What is the main reason beers finished with Brett become very dry?

Brettanomyces, commonly referred to as Brett, is known for its ability to ferment a wider range of sugars than standard brewer's yeast (Saccharomyces cerevisiae). One of the critical characteristics of Brett is its capacity to metabolize long-chain sugars, specifically those that are complex and can be left over after initial fermentation by regular yeast strains. Most strains of Saccharomyces primarily ferment simple sugars like glucose and fructose, leaving behind some residual sugars in the beer. When Brett is introduced, it continues to ferment these long-chain sugars, leading to a drier final product as these sugars are consumed and converted into alcohol and other byproducts.

This unique mechanism gives Brettanomyces its reputation for producing drier beers, which is a key aspect of many traditional styles, especially in the realms of sour and wild ales. The presence of these residual sugars and the ability of Brett to metabolize them distinguishes its drying effect from sweetness, alcohol production, or carbonation levels, making it an essential aspect of the fermentation characteristics of Brett-finished beers.