eli5: Why does apple juice expire if you leave it for a month but when you turn it into Alcohol it is okay to drink it even if it aged for months?
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Apples growing on a tree are naturally colonized on their surface by yeast. When you press the apples into juice, the yeast suddenly gets access to all the sugar in the juice and starts turning it into alcohol almost immediately. Alcohol is poisonous for most organisms (including the yeast itself; at ca 15% alcohol it stops working), and therefore the resulting cider keeps for months without spoiling – the alcohol kills any mold or other unwanted microorganisms.
Apple juice you buy at the store has been pasteurized, killing most of the yeast, giving it an actual shelf life as apple juice, and not as cider in the making. But when you open it, all kind of microorganisms get in, including mold. After some time (depending on whether you cooled it), the mold will have multiplied enough to pose a health hazard way more serious than the hangover you get from alcohol.
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