Why is my yogurt drink fizzy? What to know about carbonation

November 24, 2025 •

4 min read

The unmistakable tang of a fresh yogurt drink is a treat, but when that familiar dairy beverage starts to fizz, it often raises a red flag. A significant number of cases suggest that fizzy yogurt is the result of bacterial activity, either from deliberate or unintentional fermentation. Understanding the process behind this carbonation is crucial for differentiating a benign flavor quirk from a sign of spoilage.

Quick Summary

Fizziness in a yogurt drink is caused by the production of carbon dioxide, primarily from microbial activity. It can indicate either a normal, extended fermentation process, or, more commonly, spoilage from yeast or other bacteria due to improper storage. Consumers should use visual, smell, and taste cues to distinguish between a harmless result of live cultures and a potentially unsafe product.

Key Points

Microbial Activity: Fizz is caused by carbon dioxide production from microbial fermentation, which can be either from the intended cultures or unwanted microbes.
Over-Fermentation: Storing yogurt at temperatures that are too warm can cause the normal lactic acid bacteria to produce excess $CO_2$.
Yeast Contamination: Wild yeasts can contaminate yogurt and cause spoilage, producing carbonation along with an unpleasant, alcoholic odor.
Spoilage Indicators: Look for signs of spoilage such as mold, a slimy texture, a rancid smell, or a bitter taste before consuming.
Discard if Doubtful: When in doubt about whether a fizzy yogurt is safe, it is always best to discard it to avoid the risk of foodborne illness.

The Science Behind the Fizz: Over-Fermentation and Contamination

Fizziness in dairy products like yogurt drinks is almost always the result of a biological process that produces carbon dioxide ($CO_2$). The tangy flavor of yogurt comes from fermentation, where lactic acid bacteria break down lactose. When this process continues for too long or is supplemented by other microorganisms, the production of $CO_2$ gas can increase dramatically, leading to a noticeable carbonation.

Over-Fermentation from Existing Cultures

In a healthy, living yogurt drink, the cultures are the intended source of fermentation. If a drink is stored at a warmer temperature than recommended, these cultures become hyper-active, producing more lactic acid and more $CO_2$ than usual. The gas gets trapped in the sealed container, resulting in a carbonated sensation when opened. While this version of fizzy yogurt isn't necessarily dangerous, it may be more sour and less pleasant in taste. This is a common issue with homemade yogurt where fermentation time and temperature are not as precisely controlled.

Contamination from Wild Yeasts

Another major culprit behind fizzy yogurt drinks is contamination by yeast. Yeasts are naturally present in the air and can be found on kitchen surfaces. If a yogurt container is not sealed properly or if a utensil carrying wild yeast is dipped into it, these opportunistic organisms can begin to thrive. Yeast ferments sugars, but produces a different set of compounds than lactic acid bacteria, including carbon dioxide and alcohol. This can cause a noticeably foamy texture and an off-putting, sometimes bread-like or alcoholic smell and taste, indicating spoilage.

Improper Storage and Temperature Abuse

Commercial yogurt drinks are carefully processed and packaged to ensure the microbial balance is correct and stable. However, improper storage can disrupt this balance. Leaving a yogurt drink out of the refrigerator for too long, or storing it in a warm environment, provides the ideal conditions for bacteria and yeast to multiply excessively. This results in accelerated fermentation and a higher likelihood of spoilage. Once the product is exposed to room temperature for more than a few hours, the risk of developing harmful bacteria increases significantly.

Comparison: Safe Fizz vs. Spoiled Fizz


Feature	Potentially Safe Fizzy Yogurt (Over-fermented)	Spoiled Fizzy Yogurt (Contamination)
Appearance	May have a slightly watery layer (whey separation) on top. No mold or unusual colors.	May show signs of mold (blue, green, or black spots), thick liquid separation, or a bubbly, foamy layer.
Smell	Very sour or tangier than usual, but not rancid or foul.	Strong, unpleasant, rancid, or yeasty odor, sometimes like beer.
Texture	Thinner than normal. No significant clumping or grittiness.	Lumpy, slimy, or curdled consistency. May have sand-like particles.
Taste	More acidic and tangy, but otherwise familiar.	Very sour, bitter, or alcoholic taste. The off-flavor will be pronounced and unpleasant.
Risk	Low risk for healthy individuals, but the taste may be unappealing.	High risk for foodborne illness, including cramps, diarrhea, and vomiting.

