Malic acid will lower pH less than will tartaric acid. Malic acid is the acid found in apples and is the best choice for adjusting ciders. Malic is less sour than tartaric acid and can also be used to make acid adjustments in Rieslings, Gewurztraminer, and Muscat varieties, though many winemakers still prefer to use tartaric acid in these situations. Don't add malic acid to any wine that will undergo malolactic fermentation because only half of this malic acid will convert to lactic acid during malolactic fermentation.
3.4g per gallon adjusts acidity by +.1%.