Your grape juice is fizzy because yeast has started fermenting the natural sugars in it, producing carbon dioxide gas and small amounts of alcohol. This is the same basic process used to make wine, and it can happen in any grape juice that’s been left too long, stored too warm, or exposed to airborne yeast after opening. The fizz itself is harmless carbon dioxide, but it signals that the juice is actively spoiling and may no longer be safe to drink.
How Fermentation Starts in Juice
Grape juice is packed with sugar, which makes it a perfect food source for yeast. When yeast cells land in the juice (from the air, from the grape skins, or surviving pasteurization in small numbers), they begin breaking down each sugar molecule into two molecules of carbon dioxide and two molecules of ethanol. That carbon dioxide is what creates the bubbles and fizzy mouthfeel you’re noticing.
This process can begin surprisingly fast. In a sealed bottle, the carbon dioxide has nowhere to go, so it dissolves into the liquid under pressure, the same way a soda stays carbonated until you open it. If the juice is in a loosely sealed container, you might notice gentle bubbling at the surface instead. Either way, fizziness is the earliest and most obvious sign that fermentation is underway.
Why It Happened to Your Bottle
Several things can trigger unwanted fermentation:
- Temperature. The FDA recommends storing juice at or below 41°F (5°C). Anything warmer gives yeast a friendlier environment to multiply. Leaving juice on the counter, even for a few hours, can speed things up significantly.
- Time after opening. Once you break the seal, you introduce oxygen and airborne microorganisms. Opened juice kept in the fridge should generally be used within 7 to 10 days. A raw, unpasteurized juice has an even shorter window.
- Unpasteurized juice. If your juice was never heat-treated, it contains a much larger population of wild yeast and bacteria from the grape skins. These products are required to carry a warning label, though juice sold by the glass at farmers’ markets or juice bars may not.
- Preservative-resistant yeast. Some spoilage yeasts, particularly a species called Zygosaccharomyces bailii, are remarkably tough. They can survive common preservatives like sorbic acid, benzoic acid, and sulfites, and they thrive in acidic, high-sugar environments. This yeast is one of the most common causes of gas buildup and even bottle explosions in commercial beverages.
How to Tell if It’s Gone Bad
Fizziness alone is a strong indicator, but there are other sensory clues. Juice that’s actively fermenting often smells like bread dough rising or takes on a wine-like aroma. The taste may become noticeably sour or bitter compared to when you first opened it. You might also see the liquid looking cloudier than usual, or notice tiny bubbles clinging to the inside of the container.
If the bottle was sealed and the cap is bulging or pops loudly when opened, that’s carbon dioxide building up inside. In extreme cases, this pressure can actually rupture a container. Spoilage yeasts are described as capable of producing enough gas for “bottle explosions,” so a visibly swollen bottle should be opened carefully, pointed away from your face.
Is Fizzy Grape Juice Safe to Drink?
The short answer: it’s not worth the risk. While the fermentation itself just produces alcohol and carbon dioxide, a fizzy juice signals that microbial growth is out of control, and yeast isn’t the only organism that may be thriving. Bacteria like Salmonella and E. coli have been documented in contaminated fruit beverages. Molds from genera like Aspergillus and Penicillium can colonize fruit juice and produce mycotoxins, compounds linked to liver and kidney damage with repeated exposure.
The good news is that botulism is not a realistic concern with grape juice. The bacterium responsible for botulism cannot grow below a pH of 4.6, and grape juice typically sits well below that threshold, around 3.0 to 3.5. So while fizzy grape juice can make you sick in other ways, botulinum toxin isn’t one of them.
As for alcohol content, a partially fermented juice sitting in your fridge for a few days will contain very little, likely well under 1%. Full fermentation under ideal conditions can push alcohol levels to 12 to 18% before the yeast dies off, but that takes weeks and a much warmer environment than your refrigerator. Still, if the juice tastes even mildly alcoholic, that’s another sign it’s well past its prime.
How to Prevent It Next Time
Keep opened juice in the coldest part of your refrigerator, not the door. Use it within a week of opening. If you buy unpasteurized or cold-pressed juice, treat it with an even shorter timeline: three to five days is a safer window. Always check the expiration date before purchasing, and avoid any bottle that looks swollen or hisses when opened in the store.
For longer storage, you can freeze grape juice. Freezing halts yeast activity entirely. Pour the juice into a container with some headroom (the liquid expands as it freezes) and it will keep for months without any risk of fermentation. Thaw it in the refrigerator and use it within a couple of days.