How to Assess Your Fizzy Yogurt Drink

Check the expiration date: While not an absolute indicator of safety, a drink well past its "best by" date is more susceptible to over-fermentation and spoilage.
Use your senses: Smell the drink. Is the aroma just more sour, or is it foul and yeasty? Look for visual cues like mold, unusual colors, or a foamy layer.
Taste test (with caution): If the appearance and smell seem normal, take a very small sip. If the taste is simply a more intense tang, it is likely safe. However, if it tastes bitter, rancid, or alcoholic, discard it immediately.

Conclusion

While a gentle fizz in some specialty fermented beverages like kefir is normal, a carbonated yogurt drink is usually a sign of an issue. In most cases, it indicates either an extended fermentation period due to warmer storage or, more dangerously, contamination from unwanted yeasts or bacteria. A fizzy yogurt drink that smells or tastes rancid should always be discarded to prevent foodborne illness. For those who enjoy a carbonated twist, specific recipes exist that add sparkling water for a refreshing and intentionally fizzy drink, ensuring a controlled and safe experience.

Here is a guide to common yogurt defects from the Dairy Foods Magazine.

Common causes for fizzing in yogurt drinks

Over-fermentation: Leaving yogurt at too warm a temperature can cause existing probiotic cultures to produce excess carbon dioxide.
Yeast contamination: Airborne yeast, especially common in home kitchens, can enter an open container and cause off-flavors and carbonation.
Improper storage: Allowing a yogurt drink to sit unrefrigerated for an extended period accelerates microbial activity, increasing the risk of both over-fermentation and spoilage.
High sugar content: The presence of fruit or high amounts of added sugar can provide more fuel for opportunistic yeasts to feed on, leading to increased gas production.
Expiration: Yogurt drinks that are nearing or past their "best by" date have a higher chance of developing fizziness as microbial activity continues.

Frequently Asked Questions

If a yogurt drink is fizzy, you should first check for signs of spoilage like mold, a rancid smell, or a bitter taste. If those signs are present, it is not safe to drink. If the only issue is a slightly sourer taste and a mild fizz, it may be a result of harmless over-fermentation, but it is best to exercise caution and discard it if you are unsure.

To prevent unwanted fizziness, always store your yogurt drink in the refrigerator at a consistent temperature. Keep the container tightly sealed to prevent airborne yeast contamination and use clean utensils when serving. Consume the drink before its expiration date and avoid leaving it at room temperature for more than two hours.

Spoiled yogurt will taste noticeably off. It may have an excessively sour or bitter flavor, and in cases of yeast contamination, an unpleasant, yeasty, or alcoholic taste. A bad smell will also accompany the off-taste.

Safe, healthy fermentation, like in kefir, produces a balanced, tangy flavor. Spoilage, often caused by opportunistic yeasts or bacteria, creates excessive gas and off-flavors, resulting in a rancid smell or a bitter, unpleasant taste.

If the fizziness is due to spoilage from harmful bacteria or yeast, drinking it could lead to foodborne illness, with symptoms like nausea, cramps, and diarrhea. It is safer to discard any product with suspicious signs of spoilage.

Fizziness in homemade yogurt is often caused by over-fermentation or wild yeast contamination. Inconsistent incubation temperatures or using a contaminated starter culture can speed up the process and introduce unwanted microbes.

Yes, some traditionally fermented dairy beverages, like kefir and koumiss, contain natural yeasts that produce carbonation as part of their intended fermentation process. A fizzy sensation is part of their normal, healthy profile.