Among apple varieties, the word acidic is used to describe the flavor of the fruit, and not its actual pH. Over 2,000 apple varieties are currently available, and among them only a few cultivars are considered very acidic (also expressed as tartness). As new varieties are introduced, check with your local extension service for evaluations on the acidity of new cultivars and whether they are suitable for your area.
The tough green skins of Jersey Mac apples are blushed with pink or red hues, and they encase acidic, white flesh inside. The acid flavor isn't so intense that it cannot be eaten out of hand, but it does make this cultivar less popular than sweeter varieties. It ripens early, and is often sold as a placeholder for its sweeter parent apple, the McIntosh.
Akane is a Japanese-produced acidic apple with dark red skin and almost hard flesh that was introduced to the U.S. in the 1970s. The acidic flavor lessens the longer the apple is left on the tree, and some growers sweeten the apple by leaving it longer. However, the tart flavor stands up well in cooking. Use Akane apples quickly since they do not store well.
The NJ-109 apple is an experimental apple similar in appearance to one of its parent apples, the Golden Crisp, but carries little to none of Golden Crisp's sweetness. The genetic cross lines are rather complex, and also include Crown Prince Rudolf and James Grieve. NJ-109 is an early-fruiting variety with a strong acidic flavor.
Of the popular apple varieties sold in U.S. markets, Granny Smith is at the top of the tart, acidic chart. The tartness is an aspect that makes Granny Smith apples popular for those who enjoy sour apples or for the best flavor expression in cooked dishes, such as apple pie or apple cobbler. Granny Smith apples are a late-season, green-skinned cultivar from Australia.
The yellow-skinned, yellow-fleshed, disease-resistant Sir Prize apple variety is very acid-tart when first picked. While not prized as highly as a cooking apple as Granny Smith, the flavor mellows enough after a short period of storage to be more palatable out of hand.