During the ripening process fruit acids are degraded, the sugar content increases and the sugar/acid ratio reaches a higher value.
A high level of acidity - common in under-ripe fruit - makes the fruit taste sour. Conversely, over-ripe fruits have very low levels of fruit acid and therefore lack characteristic flavor.
For lemons, a higher level of acidity is particularly important. Lemon varieties have between 5-7% (mostly citric acid), compared with around 1% in oranges.
Crop Nutrition and Citrus Fruit Acidity
Nitrogen and potassium increase fruit acidity, whereas calcium and copper will reduce acidity. Other nutrients have little effect.