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Malic acid is the acid naturally found in apples. When fermenting juice, it is important that the pH level is within the right parameters; too high and the cider is at risk of spoiling, too low and the yeast will be unable to ferment the sugar. You should aim for a pH of between 3.2 and 3.8.
This 100g tub of malic acid in powder form has full usage instructions on the label and will allow you to increase the acidity of your juice, if required.