Although fruits get sweeter as they ripen, as starches change to sugars, some fruits are just more sour than others genetically. Citrus fruits contain citric acid. Lemons and limes contain more than oranges and watermelon . All fruits contain different types of acids. Grapes have tartaric. Apples have acetic and malic. Some of the acids turn to ketones and esters as the fruit ripens. It is the special combination of the organic acids and alcohols that gives the fruits their characteristic odors.(esters)