What makes grapefruit juice so tart?

Grapefruit juice is tart primarily due to its high levels of citric acid and the presence of other organic acids, such as ascorbic acid (vitamin C). These acids contribute to the juice's sour flavor profile. Additionally, the specific compounds in grapefruit, including flavonoids and limonoids, enhance its tartness and overall taste, making it distinctively refreshing yet tangy.