Citrus fruits, such as lemons, oranges or grapefruits, possess a characteristic tartness due to their high levels of citric acid, with a pH between 1.8 and 3.3. Alternatively, melons such as the honeydew, fall closer to the middle of the scale with pH levels between 6.00 and 6.67.
Generally speaking, most fruits fall on the acidic side of the pH scale, meaning their pH is less than 7 on a scale from zero to 14. Some fruits, such as limes, are extremely acidic and as such are considered unpleasant to eat by themselves. However, some fruits, such as the cantaloupe, can sometimes be slightly alkaline with a pH of approximately 7.13.
Additionally, acidity varies based on the variety and preparation of any given fruit. For example, the acidity of apples varies considerably based on the variety. The relatively mild Red Delicious apple has a pH of 3.9 while the Jonathan apple has a pH of 3.33. This variance creates a perceptible taste difference, making the Jonathan taste significantly more tart than the Red Delicious.
Additionally, cooking or canning fruit can have an impact on its acidity. A grapefruit can have an acidity of 3.0 when fresh, but when canned is likely never to fall lower than 3.08. Some juices, such as prune juice, decrease their acidity when removed from the fruit. Prune juice has an acidity of 3.95 compared to the prune's pH of 3.63.