Fermented Foods: Kimchi, Sauerkraut and More

What Is Fermentation?

Fermentation is an ancient food preservation process in which microorganisms such as bacteria, yeasts, or moulds break down organic compounds. The most common form is lactic acid fermentation (lactofermentation). In this process, lactic acid bacteria convert sugars into lactic acid. This lowers the pH of the food and makes it shelf-stable, because harmful pathogens cannot multiply in the acidic environment.

Fermented foods have been consumed for thousands of years (Marco et al., 2021). Sauerkraut is considered one of the most widespread forms of preserved cabbage, with origins dating back to the 4th century BC (Dimidi et al., 2019). In Korea, fish, meat, legumes, and vegetables have been preserved through lactic acid fermentation for more than 1,500 years, and kimchi is a traditional example there (Patra et al., 2016). What began as a preservation method is now at the centre of nutrition research, because fermented foods contain living microorganisms that are being studied in gut microbiome research.

An Overview of Fermented Foods

The variety of fermented foods is vast. Every culture has its own traditions:

Food	Origin	Base Ingredient	Dominant Cultures
Sauerkraut	Europe	White cabbage	Leuconostoc mesenteroides, Lactiplantibacillus plantarum, Levilactobacillus brevis
Kimchi	Korea	Napa cabbage, radish	Lactobacillus, Leuconostoc
Kefir	Caucasus	Milk	Lactobacillus, Saccharomyces
Kombucha	East Asia	Sweetened tea	Acetobacter, Saccharomyces
Miso	Japan	Soybeans, rice	Aspergillus oryzae
Tempeh	Indonesia	Soybeans	Rhizopus oligosporus
Yoghurt	Central Asia	Milk	L. bulgaricus, S. thermophilus

Important: not all fermented foods still contain living cultures. Beer, wine, and sourdough bread undergo fermentation, but the microorganisms are killed through heating or filtration.

Microorganisms: Lactobacillus and Bifidobacterium

Two genera dominate the research on fermented foods:

Lactobacillus belongs to the lactic acid bacteria, which are among the most important and most commonly used microorganisms in food fermentation (Marco et al., 2021). Several studies in humans have shown that microorganisms from fermented foods can survive passage through the stomach and reach the colon (Marco et al., 2021).

Bifidobacterium occurs naturally in the human gut. According to the Codex Alimentarius, yoghurt is made with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, and kefir with kefir grains that contain, among others, Lactobacillus kefiri, Leuconostoc, Lactococcus, Acetobacter, and yeasts. Other microorganisms may be added (FAO/WHO, 2010). Bifidobacteria produce acetate and lactate. Other gut bacteria that produce butyrate can utilise these substances (Rivière et al., 2016).

A consensus statement by Marco et al. (2021) in Nature Reviews Gastroenterology & Hepatology notes that fermentation can give rise to vitamins, amino acid derivatives, organic acids, and cofactors, as well as bioactive compounds such as bioactive peptides.

The Stanford Study

A widely cited study by Wastyk et al. (2021), conducted at Stanford School of Medicine and published in Cell, compared two dietary interventions over 10 weeks in 36 healthy adults:

• Group 1: Increased their consumption of fermented foods to 6 servings per day (yoghurt, kefir, kimchi, sauerkraut, kombucha).
• Group 2: Increased their fibre intake to over 40 g per day.

Over the course of the intervention, the study observed an increase in microbiome diversity in the fermented-foods group. The authors cite the small sample size (18 people per group) and the lack of a control group as limitations. The fibre group showed neither an increase in diversity nor a consistent change in the composition of the microbiome. In this group, microbiome-encoded enzymes that break down carbohydrates (CAZymes) increased.

This does not mean fibre is less important. The study suggests that fermented foods and fibre may act on the microbiome in different ways. The combination of both strategies is discussed in current research.

Histamine Intolerance: When Caution Is Needed

Fermented foods are not equally well tolerated by everyone. Some lactic acid bacteria produce histamine, tyramine, and other biogenic amines during fermentation (Marco et al., 2021). After consuming foods that contain histamine, such as cheese or red wine, individuals with histamine intolerance may develop symptoms including digestive complaints, headaches, or skin reactions (Comas-Basté et al., 2020).

