Without yeast, there’s no beer. The entire yeast fermentation process is what transforms sweet wort into the alcoholic, carbonated drink we love. But what exactly happens during fermentation, and how does it affect flavour? Let’s break down beer fermentation explained properly. What is beer fermentation? If you’ve ever wondered what is beer fermentation, the answer is simple: Fermentation is the metabolic process where yeast converts sugars into alcohol and carbon dioxide. In brewing terms, yeast beer fermentation is the stage where yeast consumes the sugars extracted from malted grains and produces alcohol (ethanol), carbon dioxide (natural carbonation) and flavour and aroma compounds. It’s the second major stage in brewing after mashing and boiling, and it defines whether your beer becomes a crisp lager or a fruity ale. The yeast fermentation process in beer There are hundreds of yeast strains used in brewing, but they fall into three main categories: Ale yeast (top-fermenting) Lager yeast (bottom-fermenting) Wild yeast (used in sour and spontaneous styles) Here, we’ll focus on ale vs lager yeast, as this is where most beer styles originate. Ale vs lager yeast: what’s the difference? Top fermentation (ale yeast) Ale yeast is known as top-fermenting. During fermentation, it rises to the top of the vessel and forms a thick foam. Traditionally, this was highly visible in open fermentation tanks, where brewers would skim the yeast from the surface. Fermentation temperature: 16–24°C Active fermentation time: 3–5 days Conditioning: 1–2 weeks Flavour impact: fruity, spicy, expressive Warmer fermentation temperatures encourage the production of esters (fruity aromas like banana or pear) and phenols (spicy, clove-like notes). Examples include English cask ales (e.g. London Pride), Belgian Tripels (Westmalle Tripel) and German Weissbier (banana and clove notes). Bottom fermentation (lager yeast) Lager yeast behaves differently. It ferments at cooler temperatures and settles at the bottom of the tank. Fermentation temperature: 6–12°C Active fermentation time: 7–14 days Conditioning (“lagering”): several weeks Flavour impact: clean, crisp, subtle The extended cold conditioning phase removes excess yeast-derived flavours, resulting in a smoother, cleaner profile. Examples include the Czech pilsner, Pilsner Urquell, Sapporo and classic German beer. This is why lagers are often described as “clean” as they have minimal yeast character compared to ales. Why temperature matters more than “top” or “bottom” More important than the presentation of foam is the temperature at which a beer ferments at, and for that reason a new classification of warm and cold fermented might be more relevant. It's the temperature that will have a more significant impact on the characteristics of a beer. Ale yeast likes to work in warmer temperatures. For a classic ale, that might be between 16-24ºC, with 18-20ºC being the common temperature, and it might take 3-5 days to convert the grain sugars into the intended level of alcohol. It then needs a week or two to condition. Make it colder and the yeast will be sluggish and less effective. Lager yeast prefers the cold, working best between 6-12ºC, and will ferment beer in 7-14 days, then requiring several more weeks to ‘lager’ or mature. This conditioning period is when the flavour gets cleaner as the yeast will remove some of the aromatic character naturally given out during fermentation. If the yeast is too warm then it will produce many more negative flavours. Understanding the yeast fermentation process helps you “read” a beer before you even open it. How yeast shapes flavour The flavour impact is that typically the warmer the fermentation the more yeast characteristic will be in the beer. A traditional lager, like Pilsner Urquell, Sapporo or Fourpure Pils, presents very little yeast flavour or aroma. An English Ale, especially beers like Fuller’s London Pride produce a spicy fruitiness like marmalade. Though not all ales will have aroma or flavour from the yeast, take Pohjala Must Kuld for instance. And then there are the cool-fermented ales, like Kölsch, which also then undergo an extended cold conditioning like a lager, which gives it a cleaner flavour. Or there’s California Common which is warm fermented with a lager yeast (one specially evolved for warmer temperatures) and gives out some fruitier yeast flavours. For the most aromatic expressions of ale yeast, we can look to Belgian ales or wheat beers, which are ales which ferment at a warmer temperature (maybe 25ºC) and that produces the fruity and spicy aromas and flavours (known as esters and phenols). Westmalle Tripel is wonderfully fruity, with hints of banana, then it has a peppery finish; Leffe Blonde is much more phenolic and spicy in its flavour; a classic German Weissbier like Schneider Tap 7 is smooth with banana-like fruit, while a Belgian white beer is more spicy and peppery, with those yeast aromas enhanced by the addition of coriander and orange peel. The yeast beer fermentation stage is where beer truly becomes beer. It determines: Alcohol level Carbonation Clean vs fruity flavour profile Ale vs lager classification Understanding ale vs lager yeast and the wider yeast fermentation process allows you to predict how a beer will taste before you even pour it. Next time you pick up a bottle, check whether it’s top or bottom fermented. You’ll already know what to expect.