A jar of fermenting cabbage looks like it is just sitting there, quietly turning into sauerkraut, with nothing much happening. The vegetable inside looks the same shape it started as. The liquid rises around it. Days pass.
But something far more active than simple preservation is underway. Living bacteria are consuming the natural sugars in that cabbage, and in the process, they are manufacturing nutrients the raw vegetable never contained. This is not a marketing claim built around a product. It is documented, published research, and it changes how you should think about the difference between a raw vegetable and a fermented one.
Vitamin K2 Appears Where It Never Existed Before
Raw vegetables contain essentially no vitamin K2. This matters more than it sounds, because K2 plays a critical role in directing calcium to your bones rather than your arteries.
Fermentation changes that picture entirely. Certain strains of lactic acid bacteria, the same bacteria responsible for turning cabbage into sauerkraut, produce vitamin K2 as a direct byproduct of their own metabolism. Researchers have identified specific strains of Lactococcus lactis capable of producing K2 during fermentation, with production levels varying with temperature, oxygen exposure, and the available sugar source [1]. A related study found that six strains of Lactococcus lactis produced multiple forms of vitamin K2 during fermentation, with production increasing several-fold under the right conditions [2].
Cabbage happens to be an excellent candidate for this transformation, since it naturally contains high levels of the fructose these bacteria prefer to convert into K2 [1]. Bacillus subtilis, the bacterium used to make natto, is now used industrially as a primary source of commercial vitamin K2 supplements, precisely because fermentation is an efficient way to generate this nutrient [3].
B Vitamins Multiply During Fermentation
Riboflavin and folate follow a similar pattern. A controlled study fermenting cauliflower and white beans with Lactobacillus plantarum found riboflavin increased by 76 to 113 percent after fermentation, while folate increased by 32 to 60 percent compared to the same vegetables unfermented [4]. The same study even detected a measurable increase in vitamin B12, a nutrient plant foods almost never contain on their own [4].
That number deserves more context because B12 content varies widely across fermented foods. Kimchi and kefir contain the highest naturally occurring amounts among common ferments, at roughly 1.78 and 1.06 micrograms per 100 grams [10]. Miso and tempeh contain far less, often under a quarter of a microgram per 100 grams, because the organisms actually responsible for fermenting them, koji mold in miso and Rhizopus mold in tempeh, cannot produce B12 on their own [10]. The small amount of B12 found in tempeh instead comes from incidental bacteria, Citrobacter freundii and Klebsiella pneumoniae, that happen to accompany the fermentation process rather than drive it [11].
Where researchers have deliberately raised B12 content in a ferment, they have done so by introducing a specific bacterium, Propionibacterium freudenreichii. One study found this addition more than doubled B12 levels in kimchi within a single week of fermentation [12]. A separate study fermenting soy yogurt with a particular strain of Lactobacillus reuteri reached B12 levels far higher than typical fermented foods, and the same strain corrected B12 deficiency symptoms in pregnant mice fed the fermented product [13].
The Honest Takeaway
The honest takeaway is that fermentation can meaningfully increase B12 content, but it depends heavily on which bacteria are present. Most traditional ferments, including miso, tempeh, and standard sauerkraut, do not contain enough B12 to serve as a reliable dietary source on their own.
This is not limited to a single vegetable or a single bacterial strain. Certain species, particularly Lactiplantibacillus plantarum, are now specifically studied for their ability to enhance folate content during fermentation across a wide range of traditional fermented foods worldwide [5]. Fermented legumes consistently show higher riboflavin and thiamine levels than their raw counterparts, a shift researchers attribute directly to the metabolic activity of the fermenting bacteria themselves [6].
Minerals Become Available That Your Body Could Not Reach Before
Raw vegetables and grains contain phytic acid, a compound that binds tightly to minerals such as iron, zinc, calcium, and magnesium, preventing your body from absorbing them even when they are present in the food [6]. Fermentation activates phytase, an enzyme that breaks down phytic acid and releases those minerals for use.
Human studies confirm this is not just a theory. Researchers gave people fermented vegetables alongside meals and observed a significant increase in iron absorption compared with the same meal without fermentation [7]. In animal studies, diets containing fermented quinoa and canihua produced iron levels in the liver that were 30 to 34 percent higher than diets using the same grains unfermented, along with meaningfully improved zinc bioavailability [8]. Even sourdough bread shows this effect, with phytate content dropping by at least half compared to standard whole wheat flour, leading to measurably better zinc and iron absorption [9].
The Probiotic Factor Raw Vegetables Do Not Have
Everything above happens before we even mention probiotics, and this is where fermented and raw vegetables stop being comparable at all. Raw vegetables carry no live bacterial cultures. Fermented vegetables are full of them.
These live bacteria travel through your digestive system and take up residence in your gut, where they compete with harmful microbes for space and resources. Once established, they continue producing compounds that support your gut lining from the inside, including the very same vitamins discussed above, manufactured directly in your intestines rather than delivered once and used up.
