Is Kefir Supposed to be Carbonated? Understanding the Fizz

December 3, 2025 •

4 min read

Traditional kefir is known for being a naturally carbonated and slightly effervescent beverage due to its fermentation process. This subtle fizz is a signature trait of healthy fermentation, but for many new brewers, it begs the question: is kefir supposed to be carbonated?

Quick Summary

Kefir's natural carbonation is a normal byproduct of its fermentation process, caused by active yeasts producing carbon dioxide. The level of fizz can vary significantly depending on factors like fermentation length, temperature, and container sealing, and differs between milk and water kefir.

Key Points

Natural Fermentation: The carbonation in kefir is a natural outcome of the fermentation process, caused by the yeast present in the kefir grains.
Yeast Produces Carbon Dioxide: The symbiotic yeast and bacteria in kefir grains consume sugars, releasing carbon dioxide gas as a byproduct, which creates the fizz.
Control with Secondary Fermentation: To increase fizziness, perform a second fermentation in a sealed, pressure-safe bottle, trapping the $CO_2$.
Temperature and Time Matter: Warmer temperatures and longer fermentation times lead to more vigorous yeast activity and higher carbonation levels.
Milk vs. Water Kefir: Water kefir often produces a more pronounced, soda-like fizz, while milk kefir typically has a milder effervescence.
Managing Excess Fizz: To reduce carbonation, use a loose lid during fermentation, refrigerate the kefir, or stir it to release trapped gas.

The Science Behind Kefir's Fizz

Kefir is a living food, teeming with a complex symbiotic culture of bacteria and yeast (SCOBY). This microbial community, contained within the kefir grains, is responsible for fermenting the sugars in the milk or sugar water. As the yeast consumes these sugars, it releases carbon dioxide ($CO_2$) gas as a metabolic byproduct, similar to how bread rises or beer ferments.

How Yeast Creates the Carbonation

During the initial fermentation, when the kefir is in an open jar, much of the carbon dioxide simply escapes into the air. This is why a freshly fermented batch may only have a slight tingle. However, if the kefir is sealed in a bottle for a 'second fermentation,' the gas becomes trapped. With nowhere to go, it is forced to dissolve into the liquid under pressure. When you open the bottle, the pressure is released, and the dissolved $CO_2$ rapidly precipitates out of the liquid, creating the familiar fizz and bubbles.

Factors Influencing Fizz Level

Several variables can dramatically affect how fizzy your kefir becomes:

Temperature: Warmer temperatures increase yeast activity, leading to more carbon dioxide production. If your home is warmer during the summer, your kefir is likely to be fizzier and ferment faster. Cooler temperatures slow down fermentation, resulting in less fizz.
Fermentation Time: Longer fermentation periods give the yeast more time to produce $CO_2$, resulting in higher carbonation levels, especially during a second fermentation. Shortening the fermentation time can reduce the fizz.
Sealed Containers: Using an airtight, pressure-safe bottle during a second fermentation is the most effective way to produce high levels of carbonation. A loose-fitting lid will allow the gas to escape, resulting in a flatter product.
Available Sugars: The amount of sugar present for the yeast to consume directly correlates with the amount of fizz. Adding fruits, fruit juice, or a small amount of sugar during a second fermentation, particularly with water kefir, provides a fresh food source for the yeast to produce more gas.

Milk Kefir vs. Water Kefir Carbonation

While both types of kefir become carbonated through yeast activity, there are some key differences in how the fizz develops and presents.


Feature	Milk Kefir	Water Kefir
Source of Sugar	Lactose (milk sugar)	Sucrose (sugar water) or fruit sugars
Microbial Makeup	Primarily lactobacilli and yeast like Candida kefyr and Saccharomyces cerevisiae	Primarily lactobacilli and yeast like Saccharomyces cerevisiae and Kazachstania exigua
Typical Fizz Level	Mild to moderate, often described as a slight tingle or 'dairy champagne'	Can be very strong and soda-like, especially after a second fermentation
Controlling Fizz	Loosening the lid during first ferment; agitating the finished kefir to release gas	Monitoring second fermentation closely; 'burping' bottles to release pressure

Managing Carbonation: How to Control the Fizz

For those who love the effervescence, and those who prefer a flatter drink, there are several methods to manage kefir's carbonation levels.

