Topics Food and drink

How does yeast make beer?

Yeast makes beer by eating sugar. Brewers soak malted grain, usually barley, in hot water to pull out sugars, boil that sweet liquid with hops, and cool it. Then they add yeast, a single-celled fungus, which feeds on the sugars and releases two things: alcohol and carbon dioxide. The alcohol stays in the beer, and the carbon dioxide gives it fizz.

What makes it interesting is how much the yeast decides. Different strains produce different flavors, from fruity and spicy ales to crisp, clean lagers. Temperature, time and the amount of oxygen all change what the yeast does. Yeast was used for thousands of years before anyone knew it was alive. Scientists worked out its role in the nineteenth century.

An episode on this would walk through the process from grain to glass, and explain why ales and lagers differ. It would also cover where the flavors come from and why some beers are sour. bre's hosts are AI, so they can make mistakes, and you can press Talk to ask a question about any step.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Grain to sugar: malting and mashingBarley is sprouted, dried and soaked in hot water so its enzymes turn starch into sugars the yeast can eat.
  2. Boiling with hopsThe sweet liquid, called wort, is boiled with hops for bitterness, aroma and some preservation, then cooled.
  3. Meet the yeastWhat yeast is, how it was discovered to be alive, and why it makes alcohol and carbon dioxide when it ferments.
  4. Ales versus lagersAle yeasts and lager yeasts work at different temperatures and leave different flavors behind.
  5. Where the flavor comes fromYeast makes more than alcohol. It also produces aroma compounds that can taste fruity, spicy or clean.
  6. Fizz, conditioning and packagingHow the carbon dioxide ends up in the beer, and what happens during aging before it is bottled or kegged.
  7. Why some beers are sourWild yeasts and bacteria can change a beer's character, and brewers handle them very carefully.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Cal. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Here's a picture to start with. A giant pot of warm, sweet, grainy liquid, and you drop in a pinch of living fungus. A few days later it's beer. So what actually happened in there?

  2. CalAI host

    Dinner happened. The yeast ate. And then, because every meal has a byproduct, it gave off alcohol and carbon dioxide, which is basically the whole drink.

  3. breAI host

    So the alcohol is, what, yeast waste?

  4. CalAI host

    Kind of, yeah. Hold on, how does that actually work? It isn't being polite. It's getting energy out of the sugar without much oxygen, and alcohol and CO2 are what's left over.

  5. breAI host

    Okay, here's the part nobody tells you. The brewer doesn't really make beer. They make the sugary soup and then manage the yeast.

  6. CalAI host

    Which is a lot like running a tiny restaurant where the customers are alive, picky about temperature, and billions strong.

  7. breAI host

    And the grain doesn't start out sugary, so that's our first stop. How do you get sugar out of barley?

  8. CalAI host

    Great question. It starts with a seed that thinks it's about to grow, and we trick it.

Questions people also ask

Does yeast make the alcohol in beer?
Yes. Yeast consumes sugars from the malted grain and, in the low-oxygen conditions of fermentation, produces ethanol and carbon dioxide. Without yeast, the sweet liquid would simply stay a sugary, unfermented grain drink with no alcohol.
What is the difference between ale yeast and lager yeast?
They are different species or strains that work best at different temperatures. Ale yeasts typically ferment warmer and often give fruity or spicy flavors. Lager yeasts ferment cooler and usually give a cleaner, crisper taste.
Why does beer have bubbles?
The bubbles are carbon dioxide, which yeast produces during fermentation. Some stays dissolved in the beer, and brewers can also add more by carbonating it. When you open the container, the pressure drops and the gas comes out as bubbles.
Is the yeast still in beer when you drink it?
It depends. Most commercial beers are filtered or settled so little yeast remains. Some styles, like certain wheat beers, and bottle-conditioned beers are left cloudy on purpose and still contain yeast, which adds flavor and texture.

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bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.