The Asian citrus fruit known for its sharp, sour flavor is called the kumquat. Kumquats are small, oval-shaped citrus fruits that are native to East Asia. The fruit's skin is edible and sweet, while the flesh inside is tart and acidic, creating a contrast of flavors. The sourness of kumquats is often described as intense and tangy. The fruit is commonly consumed raw, and its zest and juice can be used in various culinary applications, such as desserts, marmalades, and savory dishes.