How to Reduce Kefir Carbonation

Use a Loose-fitting Lid: During the first fermentation, covering the jar with a breathable cloth or a lid that is not sealed tightly will allow carbon dioxide to escape, preventing significant fizz.
Chill After Fermentation: Moving the finished kefir to the refrigerator immediately after straining the grains will slow down the yeast activity and halt carbonation development.
Stir Before Serving: A simple way to release trapped gas is to stir or agitate the kefir gently before pouring. This helps flatten it out, just like stirring a fizzy drink.
Refrigerate During Fermentation: If your home is very warm, you can try fermenting your kefir in the refrigerator. This longer, slower ferment will result in a much milder, less carbonated product.

Tips for a Fizzier Brew

Perform a Second Fermentation: For maximum fizz, pour your strained kefir into an airtight, pressure-safe bottle, leaving a couple of inches of headspace. Allow it to ferment for another 1-3 days at room temperature. For water kefir, add a small amount of fruit juice or sugar to boost the process.
Monitor and 'Burp' Bottles: The pressure in bottles can build quickly. Regularly 'burp' your bottles by opening and closing the lid quickly to release excess gas. This prevents over-carbonation and potential bottle explosions.
Use High-Quality Bottles: To ensure safety and a good fizz, invest in pressure-rated bottles like Grolsch-style swing-tops or canning jars with durable lids.

Conclusion

In short, the answer to "is kefir supposed to be carbonated?" is a resounding yes. The delightful fizz is a natural and healthy sign of the vibrant yeast and bacteria cultures at work during fermentation. The intensity of the carbonation is a matter of control, not concern. By understanding the factors that influence fermentation—including time, temperature, and your choice of container—you can easily tailor your homemade kefir to your perfect level of fizziness. Whether you prefer it flat, mildly effervescent, or as a sparkling probiotic soda, the bubbles are an indicator that your brew is alive and well, delivering its signature tangy taste and beneficial properties.

For more detailed information on kefir's composition and health benefits, you can consult the extensive research available, such as this review on Kefir and Its Biological Activities.

Frequently Asked Questions

No, a moderate amount of fizz is a sign of a healthy, active fermentation, not that the kefir has spoiled. It indicates that the yeast is actively consuming sugars and producing carbon dioxide.

Commercial kefir is often produced with a powdered starter or the fermentation process is stopped early to create a more consistent, non-effervescent product that is easier to manage in packaging and on shelves. Home-brewed kefir, using active grains, typically has more vigorous yeast activity.

To reduce fizziness, you can use a loosely covered jar instead of a sealed one during fermentation, ferment for a shorter period, or chill the kefir in the refrigerator immediately after straining the grains to slow down yeast activity.

After straining the grains, pour your water kefir into an airtight, pressure-safe bottle and add a small amount of fruit juice or cut-up fruit. Seal the bottle and let it sit at room temperature for 1-3 days to build carbonation before refrigerating.

Yes, it is safe. Both milk and water kefir contain a small amount of naturally occurring alcohol and carbonation as byproducts of the yeast fermentation. Very strong fizz and a yeasty smell can happen with longer fermentation periods or warm temperatures.

If milk kefir is left to ferment too long, it can become over-fermented, resulting in a more liquid texture and higher yeast activity, which causes more carbonation. This is normal, and the finished product is still safe to consume.

If your kefir is not fizzy and tastes very sweet after 24-48 hours, it may indicate that your kefir grains are inactive or need more minerals. Try a different sugar, ensure the temperature is warm enough for fermentation (mid-70s°F is ideal), or add some trace minerals.