The “What beer is made from” series is deep dive into beer’s four main ingredients and their impact on its flavour. New to the series? Start here.
Without yeast, we wouldn’t have beer. We’d simply have a glass full of bittersweet liquid.
By fermenting the sugars in that liquid, yeast makes beer.
Yeast is a single-celled microorganism that is a member of the fungus family.
It ferments malt’s sugars into alcohol and carbon dioxide, the booze and bubbles in our beer. (Hence why yeast’s scientific name, Saccharomyces, translates roughly to “sugar fungus“.)
The reaction can also produce a range of aromatic compounds, called esters and phenols, dependent on yeast strain.
Broadly speaking, there are three families of beer, grouped by the type of yeast used: ale, lager, and mixed fermentation.
From within these families, brewers select a strain to brew with, that then determines a beer’s style and influences its flavour.
First up, let’s take a look at ale yeast.
Ale yeast is more commonly known as brewer’s yeast (and more technically known as Saccharomyces cerevisiae). It’s thought to have been the primary yeast used in brewing for thousands of years.
Ale yeast ferments at warmer temperatures, around 16–20°C (60–72°F). These temperatures speed up the reaction, so ales ferment in approximately three to five days.
They also encourage the production not only of alcohol and carbon dioxide but of aromatic compounds called esters and phenols. Esters contribute fruity flavours, while phenols give beers a spicy note.
There are over 1,500 different strains of ale yeast, each varying in their ester and phenol profiles and other fermentation characteristics.
Ales are a broad family, encompassing styles as diverse as IPA, saison and German wheat beer:
- American IPAs are typically fermented with American or English ale yeast strains that produce low levels of fruity esters, which complement the fruit-forward aromas from the style’s American or New World hops.
- Belgian Saisons are fermented with an ale yeast strain that can function at much higher temperatures than normal – up to 32°C or 90°F. This strain produces high levels of phenols that bring a unique black pepper spice and low levels of citrusy esters with aromas of lemon and orange.
- Finally, German wheat beer is fermented with a specific weizen ale yeast strain that produces banana-like esters and clove-like phenols, giving this style its characteristic flavour profile.
Beers in the lager family are fermented with lager yeast, also known as Saccharomyces pastorianus. Lager yeast functions at cooler temperatures than ale yeast (around 7–13°C, 45–55°F).
At these cooler temperatures, the fermentation process takes longer, often 7 to 14 days at a minimum, followed by a further few weeks or months of cold conditioning. (Fun fact: It’s actually this long cold conditioning phase that gives lager its name, as the German word lagern means to store.)
Additionally, at these temperatures, there is minimal ester and phenol production by lager yeast, hence why lagers are said to have a “cleaner” fermentation profile than ales. (There’s no fruit or spice here.)
There is also less diversity amongst lager yeast strains. But that doesn’t mean all lagers are same though!
There are pale lagers like Czech pilsner, dark lagers like the Munich dunkel and strong lagers like doppelbock.
Historically, all kinds of different microbes – from brewer’s yeast, to wild yeast and bacteria – played a part in fermentation. (Mostly because there was no way to avoid them!)
These days, we understand much more about yeast and can now isolate and culture specific strains to brew with, as you’ve just heard. Hence why in most breweries, wild yeast and bacteria are considered contaminants.
But some brewers actually welcome wild microbes into their brews because of the layers of complexity they lend.
Beers in the mixed fermentation family use a combination of brewer’s yeast, plus wild yeast and bacteria, to give sour or funky flavours to a range of styles.
Here are a few different microorganisms that are often involved in mixed fermentation:
- Brettanomyces is a wild yeast that produces complex “funky” and earthy flavours found in Belgian styles like lambic, gueuze and some saisons.
- Lactobacillus is a strain of bacteria that produces lactic acid, giving styles like Berliner weisse and gose a sharp, tangy acidity you may be familiar with from Greek yogurt.
- Finally, Acetobacter is a bacteria that produces acetic acid, also known as vinegar, which adds a unique sharp edge to styles like the Flanders red.
Mixed fermentation often takes place at ambient temperatures and can take from months… to years!
Depending on the desired outcome, wild yeast and bacteria may be introduced at several different points during the brew… but we’ll talk more about these techniques in the ‘How beer is made’ series shortly.
In sum, without yeast, we wouldn’t have beer. As beer is, by definition, a fermented alcoholic beverage.
By selecting a strain of yeast to brew with, brewers determine what family their beer falls into – ale, lager, or mixed fermentation – which then sets expectations for the flavours we’ll find.
But, despite its importance in making beer, yeast isn’t the only contributor to beer’s flavour. Beer’s other ingredients – malt, water and hops – all play their part.
Let’s move on to ingredient #4: hops.
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