My brother had this experience with his young daughter. Bottle of apple juice left out on the counter that should have been refrigerated. Young girl kept going back to get a few glasses after a meal. She started acting a little goofy. He sniffs the container and tries a splash. They had accidentally created apple jax by not refrigerating the juice.
Your juice got 'contaminated' with the homemade wine yeast that you have floating in the air around your house. Those yeast then started multiplying and producing alcohol.
Be careful the 'juice' could have more than just yeast in it multiplying.
Normally if you finish your juice within 5days of opening it. The preservatives in it will keep the yeast and other microbes population low, esp in the fridge with low temperature. But if you had taken a break and let the juice stay in the fridge for 10+days or you had taken it from outside the fridge with the near freezing temp, the yeast will proliferate and make alcohol.
Also you dont need special grape juice or any juice to make alcohol. Just sugar and water. The sugars can come from any fruit, or even flour/barley (beer), sugarcane (rum), potatoes(vodka) or rice(sake).
when i worked at a natural grocery, we would frequently get juices and ciders that would start fermenting and puffing up, even awhile before their 'sell-by' date, and never opened. i brought more than a few home to fortify a little extra with honey and get them under an airlock.
I've had something similar happen with some apple juice from the store. One night I forgot to put it back in the refrigerator and left the bottle on the counter until the next morning, then put it back in the fridge. Few days later I went to pour another glass and noticed it was fizzy, so I smelled and tasted it.
It actually was pretty good; considering it was made from a $1 bottle of apple juice
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