What kind of acid is in kefir?

January 8, 2026 •

4 min read

Fermentation of milk with kefir grains naturally lowers the pH to an acidic level of around 4.2 to 4.6, giving the beverage its characteristic tang. But what kind of acid is in kefir, and what makes it such a probiotic powerhouse? The answer lies in the dynamic metabolic activity of its complex microflora.

Quick Summary

Kefir's primary acid is lactic acid, produced by its lactic acid bacteria fermenting lactose. Acetic and citric acids are also present, combining to give kefir its signature tangy flavor and contributing to its numerous health benefits.

Key Points

Lactic Acid is Key: The most prominent acid in kefir is lactic acid, produced by bacteria fermenting milk sugar, which gives the drink its signature tangy taste.
Acids Protect and Preserve: The acidic environment created by lactic acid and other organic acids prevents the growth of harmful bacteria, acting as a natural preservative.
Acetic Acid Adds Sharpness: Acetic acid bacteria also present in the grains produce acetic acid, which adds a sharper, more pungent, and vinegar-like note to the flavor profile.
Minor Acids Contribute to Complexity: In addition to lactic and acetic acids, other organic acids such as citric and pyruvic acid are found in smaller amounts, contributing to kefir's overall flavor complexity.
Fermentation Influences Acidity: Factors like fermentation time, temperature, and milk type all influence the final concentration and balance of acids in kefir.
Acid Profile and Health Benefits are Linked: The organic acids are part of a suite of bioactive compounds in kefir that contribute to its numerous health benefits, including its antimicrobial and anti-inflammatory effects.

The Symphony of Acids in Kefir

At its core, kefir's acidic profile is a direct result of fermentation. The kefir grains, a symbiotic community of bacteria and yeasts, consume the milk's sugars and produce a range of organic acids as metabolic byproducts. While lactic acid is the most prominent, a variety of other organic acids play a crucial, if smaller, role in the final flavor and biochemical properties of the finished drink. The specific balance of these acids is influenced by factors such as fermentation time, temperature, and the composition of the microbial consortium within the grains.

Lactic Acid: The Main Player

Lactic acid is the dominant acid in kefir, responsible for its pleasant, sour taste, similar to yogurt but with a thinner consistency. The production of lactic acid begins when lactic acid bacteria (LAB) within the kefir grains ferment lactose, the natural sugar found in milk. This process is not only responsible for kefir's tangy flavor but also for its reduced lactose content, making it generally more digestible for individuals with lactose intolerance. Key LAB species involved include Lactobacillus kefiranofaciens and Lactobacillus kefiri, among many others. The accumulation of lactic acid also serves as a natural preservative by lowering the pH, which inhibits the growth of harmful bacteria and extends the product's shelf life.

Acetic Acid: Adding the Sharpness

While present in lower concentrations than lactic acid, acetic acid is another important contributor to kefir's flavor profile, adding a sharper, more pungent, and sometimes slightly yeasty or vinegar-like note. This acid is produced by acetic acid bacteria (AAB), such as Acetobacter aceti and Acetobacter rasens, which are also part of the kefir grain's microbial community. Acetic acid is particularly prominent in water kefir, and its concentration in both water and milk kefir can be influenced by fermentation conditions, especially the amount of oxygen available during fermentation.

Other Contributing Acids

In addition to lactic and acetic acids, research has identified other organic acids that contribute to kefir's complexity. These include:

Citric Acid: Present in kefir, with its concentration potentially influenced by the fermentation substrate.
Pyruvic Acid: Also detected in kefir samples, contributing to the overall flavor.
Propionic Acid: Produced by Propionibacteria, which can further break down some lactic acid, similar to their role in certain cheeses.

Health Benefits Driven by Kefir's Acids

The organic acids in kefir do more than just provide flavor; they are key drivers of its health benefits. Lactic acid has proven antimicrobial properties, inhibiting the growth of pathogenic bacteria. Acetic acid also contributes to this antimicrobial effect. This protective function, along with the action of beneficial probiotics, is part of why kefir has been shown to have antimicrobial and gut-health-modulating effects.

Furthermore, some studies have explored the anti-inflammatory effects of kefir, attributing them in part to its unique blend of metabolites, including its organic acids. These bioactive compounds work synergistically to support the body's immune system and contribute to overall wellness.

