The Metabolic Process of Fermentation

Learn the metabolic process underpinning the specific cycles that create fermented food and drink.

| Fall 2019

Shutterstock/Megan Betteridge

Take a stroll through any supermarket, and you’ll find a variety of fermented products. Wine and beer are fermented from fruits and grains, respectively. Yogurt, sour cream, and some cheeses are fermented dairy products. Chocolate is made from fermented cacao beans. Sauerkraut and kimchi are fermented cabbage. The little bubbles inside leavened bread are the result of fermentation. So is the slight tang in sourdough. The acetic acid in vinegar comes from fermentation. And fermentation isn’t just limited to the foods we eat; it takes place all around us. Odds are good that the gas in your car contains ethanol — the product of industrial fermentation. Even your own muscle cells will start to ferment if you work them hard enough.

People who ferment foods or run industrial fermentations often loosely define fermentation as a process in which microorganisms — such as yeast or bacteria — break down organic matter in a sealed container. This leaves out fermentations that occur in the wild and in multicellular organisms — especially the muscles of vertebrates — but covers the fermentations intentionally performed by humans.

Adobe Stock/Yulia Furman

Biologically, fermentation is a metabolic process in which a carbohydrate is broken down anaerobically, resulting in the release of energy. In layman’s terms, fermentation is a process that occurs inside living organisms, in which molecules that contain both carbon and hydrogen (such as sugars) are broken down in an environment that lacks oxygen. As a result, energy that the organism can use is released. This is a complex process, and I’ve discussed the molecular details first. If you’d rather get the conceptual overview first, scroll down to “Why Do Some Organisms Ferment?”, and then come back. In some food fermentations, the most important actions of the microorganisms are not biological fermentations at all. For example, the breakdown of proteins and fats in fermented fish may contribute more to the desired qualities of the food than the breakdown of carbohydrates does. Sometimes this process is called “ripening” instead of “fermentation.”

Fermentation is one of the many interrelated metabolic pathways that keep life going on Earth. The best introduction to fermentation, I think, is to see how it fits into that big metabolic picture. Fermentation requires carbohydrates — often the sugar glucose — and is an alternative to aerobic cellular respiration in many organisms. We’ll take a quick look at photosynthesis, which produces glucose, and at cellular respiration before getting into the nitty-gritty of fermentation.



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