Yeast is the “magic” ingredient that converts sugary sweet wort into ethanol, carbon dioxide and a host of other compounds that lend flavor and aroma to beer. Without an understanding of yeast’s role in fermentation, beer making truly is a mysterious process.
Until recently, ale yeasts (Saccharomyces cerevisiae) were most commonly used in craft beer production. Known as top fermenting because they rise to the surface during fermentation, creating a foam brewers call krausen, ale yeasts generally ferment at warmer temperatures (between 62˚-75˚f) and create beer with readily noticeable flavors from the byproducts of yeast’s metabolism, respiration and reproduction.
If the beer is improperly fermented these compounds at elevated levels can reveal poor brewing practices and a lack of sanitation. But when the brewer does his job, these flavor compounds are subtle, and when balanced with the residual malt sugars and hop bitterness create good craft beer.
In the past decade, American brewers also have been experimenting with wild yeasts, open fermentation, or the introduction of a variant of Brettanomyces, another genus of yeast that nearly all brewers save Belgian Monks used to avoid. With wild yeast, the brewer has no control over the fermentation byproducts; the yeast that’s in the air spontaneously ferments the beer, resulting in flavor profiles that vary from batch to batch.
