. . .but oranges do not?
So it doesn’t go bad. Ever leave an orange out for a week or two, until it turns green? That’d be your mold growing.
It’s just like the human body. Once you take the skin off, you gotta watch out for infection.
Notice how a peeled banana, or a peeled potato, or a peeled apple, turn black after a while? Oxidation my friend! Fruits have a protective covering for a reason. Remove it, and they rot.
It will start to ferment.Natural yeasts in the air and the juice will start this process and you will end up with a liquid which has CO2 bubbles in it and with a slight yeast flavour. This will even happen to frozen juice when you defrost it.one reason for pasturising juice.
Man, orange juice gets nasty if you pour some in a cup and leave it overnight. Kinda cool looking, since it separates from the water, but gross.
OK, we’re on the right track–but these answers seem to address exposure to the air vs. temperature. Suppose I took the juice and sealed it in a vacuum-packed container. Would I still have to refrigerate it?
If you managed to do so without introducing any fungal spores or bacteria (some of which don’t mind being in an airless environment, or positively thrive that way) then yes, it would.
Oh, hang on; enzymes -I forgot about enzymes which might be naturally present in the fruit nd would cause it to break down of it’s own accord.
That’s pretty much the idea behind aseptic packaging, such as is used for Parmalat milk and juice boxes.
I don’t know offhand if it has been used for orange juice, and I don’t really feel like Googling it right now…
Perhaps it doesn’t need to be refrigerated, at least if you plan on drinking it soon. I have a couple Romanian office mates who regularly bring two-litre cartons of orange juice, drink some, and then leave the cartons on their desks overnight. They drink the rest over the next couple days. They do this despite the fact that there is a perfectly functional fridge across the hall.
Perhaps the orange juice is resistant to spoilage because of its high sugar content and acidity, and perhaps because it has been pasteurized.