The term 'apple acid' refers to four different acids found in apples: malic acid, amino acid, boric acid, and oleic acid. Malic acid is a type of organic acid found in many fruits, including apples, and is responsible for the tart taste of the fruit. Amino acid is an essential nutrient found in apples that helps with muscle growth and repair. Boric acid is a naturally occurring mineral found in apples that helps to regulate pH levels. Finally, oleic acid is a type of fatty acid found in apples that helps to keep the fruit moist. All four of these acids are important components of apples and contribute to their flavor and texture.