Cutting into a watermelon is usually a refreshing moment — bright red flesh, a sweet aroma, and plenty of juice. But if you slice one open and notice foam bubbling or oozing from the inside, stop immediately and throw it away.
This isn’t a harmless oddity. It may be a sign that the watermelon has begun to ferment and spoil, and eating spoiled fruit could make you sick.
Why Foaming Watermelon Is a Red Flag
Watermelon is naturally high in sugar, which makes it susceptible to fermentation when microorganisms such as yeast and bacteria are present. As these microorganisms break down the fruit’s sugars, they can produce gases such as carbon dioxide.
As gas builds up inside the watermelon, it may mix with the fruit’s liquid and escape as foam or bubbling liquid through cracks in the flesh or rind. In other words, the watermelon may effectively become a small fermentation chamber.
A watermelon showing obvious signs of fermentation or spoilage should not be eaten. Some harmful bacteria can cause foodborne illness, and you cannot reliably determine whether a spoiled fruit is safe simply by removing the affected area.
One concern is that spoilage can begin inside the fruit, meaning the rind may still look relatively normal while the flesh has already deteriorated.
Other Signs Your Watermelon Has Gone Bad
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