They’re also a good source of copper, magnesium, potassium and manganese. Sour cherries contain vitamins A, B, C and K and lots of fiber. The supplement is also available in powder form. The cherries can be boiled to extract the pulp, and then mashed. Tart cherry juice often has added sugar for sweetening. They are available dried, juiced or frozen. Tart cherries are also known as Montmorency, dwarf or sour cherries.