The most obvious sign that the juice has spoilt is the smell. A potent, sour, or fermented smell is your cue to throw it in the compost heap! Another telltale sign is a change of color. If the fresh juice has gone brown, it means it has oxidized, and there's a high chance the juice has gone rancid. An increase of 'bad' bacterias the reason why fruits and vegetables go rotten. A simple way to minimize bad bacteria is to make sure you wash all of your fresh produce thoroughly before using it. This will help your juice to last longer.