Short answer: yes, yeast is a fungus, but not a mushroom or visible mould: it is a single-celled microorganism that transforms sugars into alcohol and carbon dioxide. In beer, that "breathing" is called fermentation, and it is what turns wort into liquid life.
1) What yeast really is
Yeast belongs to the Fungi Kingdom. Unlike other multicellular fungi, yeast is single-celled. In beer, the key species are Saccharomyces cerevisiae (Ales) and Saccharomyces pastorianus (Lagers). Its metabolism consumes wort sugars and produces alcohol, CO2 and aromatic compounds (esters, phenols) that define sensory character.
2) How it works: alcoholic fermentation
During fermentation, yeast enzymes transform glucose, maltose and maltotriose into ethanol and CO2. Temperature, pitching rate, initial oxygen and nutrients determine profile cleanliness, attenuation (dryness) and the controlled appearance of fruity or spicy aromas.
3) Types of brewing yeast (and the flavours they bring)
Ale (top fermentation)
Ferments at 18-25 °C. Fruity profile (esters), sometimes spicy (phenols). Typical of IPA, Stout, Saison and Belgian Ale.
Lager (bottom fermentation)
Ferments at 8-12 °C. Clean and crisp profile, highlighting malt and hops. Typical of Pils, Helles and Bock.
Brettanomyces
"Wild" yeast that brings funky notes (leather, ripe fruit, barnyard). Found in Wild Ales and blends with lactic bacteria.
Kveik
Traditional Norwegian strains; they ferment very fast and tolerate high temperatures, with a clean fruity profile. Widely used in modern craft.
4) Why does it matter so much in craft beer?
Industry seeks to standardise and neutralise; in the craft world, yeast is identity. Each strain leaves its signature: from a juicy NEIPA (tropical esters) to a dry, spicy Saison or a clean Stout that lets the malt shine.
5) Wild yeasts and mixed fermentation
Beyond Saccharomyces, craft projects explore mixed fermentations with Brettanomyces and bacteria (Lactobacillus, Pediococcus) to create complex Sours (Gueuze, Lambic, Wild Ale). It is a living ecosystem where time and wood (barrels, foeders) add aromatic layers.
6) Control and good practice
- Yeast health: oxygenating the wort, proper nutrition and correct cell count (pitch rate).
- Temperature: fruitier profiles at higher temperatures (Ales); cleaner profiles lower down (Lagers).
- Diacetyl rest: at the end, a slight temperature rise allows yeast to "clean up" buttery notes.
- Cold crash: chilling to sediment and clarify, without "switching off" the beer.
7) A little history (and why it changed the world)
Louis Pasteur demonstrated in the 19th century that fermentation is the work of living microorganisms, founding modern microbiology. Since then, domesticating yeast has been one of the great arts of brewing: selecting strains, growing them, reusing them. In other words: without yeast, there is no beer; without control, there is no style.
8) Conclusion: yes, it is a fungus... and it is the soul
Yeast is a single-celled fungus that makes beer possible. In brewing craft, it is also authorship: it decides aromas, texture and finish. Caring for it, understanding it and respecting it means drinking better.
Frequently asked questions
Is yeast a fungus?
Yes. It is a single-celled fungus from the Fungi Kingdom. In beer, the key species are S. cerevisiae (Ale) and S. pastorianus (Lager).
What does yeast do in beer?
It ferments wort sugars and turns them into alcohol, CO2 and aromatic compounds that define the style.
What is the difference between Ale and Lager yeast?
Ale ferments warm (18-25 °C) and brings a fruity profile; Lager ferments cold (8-12 °C) and is cleaner and drier.
What is a "wild" yeast?
Brettanomyces and other non-domesticated microflora that generate funky profiles. They are used in Wild Ales and mixed fermentation.
Can beer be made without yeast?
Not realistically: all beer depends on yeasts. In spontaneous fermentation, the flora comes from the environment, but yeasts are still involved.
