Apple juice begins to ferment between 12 hours after production and a few days, if kept in a sterile container in sufficiently warm weather. The fermentation process continues for between seven days and a few months, depending on the temperature of the surroundings.
Apple juice is often allowed to ferment to produce hard cider or vinegar. The fermentation process of apple juice can be measured with a hydrometer. As the fermentation process continues, a layer of carbon dioxide forms over the liquid and protects it from spoilage. The greater amount of air exposed to the juice, the faster the fermentation process transpires.