AHAs most commonly used in cosmetic applications are typically similar to those found in food products including glycolic acid (found in sugar cane), lactic acid (found in sour milk), malic acid (found in apples), citric acid (found in citrus fruits) and tartaric acid (found in grape wine) — though the source of the AHAs in cosmetics is predominantly synthetic or from bacterial or fungal fermentations. For any topical compound to be effective, including AHA, it must penetrate into the skin where it can act on living cells. Glycolic acid, having the smallest molecular size, is the AHA with greatest bioavailability and penetrates the skin most easily; this largely accounts for the popularity of this product in cosmetic applications.[citation needed]