Mike Beckett, of Featherston, asks :-

When a yeast is added to fruits or malt it changes the sugars into alcohol. However when added to ginger and sugar the resulting fermentation is non-alcoholic. What is the chemical difference in the fermentation?

Ralph Bungard, a biochemist who is brewer and owner of Three Boys Brewery, responded.

There is no difference in the chemistry of a fermentation between a fruit or malt-based beverage and a ginger beer. The real difference in the alcohol content between ginger beer and malt beer or fruit wine is actually a consequence of how much “true” fermentation occurs.

Yeasts, just like us humans, are good at getting energy out of sugar. In their ideal world, they do this by breaking down sugar in gradual steps in a series of linked biochemical reactions that involve oxygen. The result is a controlled and relatively efficient extraction of energy and the release of the leftovers as carbon dioxide and water.

True fermentation only comes about when there is not enough oxygen to efficiently do all these biochemical steps. In this dire situation, yeast resorts to the biochemistry we call fermentation. Although not as efficient as sugar metabolism in the presence of oxygen, fermentation allows yeast to continue to extract some energy out of sugar - pretty much just enough to survive.

For ginger beer makers, true fermentation only really starts when the ginger beer is bottled and oxygen is excluded when the bottles are capped. In this oxygen-free world, alcohol is produced along with the other bi-product of fermentation, carbon dioxide. As carbon dioxide builds up, the ginger beer becomes carbonated or “fizzy”. However, the pressure build-up of carbon dioxide happens very quickly and the fermentation required to get very high pressure in a bottle is relatively small - so small that even the most explosive of bottle-fermented ginger beers will have less than 1% alcohol before they explode or simply stop fermenting as the pressure becomes so high that even yeast can no longer metabolise.

It turns out that the secret to getting lots of alcohol is to torture the yeast by depriving them of oxygen for as long as possible! Perhaps torture is the wrong word, but brewers and winemakers keep oxygen away from their yeast for prolonged periods allowing the build-up of significant levels of alcohol. Ginger beer makers tend to only truly ferment for that short period in the bottle, allowing only a little alcohol to accumulate.

To say that your home-brewed ginger beer is alcohol free is a little misleading. Even commercially available, traditional fermented ginger beers contain a little alcohol. That also applies to other fermented drinks like Kombucha, the fermented tea drink. In New Zealand, as long as alcohol levels stay below 0.5%, then they can be legally sold alongside non-alcoholic fizz.