How do bacteria produce acids

Healthy eating : When bacteria fart acid

Fermentation has actually been known for centuries: in this way, perishable food is preserved for a long time. Kefir is valued in Russia, the Swedes love their fermented herring called surstromming, in Korea kimchi is on the table with almost every meal and Japanese cuisine is indispensable without miso.

And we Germans? Eat sauerkraut, of course. But apart from these national identification dishes, it has become quiet in the domestic fermentation vessels of this world in the past few decades. This is changing a little at the moment. For some time now, star chefs have been experimenting with flavors that they achieve through fermentation and serving pickled fruits and vegetables. And glasses with colorful creations are also increasingly found in private pantries: beetroot and cauliflower, for example, or zucchini squash.

We are keen on a degradation product

Fermentation is a metabolic process of bacteria. These eat the sugar, in other words: The carbohydrates from the food metabolize it and gain their energy in the process, and when that is done, they fart out acid. So it's a breakdown product that we're so keen on when fermenting.

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The people who pickled cabbage and carrots centuries ago were less keen on the sour and spicy aroma, they wanted one thing above all: a secure supply of food. The biggest plus back then was that when fermenting - if you handle it correctly - little breaks. "The resulting acid makes the food much safer, because many pathogenic bacteria can no longer multiply in this way," says Charles Franz, head of the Institute for Microbiology and Biotechnology at the Max Rubner Institute, Federal Research Institute for Nutrition and Food (MRI), in Kiel. "The process lowers the pH value of the vegetables to 4 or below 4.5, most pathogens such as Escherichia coli or Salmonella stop multiplying at a pH value of 4 to around 4.6."

Cocoa beans are also fermented

In principle, anything can be fermented. If organic substances are converted, raw sausages are produced from meat, and buttermilk, sour milk, yoghurt or cheese from milk. The beans for coffee and chocolate are fermented, the sourdough bread, wine and beer would not exist without the fermentation process.

The specialists for fermentation are lactic acid bacteria. You don't need oxygen to reproduce, but you don't mind it either. The group is very heterogeneous, there are lactic acid bacteria which, while they are feasting on sugar, produce not only lactic acid but also other by-products in significant quantities: alcohol or acetic acid, for example. The bacterial groups staphylococci and micrococci also play a role in fermentation. Which types of bacteria gain the upper hand in the fermentation vessel depends, among other things, on the temperature and the amount of bacteria present or added. The result varies accordingly every time.

There is also alkaline fermentation

"In addition to lactic acid fermentation, there is also alkaline fermentation, in which mostly the bacterium Bacillus subtilis is used," says Franz. The bacterium breaks down proteins, the food becomes alkaline, not acidic. For example, the Japanese specialty natto is made from cooked soybeans.

A third known form is acetic acid fermentation, when acetic acid bacteria transform wine or other alcohols such as brandy, sherry or industrial ethanol into vinegar. However, since oxygen is consumed here, it is not fermentation in the narrower sense.

Lactic acid bacteria are everywhere

So how do bacteria actually get into food? When fermenting for domestic use, you can do this all by yourself, because lactic acid bacteria are everywhere: on plants, on our skin, in the body. "Ready-made starter cultures - freeze-dried or as a liquid concentrate - are added to industrially manufactured milk and meat products," explains Franz, "you consciously add a lot of bacteria to prevent potentially pathogenic pathogens from multiplying." The disadvantage of this controlled fermentation is that the end product always tastes similar.

Those who ferment at home can undoubtedly count on a broader taste spectrum. The more sugar a vegetable contains, the easier it is to ferment. The only other ingredient you need is salt. As a rule of thumb, experts recommend around one to three percent of the weight of the chopped vegetables. In principle, vegetables ferment without salt, but you can taste that too. In addition, it does not last that long. The less salt is used, the faster the fermentation takes place. More salt ensures more acidity and crisper vegetables, which is important for courgettes and cucumbers, for example, otherwise they will quickly become mushy.

The food becomes more digestible for us

The enjoyment of fermented dishes not only broadens the culinary horizon. It also helps health. "Plants have cells with a large number of long sugar compounds that we cannot digest well," says Franz. "These are used and broken down by the lactic acid bacteria, which makes the food more digestible for us."

Vegetables that have been fermented were still one of the few sources of “fresh” vegetables a few decades ago. Vegetable products are not processed much by fermentation, so they retain a lot of their original nutrients. These include, for example, so-called secondary plant substances, which we need everywhere in the human organism to keep it healthy and functional. "Such substances are of very high nutritional quality," says Bernhard Watzl, who heads the Institute for Physiology and Biochemistry at the MRI. The same goes for fiber.

We eat too little fiber

Many of the herbal products that we eat are heated to such an extent that the fiber structure is completely changed. "In general, we eat too little and, above all, too little of different types of fiber," says Watzl. "Instead of the recommended 400 grams of vegetables per day, we only consume 150 grams on average. That is why anything that helps to increase this amount is positive."

Scientists now know: the greater the diversity of bacteria in the intestine, the better. Because an intestinal flora that is richly colonized with bacteria keeps people healthy. Anyone who eats fermented foods and thus also a lot of bacteria automatically contributes to a more diverse bacterial community in the intestine.

Dietary fiber serves as food for intestinal bacteria

If the fermented food is a vegetable, the effect is twice as good: dietary fiber serves as food for the intestinal bacteria; when broken down, they produce certain fatty acids that can strengthen the immune system or protect vessels from calcification, for example. "You can also see this very nicely with dairy products: If the lactose has already been broken down by bacteria during fermentation, products such as yoghurt or cheese are much easier to digest for people with lactose intolerance," says Watzl. He advises to incorporate fermented foods into the menu on a regular basis - "preferably every day": "You shouldn't heat such products for a long time, otherwise many good ingredients will break."

Tips for the kitchen


Fermenting vegetables yourself is child's play. In fact, the little ones can help wonderfully. For homemade sauerkraut, cut or grate white cabbage into fine strips and remove the stalk. Salting: Add 2 grams of salt per 100 grams of cut cabbage. Knead vigorously and let stand for half an hour so that juice forms. Pack as tightly as possible in the fermentation vessel and pour in the resulting liquid. Place two or three larger white cabbage leaves on top and weigh them down with a clean stone or glass weight. Close the vessel, if the lid does not have a valve, just put it on so that the gases can escape. The salt removes liquid from the vegetables. So much should have formed within the first few days that the cabbage is completely covered. This is important to prevent mold and other pathogens from growing. The container with the cabbage should stand at around 16 to 18 degrees Celsius for one to two weeks. When the sauerkraut is ready depends on your personal taste: the longer it ferments, the more intense and sour it becomes.

If you are still looking for recipes, you can read on in the “Noma-Handbuch Fermentation” by René Redzepi, David Zilber and Evan Sung. The authors explain how to make and cook koji, kombucha, shoyu, miso, vinegar, garum, lactic acid pickled and black fruits and vegetables. Author René Redzepi is the chef, founder and co-owner of the Noma restaurant in Copenhagen, which was named the “best restaurant in the world” by the British trade magazine “Restaurant” in 2010, 2011, 2012 and 2014. Co-author David Chaim Jacob Zilber has been working at Noma since 2014, and has headed the Noma Fermentation Lab since 2017. He is a cook and photographer. The book has 456 pages and was published by Antje Kunstmann (ISBN: 978-3- 95614-293-2). It costs 40 euros.

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