Why Fermentation Changes Flavor

Biotransformation helps explain why microbes do more than just turn sugar into alcohol or acid.

• Fermentation
• Brewing
• Food Science

It’s easy to think of fermentation as simply the conversion of sugars into other compounds like alcohol and vinegar (and $CO_2$), but if that were the case, lots of fermented drinks would taste exactly the same, or many more of the flavors present in the pre-fermentation form would carry through the post-fermentation form.

The secret to why the flavors change is something called “biotransformation”, a process that is only partially understood — we understand what happens, and to a certain extent how it might happen, but it’s still an active area of research (for example, this article from White Labs).

However, even without knowing exactly why and how it works, it’s been leveraged to develop some incredible “Juicy IPAs” (via “hop biotransformation”, where the flavor- and aroma-producing compounds in the hops that haven’t already been converted into bitter compounds get converted into new and interesting flavor components and giving rise to the Hazy/Juicy IPA’s signature thick, cloudy appearance).

This is also what makes Kombucha and Ginger Beer (alcoholic and non-alcoholic) interesting. Both beverages are made by adding significant amounts of sugar to a tea (term applied loosely in the case of the ginger root) made from the tea leaves or ginger root. The “tea” itself is primarily to extract the flavor characteristics from the leaves/rhizome, but the sugar is necessary for fermentation because tea leaves and ginger root both have negligible amounts of sugar; certainly not enough to ferment.

So if the fermentable part of both beverages is provided by added sugar, why not just ferment the sugar separately and then add tea or ginger? The answer is biotransformation.

By adding sugar to the tea and fermenting it all together, the bacteria and yeast have a chance not only to turn the dissolved sugars into acid and alcohol, but also to simultaneously convert the flavor compounds into other flavor compounds that end up being more pleasant to drink.

Furthermore, yeast and bacteria go dormant once they’ve exhausted their food supply (sugar), so adding flavor compounds after fermentation has occurred would not initiate biotransformation.