Grape Juice

How to Acquire
Put Grapes into a Winepress use it. This will put grape juice in the press (shown as a bar); two full pressings will fill the press with juice.

With an empty bucket selected, right-click the press to fill the bucket with juice.

Use
Grape juice is placed in barrels to ferment into Wine or, if left in too long, Vinegar.

If you add more grape juice later, the fermentation doesn't reset but averages.

Source:
 * Loftar wrote: Nope, it's not reset; the time it will take for the fermentation to finish is the average of the time for the fresh and the half-fermented grape-juice to complete, weighted by the amount of each. In other words, if you have a barrel with Ae liters of grape-juice which will take De units of time to ferment, and put in An liters of fresh grape-juice which will take Dn units of time to ferment, the time it will take for the mixture to complete will be ((De * Ae) + (Dn * An)) / (Ae + An).

Quality
The quality of grape juice depends on the grapes used and is soft-capped by the quality of the winepress