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?


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?

In: 3

Because yeast isn’t the only small creature that likes to grow in a sweet medium. Making wine out of fruit juice requires strict sanitation/sterilization.

I don’t think that’s true. If you leave apple juice out and it spoils, you can’t then ferment it and magically all the bad bacteria is gone. There is just alcohol and bad bacteria. This is why people who brew alcohol spray everything with with Star San.

Proper fermentation involves preparing the medium by restricting all other unwanted microbes and allowing yeast (naturally occurring or added by the winemaker) to convert sugar into alcohol. Once fermentation into alcohol has occurred to a certain extent, the result is somewhat toxic to most microbial growth and will keep for a long time. (this is why alcohol is a very good disinfectant)

Randomly allowing apple juice to spoil is rather unreliable as there is a good chance that the juice spoils through unwanted microbial growth rather than yeast fermentation.

To just simplify what others have said:

Let apple juice spoil: let all the “bad” germs in and around the juice have a shot at colonizing the juice. Drink it, you drink the “bad” germs.

Turn apple juice into alcohol: Turn into alcohol by killing all the “bad” germs that are currently in the juice, and then convert the sugars in the juice into something that’ll keep the “bad” germs out in the future (alcohol).

As a historic perspective, we took the first process, and slowly discovered what we could do to increase the odds that the process would result in clean alcohol. Learned how to kill the “bad” germs. Learned how to keep the “good” germs (yeast) that do the converting for us. etc, etc.

Turning it to alcohol is controlled spoilage. Like turning milk to yogurt or cheese, or making saurkraut.

And yes it can spoil further becoming unsafe to eat.

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.

Alcohol is toxic to microbes, so it works great as a preservative to stop things from going bad.