If you want to consume fruit juice after several days:
- the more it tastes fermented, the more likely it will give you
- the more it is carbonated / bubbly from fermentation, the more it can cause indegestion.
- if you boil it or put alcohol in it, it will be a bit safer, but it can still cause indegestion.
- the fruit substrate bacteria aren't notably beneficial especially in volume.
- having a sip is not very noxious unless you have a weak constitution.
Having a small amount of natural fermentation of non-sugary vegetables or couscous provides the digestive system with beneficial bacteria the digest veg and carb inside you, but sugary food bacteria are not metabolically stable in teh gut except for special fermentation granules like fruit kefir... which can make naturally bubble lemonade, unfortunately, if you drink too much fruit kefir, then your entire gut carbonates too much and causes gas issues.
Fermented food varies depending on the pH and substrate. For example, lactose has evolved in animals because it could not be digested by all the microbes that could previously digest glucose/galactose which is found in plants. So milk is often used as a substrate for biological cultures which are fairly symbiotic with humans.
Fruit is normally subject to funghi and microbiota that are not particularly dangerous to humans like meat is, because they don't particularly thrive in the human body, but they can give you indegestion and the runs/dysentery.