This is the piece a shelf-stable, pasteurized jar of store-bought sauerkraut cannot offer you. Pasteurization heat kills the live bacteria the entire fermentation process was built to create. The vitamins produced during fermentation may remain, but the probiotic benefit- the ongoing gut colonization- does not survive that heat.
Bringing This Into Your Own Kitchen
Getting started at home takes far less equipment and effort than the science above might suggest.
Everything begins with salt. Chop your cabbage, or whichever vegetable you are working with, and mix in sea salt at roughly two percent of the vegetable’s total weight. Within a short time, that salt pulls moisture out of the vegetable itself, and that released liquid becomes the brine your bacteria will live in for the rest of the process.
From there, the vegetable needs to stay fully submerged beneath that brine inside a clean jar. A simple fermentation weight holds everything down, since any exposed piece is at risk of developing mold. A lid that vents built-up gas without letting outside air back in means you don’t have to check the jar constantly.
Once it is packed and sealed, the bacteria take over, and your job becomes waiting. A few days on the counter, checked occasionally for taste, and you have a jar full of nutrients your body could not have accessed from the raw vegetable, along with a living culture ready to support your gut the moment you take your first bite.
What Your Grandmother Already Knew
None of this would have surprised the generations who kept a fermentation crock going in their own kitchens without ever hearing the words phytase or menaquinone. They trusted a process they could not name scientifically, because it fed their families well and it worked.
Science has simply caught up to confirm what that trust already understood. The jar on your counter is not just holding vegetables in stasis. It is quietly building nutrition that was never there to begin with, one day at a time.
If you’re new to fermentation, this complete beginner’s guide walks through everything from brine percentages to troubleshooting your first batch.
Continue the Journey
If you want a gentle first ferment to start with, Healthy Yogurt: Make It at Home Easily walks through the same live-culture principles covered here, applied to dairy instead of vegetables.
If you are working to identify which changes actually move the needle for your own health, The Pattern Detective Approach to Health: Why Changing One Variable at a Time Is the Wisest Move offers a framework for testing something like daily fermented vegetables in your own routine with real clarity.
Healthy Living Picks
If you are setting up your own fermentation station, my Healthy Living Picks includes the jars, weights, and airlock lids I recommend for getting started without guesswork.
As an Amazon Associate, I earn from qualifying purchases.
References
- Long-chain vitamin K2 production in Lactococcus lactis is influenced by temperature, carbon source, aeration and mode of energy metabolism. Microbial Cell Factories. https://link.springer.com/article/10.1186/s12934-019-1179-9
- Long-chain vitamin K2 production in Lactococcus lactis is influenced by temperature, carbon source, aeration and mode of energy metabolism. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6683496/
- Review: Bacterially produced vitamin K2 and its potential to generate health benefits in humans. ScienceDirect. https://www.sciencedirect.com/science/article/abs/pii/S0924224424001377
- Fermentation of Cauliflower and White Beans with Lactobacillus plantarum: Impact on Levels of Riboflavin, Folate, Vitamin B12, and Amino Acid Composition. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7266841/
- Folate-Containing Fermented Foods of Plant and Animal Origin: The Value of Traditional Foods to Tackle Folate Deficiency. Food Science & Nutrition. https://onlinelibrary.wiley.com/doi/10.1002/fsn3.72126
- Nutritional Benefits of Fermented Foods. Agriculture Notes by Agriculture.Institute. https://agriculture.institute/food-chemistry-and-physiology/nutritional-benefits-fermented-foods/
- Increased iron bioavailability from lactic-fermented vegetables is likely an effect of promoting the formation of ferric iron. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4737790/
- Fermented Quinoa and Canihua in Plant-Based Diets Increase Iron and Zinc Bioavailability in Growing Rats. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11606888/
- Fermentation Nutrients & Bioavailability. Edible Alchemy Blog. https://ediblealchemy.co/how-fermentation-can-unlock-your-foods-full-nutrient-potential/
- Bioenrichment of Vitamin B12 in Fermented Foods. ResearchGate. https://www.researchgate.net/publication/301353870_Bioenrichment_of_Vitamin_B12_in_Fermented_Foods
- Vitamin-B12 enrichment in tempeh by co-culture with Propionibacterium freudenreichii during fermentation. bioRxiv. https://www.biorxiv.org/content/10.1101/2022.11.06.515253.full.pdf
- Vitamin B12 and ascorbic acid in kimchi inoculated with Propionibacterium freudenreichii ss. shermanii. ResearchGate. https://www.researchgate.net/publication/229983913_Vitamin_B12_and_ascorbic_acid_in_kimchi_inoculated_with_Propionibacterium_freudenreichii_SS_shermanii
- In situ production of active vitamin B12 in cereal matrices using Propionibacterium freudenreichii. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5778212/