Comparing Key Acids in Kefir


Characteristic	Lactic Acid	Acetic Acid	Citric Acid
Primary Microorganism	Lactic Acid Bacteria (LAB)	Acetic Acid Bacteria (AAB)	Result of bacterial metabolism
Flavour Contribution	Prominent sour, creamy tang	Sharper, slightly vinegar-like note	Minor contributor, tartness
Source	Fermentation of milk's lactose	Fermentation of sugars, requires oxygen	Present naturally in milk, may be metabolized
Relative Concentration	Highest concentration	Lower concentration than lactic acid	Variable, generally lower

Factors Influencing Kefir's Acid Profile

The final composition and concentration of acids in kefir can differ significantly depending on several variables, highlighting the nuanced nature of this fermented beverage.

1. Fermentation Time: A longer fermentation period generally leads to a lower pH and higher levels of lactic acid, as the LAB have more time to convert lactose. This can result in a more tangy and less sweet product. Longer fermentation also gives AAB more time to produce acetic acid, especially if oxygen is present.

2. Temperature: Kefir can be fermented at various temperatures, which influences the dominant microbial activity. Some studies have shown that temperature affects the balance of different acids.

3. Milk Type: The type of milk used (cow, goat, sheep, soy, etc.) provides different sugar and nutrient profiles for the microorganisms to ferment, leading to variations in the final acid composition. For instance, cow's milk kefir may have different concentrations of acetic and citric acid compared to soy milk kefir.

4. Grain Health and Microbiota: The health and specific microbial composition of the kefir grains themselves are crucial. Since the grains contain a unique mix of bacteria and yeasts, the ratio of LAB to AAB will directly impact the final acid balance. For optimal and consistent results, maintaining healthy, active kefir grains is essential.

Conclusion: More Than Just Sourness

The answer to "what kind of acid is in kefir?" is not a simple one, as it contains a complex blend of organic acids. While lactic acid is the most abundant and well-known, other acids like acetic, citric, and pyruvic also play vital roles. This harmonious blend of organic acids is a result of the symbiotic microbial activity within the kefir grains, contributing to its distinct flavor, extended shelf life, and notable health benefits, including antimicrobial and anti-inflammatory properties. Understanding this complex chemical nature offers a deeper appreciation for this ancient probiotic drink. To learn more about the science of fermentation and its health implications, you may wish to visit the National Institutes of Health.

Frequently Asked Questions

Both kefir and yogurt are acidic, but their pH levels and specific acid profiles can differ. While yogurt is primarily fermented by bacteria to produce lactic acid, kefir involves a wider variety of bacteria and yeasts. This can result in a more complex flavor profile, though the overall acidity level is often comparable to or slightly more acidic than yogurt.

Some people find that the probiotics in kefir can help balance the gut and aid digestion, potentially alleviating symptoms of acid reflux, but research is mixed. As an acidic food, it might worsen symptoms for some individuals. It is best to consult a doctor to see if incorporating kefir is right for your condition.

Yes, fermentation time has a significant impact on kefir's acid content. A longer fermentation period typically results in higher levels of lactic acid and a lower pH, which makes the kefir tangier and more sour. Shorter fermentation times yield a milder, sweeter product.

Water kefir also contains lactic and acetic acids, but the microbial composition and substrate (sugar water instead of milk) mean the final acid profile is different. For example, water kefir contains lower levels of acetic acid compared to some milk kefirs, but the specific balance depends on the grains and fermentation conditions.

If your homemade kefir is too sour, it’s likely a result of over-fermentation. This happens when the kefir grains are left in the milk for too long, allowing the lactic acid bacteria to produce excessive lactic acid. You can reduce the sourness by shortening the fermentation time, using fewer grains, or fermenting in a cooler spot.

Yes, the organic acids in kefir offer several health benefits. They contribute to its antimicrobial properties, helping to inhibit the growth of harmful pathogens. The acids also play a role in modulating the gut environment, which is linked to immune function and overall digestive health.

You should generally not add citrus fruit directly into the primary fermentation of milk kefir. While kefir grains are acid-resistant, excessive citric acid from fruits can lower the pH too drastically and damage the microbial cultures. It is best to add citrus after straining the finished kefir.