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Fermentation: behind-the-scenes

Biochemistry stuff I barely understand Published on thought

In the last few years, I have superficially dabbled with fermenting my own food, making kimchi, Sauerkraut, kombucha or kefir. I have also learned that so many of the things I drink or eat are in fact fermented — think cheese, wine, beer, yogurt, bread.

Since Wednesday, I have been diving deep into the intricate world of fermentation from a complete other direction. I stepped outside my comfort zone and had a look at this whole fermentation shenanigan from the point of view of biochemistry.

Here are the things I've learned. (Disclaimer: I am mostly quoting Wikipedia as that's where my fermentation rabbit hole started. Please get back to me if I wrote anything wrong-I-don't-really-know-what-I'm-talking-about.)

What is fermentation?

In biochemistry, fermentation is the process where living organisms extract energy from carbohydrates (e.g. starch or sugar) in the absence of oxygen. The food industry sets a broader scope, as it is more largely defined as a metabolic process that produces chemical changes in organic substances through the action of enzymes. There are tons of different fermentation processes that can produce a number of things: alcohol (a.k.a. ethanol), acid, carbon dioxide, and others.

For instance, yeasts are used in wine and beer-making to produce alcohol, while bacteria are used for yogurts to produce acid. Sometimes, a mix of both yeasts and bacteria can be used in tandem as well.

Let's have a look at a few types of fermentation used in the food-industry — because who cares about the other types anyway.1

Lacto fermentation

Lacto fermentation, also called lactic acid fermentation, is the process where lactic acid bacteria (LAB) convert sugars into energy, with lactic acid (and sometimes other stuff as well) as by-products. This occurs in an anaerobic environment2.

Common examples of lacto fermentation are kimchi, Sauerkraut or yogurt. LAB already naturally exists in many vegetables, so you don't need to add external bacteria for the fermentation to occur. The bacteria keep on living in the final product and make them probiotic, which is good for your gut health.

Let's use the Sauerkraut recipe as an example. At its simplest, you only need to mix cabbage with water and let it rest at room temperature to kickstart the fermentation process. Notice how the recipe insists on preventing the minced cabbage from coming into contact with the air. The reason is to keep the environment anaerobic, where LAB thrive whereas other *bad bacteria* would die. As the fermentation goes on, all the glucose present in the cabbage is transformed into lactic acid, making Sauerkraut sour.

Another example of lacto fermentation is cheese. The basic concept of cheese is to divide milk into solid curds and liquid whey. To do so, we need to either add an acid element to the milk (e.g. vinegar) or mix in LAB that will consume the lactose and transform it into lactic acid.3

Ethanol fermentation

Ethanol fermentation or alcoholic fermentation is the process where yeasts — a.k.a. mushrooms — convert sugars into energy in an anaerobic environment (same old, same old). However, this time, the by-products are ethanol and carbon dioxide.

In the case of food, ethanol makes things — well — alcoholic, whereas carbon dioxide produces bubbles. In short, this type of fermentation is used for wines, beers, whiskeys, rums and tons of other alcoholic beverages.

In most cases, the brewers will let the carbon dioxide escape during a primary fermentation. When needed, a second fermentation (sometimes with extra yeast) can be done in the final bottle. As the carbon dioxide doesn't have anywhere to go, it is integrated into the liquid as bubbles — yay! Beers and sparkly wines are perfect examples of this second fermentation.

With ethanol fermentation, the yeasts die after a while: either when all the sugars have been converted into ethanol or when the alcohol content has reached 15% alcohol per unit volume — strong enough to kill all types of yeasts. When a beer is unfiltered, you can see the sediment of dead yeasts remaining at the bottom of the bottle.

Winemakers also use this yeast property to make fortified wines (e.g. Porto wine). They can stop the fermenting process by adding some extra alcohol in the wine. This usually results in a sweeter and stronger wine. The fermenting process can also be stopped by simply dropping the temperature of the tank.

Fun fact: ethanol fermentation also takes place in some species of fish where it provides energy when oxygen is scarce.

Malolactic fermentation

Malolactic fermentation is a process in winemaking. It is usually done after the first ethanol fermentation, by introducing LAB into the must. Those bacteria will consume the tart-tasting malic acid, naturally present in grape must, and produce softer-tasting lactic acid and carbon dioxide.

The final aromas are said to be less acid, apple-like and more buttery, milky, oily or vanilla-like. Most red wines undergo malolactic fermentation, as well as some fuller white wines like Chardonnay.

LAB can be very sensitive to the sulfites (sulfur dioxide) added during the winemaking process. When choosing to not use sulfites, wild LAB native to the grapes can initiate uncontrolled malolactic fermentation that might give an off-taste to some natural wines.

Carbonic maceration

Carbonic maceration is a fermentation technique usually used for specific types of wines (e.g. Beaujolais). It results in wines with low tannins that should be drunk young.

It actually falls under the ethanol fermentation family. The difference is that instead of crushing the grapes, most of the fruits are kept whole in a sealed container. The fruits at the bottom are crushed­ — because of duh gravity — and undergo conventional ethanol fermentation. The carbon dioxide produced saturates the air, creating an anaerobic environment4. The carbon dioxide gas stimulates the fermentation process, which occurs inside of each berry.

Carbonic maceration started as a winemaking technique, but people have started using it during coffee preprocessing as well. Before the coffee berries are ready to be roasted, they need to first be fermented, so that the fruit pulp can be removed from the seed. The method is similar: keep the berries intact in a closed vessel to undergo a carbonic maceration. In the case of coffee, the vessel is pumped with extra carbon dioxide to begin with. The resulting coffee profiles are often described as more intense.

Aerobic fermentation

Aerobic fermentation is the process where acetic acid bacteria (AAB) metabolize sugars or ethanol in an aerobic environment5. Its by-product is acetic acid. As there is oxygen involved, it is more accurate to call this fermentation an oxidation process.

This type of fermentation is used for the production of vinegar. If making wine vinegar, the ethanol present in the wine is converted into acetic acid.

This is also why we usually use sulfites in the winemaking process. Sulfites prevent oxidation, thus avoiding the wine transforming into vinegar.

Special mention: SCOBY

A SCOBY is a symbiotic culture of bacteria and yeast. It is a culinary fermentation culture (or starter) where we find both yeasts and bacteria living in harmony — isn't it beautiful?

Common examples of food / drinks made from a SCOBY are kombucha, kefir, sourdough bread, vinegar, soy sauce or sour beers.

A SCOBY is a mix of yeast, LAB, and AAB.

The SCOBY, when mixed with water and sugar-rich ingredients, undergo multiple types of fermentation and oxidation. To be completely honest, I haven't fully understood what ferments what and when. But all in all, yeasts consume sugars and produce ethanol. The LAB (anaerobic) consume sugars and produce lactic acid. The AAB (aerobic) consume either sugars or ethanol and produce acetic acid. The end results is often acidic and just a tiny tiny bit alcoholic.

That's it for today's episode on food fermentation! I might update it in the future, as I learn more about this fascinating world!


  1. Actually, fermentation also occurs in other super interesting cases. For instance, during intense exercise, Mammalian muscles will undergo fermentation and transform some of those stored sugars into energy and lactic acid as a by-product. I now finally understand why my arms hurt after an intense climbing session!

  2. An anaerobic environment has no oxygen.

  3. A lot of cheeses are also made with rennet, a set of enzymes, that will also divide the milk into curds and whey. The bonus effect is that the curds will be less fragile and more gel-like.

  4. Carbon dioxide is heavier than oxygen. As it saturates the tank, oxygen is pushed out from the tank through a one-way valve.

  5. They like oxygen.

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