Anyone who regularly experiences discomfort after eating fermented foods should seek medical advice.

Not all fermented foods contain the same amount of histamine. Fresh yoghurt and kefir are generally better tolerated than products that have been aged for a long time, such as sauerkraut or kombucha.

Fermented vs. Pasteurised: What to Look for When Buying

Many commercially available fermented products are pasteurised after fermentation. This kills the living cultures through heating. It extends shelf life, but the living cultures are no longer present afterwards.

What to look for:

• Refrigerated section, not the shelf: Unpasteurised products are found in the refrigerated section. Sauerkraut in tins or jars stored at room temperature is almost always pasteurised.
• Check the ingredients list: Does the product contain vinegar instead of natural acidity? Then it was not fermented but pickled in vinegar.
• Label claims: Terms such as “raw”, “unpasteurised”, “with live cultures”, or “natural” indicate a product that has not been heat-treated.
• Make your own: Sauerkraut and kimchi can be made at home with minimal effort. All you need is vegetables, salt, and a jar.

Practical Tips: Integrating Fermented Foods into Daily Life

The Stanford study used 6 servings of fermented foods per day. That is an ambitious target. For beginners, smaller amounts that can be gradually increased are a good starting point.

• Morning: Natural yoghurt or kefir for breakfast. Combine with oats, nuts, and fruit.
• Lunch: 2-3 tablespoons of sauerkraut or kimchi as a side dish. Goes well with rice, salads, or bowls.
• Evening: Miso soup as a starter. Dissolve miso paste in warm (not boiling) water to preserve the cultures.
• Between meals: A small glass of kombucha or water kefir as an alternative to sugary drinks.

Important: fermented foods should not be heated, because high temperatures kill the living cultures. When adding sauerkraut as a side dish, add it after cooking rather than during.

Conclusion

Fermented foods combine a tradition spanning thousands of years with modern nutrition research. The Stanford study by Wastyk et al. (2021) observed an increase in microbiome diversity in the group with a high intake of fermented foods. Quality is key: only unpasteurised products contain living cultures. Gradually integrating fermented foods into everyday meals adds variety to the plate.

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Sources:

• Wastyk HC, Fragiadakis GK, Perelman D, Dahan D, Merrill BD, Yu FB, Topf M, Gonzalez CG, Van Treuren W, Han S, Robinson JL, Elias JE, Sonnenburg ED, Gardner CD, Sonnenburg JL (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137-4153.e14. DOI: 10.1016/j.cell.2021.06.019
• Marco ML, Sanders ME, Gänzle M, Arrieta MC, Cotter PD, De Vuyst L, Hill C, Holzapfel W, Lebeer S, Merenstein D, Reid G, Wolfe BE, Hutkins R (2021). The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nature Reviews Gastroenterology & Hepatology, 18(3), 196-208. DOI: 10.1038/s41575-020-00390-5
• Dimidi E, Cox SR, Rossi M, Whelan K (2019). Fermented Foods: Definitions and Characteristics, Impact on the Gut Microbiota and Effects on Gastrointestinal Health and Disease. Nutrients, 11(8), 1806. DOI: 10.3390/nu11081806
• Patra JK, Das G, Paramithiotis S, Shin HS (2016). Kimchi and Other Widely Consumed Traditional Fermented Foods of Korea: A Review. Frontiers in Microbiology, 7, 1493. DOI: 10.3389/fmicb.2016.01493
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• Codex Alimentarius Commission, FAO/WHO (2003, revised 2008 and 2010). Standard for Fermented Milks. CODEX STAN 243-2003.
• Rivière A, Selak M, Lantin D, Leroy F, De Vuyst L (2016). Bifidobacteria and Butyrate-Producing Colon Bacteria: Importance and Strategies for Their Stimulation in the Human Gut. Frontiers in Microbiology, 7, 979. DOI: 10.3389/fmicb.2016.00979
• Comas-Basté O, Sánchez-Pérez S, Veciana-Nogués MT, Latorre-Moratalla M, Vidal-Carou MDC (2020). Histamine Intolerance: The Current State of the Art. Biomolecules, 10(8), 1181. DOI: 10.3390/biom10081181