Topics Food and drink

How does fermentation work?

Fermentation is what happens when microbes such as yeast and bacteria break down sugars for energy without using oxygen, and leave useful leftovers behind. Depending on the microbe, those leftovers can be alcohol, carbon dioxide gas, or acids such as lactic acid. That is the chemistry behind beer, wine, bread, yogurt, sauerkraut, kimchi and many other foods.

What makes it interesting is that people have used it for thousands of years without knowing microbes existed. The same basic move, microbes eating sugar, gives you very different foods depending on who is eating, what they are eating, and the conditions. Fermentation also changes flavor, texture and how long food keeps, though it can go wrong, and not every ferment is safe just because it is sour.

An episode on bre, whose hosts are AI and can be wrong, would walk through the sugar-to-products chemistry, the main kinds of microbes, why acid and alcohol hold back other organisms, and how the same idea shows up across cuisines. You can press Talk and ask a question mid-episode, like what separates fermenting from rotting.

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. Microbes eating sugarThe core idea: yeast and bacteria break down sugars to get energy, and what they release is what we taste, drink or eat.
  2. Three main productsAlcohol and carbon dioxide from yeast, lactic acid from lactic acid bacteria, and acetic acid in vinegar-making, which also needs oxygen at a later step.
  3. Why no oxygen is neededFermentation is a way to get some energy from sugar without oxygen. It yields less energy than breathing with oxygen does, which is why microbes work so hard at it.
  4. Fermented foods around the worldBread, beer, wine, yogurt, cheese, kimchi, soy sauce, tempeh: different ingredients and microbes, one shared principle.
  5. Fermenting versus spoilingAcid, alcohol and salt can hold back unwanted microbes, but ferments can fail, so conditions and preparation matter.
  6. Before anyone saw a microbePeople fermented for millennia before microscopes. How they learned is largely lost to history, and the episode says where that is uncertain.

How the episode might open

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

  1. breAI host

    Picture a jar of shredded cabbage with some salt, left on the counter for a few days. Nobody heated it. It comes out sour and bubbly. Something changed it, and that something is alive.

  2. ArloAI host

    Microbes. Bacteria, mostly. They eat the sugar. That's it. Tiny things eating sugar.

  3. breAI host

    That's the heart of it. And what they leave behind is the point. In the cabbage it's lactic acid, which is the sour. With yeast in dough or beer, it's carbon dioxide and alcohol.

  4. ArloAI host

    So alcohol is waste. Flatly: yeast's trash is our drink.

  5. breAI host

    Fair way to put it. And at high levels, alcohol holds back a lot of other microbes, including some yeast. Acid does something similar to bacteria.

  6. ArloAI host

    Who counted that? How do we know it holds them back?

  7. breAI host

    Microbiologists have measured it in labs for well over a century. The exact limits vary by microbe and conditions, so we'll stay with the general pattern.

  8. ArloAI host

    Fine. And ancient people did this with no microscope. How'd they know it worked?

  9. breAI host

    Probably trial and error. I don't know the details, and I don't think anyone fully does.

Questions people also ask

What is fermentation in simple terms?
It is microbes, usually yeast or bacteria, breaking down sugars for energy without needing oxygen. The leftovers can be alcohol, carbon dioxide or acids. Those byproducts give fermented foods their flavor, fizz or sourness, and can change how long the food lasts.
Is fermentation the same as rotting?
No, though both involve microbes. Fermentation relies on particular microbes and conditions that favor them, such as salt, acid or alcohol. Spoilage is unwanted microbes taking over. A ferment can still go wrong, so smell and appearance alone are not always a reliable guide.
Does fermentation need oxygen?
Not for the core process. Fermentation lets microbes get some energy from sugar without oxygen. Some ferments are kept sealed or submerged for that reason. Vinegar is a partial exception, since the bacteria that turn alcohol into acetic acid do need oxygen.
What foods are made by fermentation?
Many common ones: bread, beer, wine, yogurt, cheese, sauerkraut, kimchi, soy sauce, miso, tempeh, vinegar and some chocolate and coffee processing. The microbes and ingredients differ, but each relies on microbes changing sugars or other compounds in the food.

Related topics

More: all 300 topics, food and drink, or the longer reads on /learn.

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.