporophore of the black mould. © Bruce Watt, University of Maine, Bugwood.org

Aspergillus niger requires a lot of oxygen

In contrast to alcoholic fermentation as with beer, or even to lactic acid fermentation in yogurt, A. niger requires oxygen in order to convert the sugar into the target product citric acid.

A large surface area and thin layer thicknesses of the sugar solution provide enough oxygen from the surrounding air. For this reason, in 1939 Boehringer Ingelheim began using flat tanks for its biotechnological citric acid production.

The signs of usage on the tank stopper displayed here are a good indication of what thicknesses were used. Nowadays the surface process has been replaced by technically complex bioreactors that can guarantee the optimal supply of oxygen.

Boehringer Ingelheim ceased all citric acid production in 1982.