Understanding Fermentation: The Science Behind Brewed and Preserved Food

Fermentation is a metabolic process in which microorganisms, usually bacteria or yeast, convert sugars and starches into other compounds such as acids, alcohol or gases. It happens in the absence of oxygen, or with very little oxygen present, which distinguishes it from simple spoilage or oxidation. While the idea of letting food sit and change through microbial activity might sound uncontrolled, traditional and modern fermentation practices rely on careful management of salt, temperature, time and starter cultures to guide the process toward a specific, desirable outcome.

The Main Types of Fermentation

There are three types of fermentation that appear most often in food and brewed beverages.

Lactic acid fermentation: Lactic acid bacteria convert sugars into lactic acid. This is the process behind sauerkraut, kimchi, traditional pickles, yogurt and some sourdough breads. The resulting acidity gives these foods their characteristic sour taste and also helps preserve them.
Alcoholic fermentation: Yeast converts sugars into ethanol and carbon dioxide. This is the basis of beer, wine and the rise in bread dough. In beer brewing, malted grain is first mashed to release fermentable sugars, which yeast then converts during fermentation.
Acetic acid fermentation: Acetic acid bacteria convert alcohol into acetic acid, which is how vinegar is produced. This process is also involved in the production of kombucha, where a symbiotic culture of bacteria and yeast, often called a SCOBY, first ferments sweetened tea into a mildly alcoholic liquid before acetic acid bacteria continue the process to produce the tangy, slightly effervescent drink most people recognize as kombucha.

Starter Cultures and Wild Fermentation

Some fermented foods rely on wild fermentation, where naturally occurring microorganisms on the ingredient or in the environment drive the process. Traditional sauerkraut, for example, often relies on the bacteria already present on cabbage leaves. Other fermented foods and drinks use a starter culture, a concentrated batch of specific microorganisms added deliberately to ensure a consistent and predictable result. Yogurt, kombucha, sourdough bread and most commercial beer rely on starter cultures, whether that is a yogurt culture, a SCOBY, a sourdough starter or a specific strain of brewing yeast.

Temperature, Salt and Time

Three variables largely determine how a fermentation unfolds: temperature, salt concentration and time. Warmer temperatures generally speed up fermentation but can also favor the growth of unwanted microorganisms if not controlled. Salt is used in many vegetable fermentations to suppress spoilage organisms while allowing salt tolerant lactic acid bacteria to thrive. Time determines how far the fermentation progresses, a kimchi left to ferment for a few days will taste different from one left for several weeks, just as a kombucha brewed for five days will taste sweeter and less acidic than one left for two weeks.

Common Fermented Foods and Drinks

Sauerkraut and kimchi, both lactic acid fermented vegetables
Yogurt and kefir, fermented dairy products
Sourdough bread, which uses a mix of wild yeast and lactic acid bacteria
Beer and wine, both relying on alcoholic fermentation
Kombucha, which combines alcoholic and acetic fermentation
Traditional sorghum based fermented beverages, found in various forms across Southern Africa

Why Understanding Fermentation Helps

Knowing the type of fermentation behind a food or drink helps explain its taste, texture and how it should be stored. Lactic acid fermented foods tend to be tangy and are often kept refrigerated once opened to slow further fermentation. Alcoholic and acetic fermentations continue to develop even after bottling unless the product is pasteurized, which is why some bottled kombucha or unfiltered beer can change character over time. Understanding these basics gives a practical foundation for reading labels, choosing storage methods and appreciating why a brewed or fermented product tastes the way it does.