The World Federation One Stop Fiqh

Ruling 196

Grape juice becomes unlawful to drink if it bubbles either by heating or by itself [through fermentation]. If grape juice bubbles so much that two-thirds of it reduces and only one-third of it remains, it becomes lawful to drink. Furthermore, if it is established that the grape juice is intoxicating, as some [jurists (fuqahāʾ)] have said with regard to when it bubbles by itself, it can only become lawful to drink if it turns into vinegar. As mentioned in Ruling 110, grape juice does not become impure by bubbling unless it turns into wine.