An article in The Chemical Engineer Today magazine from May 2012 describes a new process to dealcoholise beer whilst not stripping the pleasant tastes out at the same time.
Alcohol is made by yeast converting sugars into alcohol, but also makes many other desirable flavour compounds. Low-tech methods of making low alcohol beer include;
* Altering the mash of the malted grains to produce less simple sugars & more long-chain sugars. Yeast can only use simple sugars for metabolism, so the larger sugars are not converted into alcohol.
* Stopping fermentation part-way through by removing the yeast.
More involved methods include letting the beer ferment out normally & then removing the alcohol;
* Distilling the beer - alcohol has a lower boiling point than water, so heating the beer evapourates proportionally more alcohol than water, leaving a lower alcohol beer behind. Heating the beer damages many of the desirable flavour compounds. Reducing the pressure lowers the boiling points of the liquids, so less heat can be used which in turn damages the desirable flavour compounds less.
The new method involves bubbling carbon dioxide through the beer - something already done in many breweries to alter the carbonation level of the beer just before packaging. As the bubbles of pure CO2 rise through the beer, some alcohol vapour is transferred to the bubbles down a concentration gradient according to Dalton's Law. Continually bubbling fresh, pure CO2 from the base of the beer vessel & removing the CO2 from above the liquid's surface, will eventually remove all the alcohol. The advantage of this method is that it can be done at cold temperatures so heat sensitive flavour compounds aren't destroyed.