Microbial fuel cells brew low-alcohol beer

ENGINEERS in Hungary have developed a process which produces low-alcohol beer in a microbial fuel cell (MFC), which simultaneously produces small amounts of electricity.

Sales of low-alcohol and non-alcoholic beer are growing, due to increasing focus from consumers on the health benefits of lower alcohol consumption, as well as the tightening of drink-driving laws. Manufacturers use a variety of physical and biological techniques to lower the alcohol content of beer, including distillation and using special yeast strains. However, these techniques tend to adversely affect the beer’s flavour. The researchers at Corvinus University of Budapest, say that their technique, where the yeast are effectively driven to use up the alcohol, does not have such a great effect on the taste. Beer made this way would therefore be more acceptable to consumers.

Beata Hegyesne-Vecseri and the team began with a standard strain of bottom fermenting lager yeast and a conventional hopped wort, the liquid extracted from malt and hops used for brewing beer, which contains the fermentable sugars. They used dual-chamber MFCs, in which the anode and cathode chambers are separated by a membrane. Graphite plates were used as the electrodes. The researchers tested different sizes of graphite plates and the addition of riboflavin, to act as an extra-cellular mediator. The MFCs ran at 15?C for two weeks, a fairly typical brewing time for beer.

Within the fuel cell, the yeast initially ferment the sugar to produce alcohol. Once the level of sugar falls below a certain level, around 4–5 g/100 ml, the yeast begin to metabolise the alcohol as a carbon source, a process which requires an electron acceptor. Usually this would be oxygen, but in this case, the anode acts as an electrode acceptor instead, thus creating a current, as well as blocking the ethanol formation pathway.

The researchers found that the larger the anode area, the more electricity was produced. This is because the yeast need direct contact with the anode to pass on any produced electrons. The greater the surface area of the anode, the more yeast can attach. A 12 cm² anode produced beer with an ethanol content of around 2.3 V/V%, compared to the control beer, with an ethanol content of approximately 3.5 V/V%. Increasing the anode to 24 cm² reduced the alcohol content to around 2.1 V/V%, and generated a current of 12.5 mA/m², around three times that of the smaller anode.

Riboflavin acts a mediator, effectively shuttling electrons between the yeast and the anode. Therefore, the larger the quantity of riboflavin, the more electrons were transferred, and the greater the current generated, as the yeast no longer needed direct contact with the anode. Adding 100 µM of riboflavin to the MFC with the 24 cm² anode produced beer with an ethanol content of around 1.5%, and generated a current of 86.4 mA/m².

“Although in this study only low alcoholic beer was produced in the MFC, with the proper enlargement of anode surface and/or the addition of the proper amount of electron shuttles, non-alcoholic beer may be achieved as well,” write the researchers.

Extending the fermentation time may also force the yeast to metabolise more of the ethanol.

Food and Bioproducts Processing DOI: 10.1016/j.fbp.2016.01.012