What is Fermentation?

You might be familiar with the term fermentation, and know that it has something to do with brewing or microbes or something. In fact, fermentation is a crucial process for making such culinary essentials as bread, beer, wine and cheese. Some forms of fermentation also give us probiotics, beneficial bacteria that aid in digestion. Note that fermentation is not the same thing as pickling. We'll talk about the difference toward the end of this piece.

What Is Fermentation?

Fermentation is a metabolic process where living organisms consume carbohydrates (such as starch or sugar) and produce alcohol or acid. Yeasts, like the ones used for baking bread and making beer and wine, produce alcohol, while bacteria, like the ones in yogurt, produce acid.

How Does Fermentation Work?

Fermentation is complicated, but most significantly, all organisms need oxygen to convert glucose, a simple sugar, into energy for the organism's cells to use. (In the case of yeast and bacteria, the entire organism consists of a single cell, but the process also goes on in larger organisms like humans, as well as in plants.)

Microorganisms like yeasts and bacteria don't have lungs to breathe in oxygen, but they pull it out of the atmosphere around them or from the liquid environment in which it lives (assuming the liquid contains oxygen, as water does), and life goes on. 

But what if there's little or no oxygen in the environment? In that case, these organisms need to pull oxygen out of the environment in a different way, called fermentation.

Benefits of Fermentation

Fermentation in food provides several benefits. It aids in preservation, since the alcohols or acids it produces prevent the growth of the bacteria that cause food spoilage. Foods like cheese, sour cream, yogurt, sauerkraut and kombucha are examples of this. Fermentation adds flavor to foods, such as the tanginess of yogurt, the zest of kombucha, and the characteristic funkiness of traditional miso paste. And because foods like yogurt and kombucha contain live cultures of beneficial bacteria called probiotics, fermentation also enhances so-called "gut health," by replenishing the beneficial bacteria in our intestines that aid in digestion.

Note that while fermentation plays a role in making beer, wine and bread, these foods do not contain live cultures, so there they don't provide any "gut health" benefits.

The Fermentation Process

There are two main types of fermentation: ethyl alcohol fermentation and lactic acid fermentation. 

Both involve organisms breaking down the carbohydrates they consume to pull out the oxygen that helps make up each carbohydrate molecule. In doing so, they produce waste products, lactic acid in the case of bacteria, and a combination of ethyl alcohol and CO2 gas in the case of yeast.

In ethyl alcohol fermentation, yeast organisms consume sugar and produce ethyl alcohol, which is what gives beer, wine and spirits their kick, and CO2 gas, which is what causes bread to rise.  

Lactic acid fermentation is carried on by the bacteria in yogurt, such as lactobacillus acidophilus. In this process, the bacteria consume lactose, a sugar in milk, and convert that into oxygen for themselves with a waste product of lactic acid. It's this acid that gives yogurt its tangy flavor.

Lactic Acid Fermentation

Lactic acid fermentation is the type of fermentation at work when making fermented foods like kimchi and sauerkraut. How it works is that foods like cabbage are submerged in a salty brine. Salt kills bacteria that cause food spoilage, but doesn't harm lactobacillus bacteria, which are present pretty much everywhere, including on cabbage and other foods. So after a while of soaking in this salty brine, the lactobacillus go to work converting the carbohydrates in the vegetables into lactic acid, which in turn aids in preservation while also imparting a tangy flavor. Tofu, miso, soy sauce, and even ketchup, pepperoni, and hot sauce are also made using lactic acid fermentation.

Ethyl Alcohol Fermentation

Ethyl alcohol fermentation is what gives us beer, wine, sprits as well as bread. Note that sourdough bread uses a combination of ethyl alcohol and lactic acid fermentation. The yeast gives the bread its rise while the lactobacillus gives it its sour flavor. 

Beer and wine typically undergo two stages of fermentation, known as primary and secondary fermentation. The primary stage triggers a rapid production of alcohol, while the secondary stage is slower and helps to develop more complex flavors.

Acetic Acid Fermentation

Some people regard acetic acid fermentation as a third type of fermentation, although it is arguably a version of ethyl alcohol fermentation. It's the process that turns wine to vinegar, so if you were wanting to make wine, it's not good, but if your goal is vinegar, then it's great.

How it works is that certain bacteria, known as acetic acid bacteria, oxidize alcohol and sugar to to form acetic acid, which, when highly diluted, is simply ordinary vinegar. In addition to vinegar, this process is also a secondary fermentation stage in making kombucha, following the initial stage of ethyl alcohol fermentation.

Fermenting Vs. Pickling

Often fermentation is confused with pickling, because both are forms of food preservation and both produce foods with a sour flavor. The difference is that with fermentation, lactobacillus acidophilus bacteria are producing acetic acid as part of the fermentation process, and it's this that gives the food its sour flavor. With pickling, foods are immersed in a solution that includes vinegar, which imparts a sour flavor to the food, even though no actual fermentation is taking place. It's just soaking in vinegar.