A 50g tub of Precipitated Chalk (Calcium Carbonate, also known as E170) by Young's.
Used to reduce acidity in wines. The addition of 7g per gallon will lower the acidity by about 1.5 ppt.
The liquid will start to foam because gas is given up by the chalk as it neutralizes some of the acid. So be sure to do this in a vessel from which some of the juice has been removed, to allow adequate headspace for the foam.