Alpha-hydroxy acids are naturally occurring acids that are derived from plant sugars. Alpha-hydroxy acids are often used in cosmetics to increase skin cell turnover, exfoliate dead cells from the surface of the skin, and improve the skin's ability to maintain moisture. Alpha-hydroxy acids are also synthetically manufactured and used in applications ranging from food to medical to industrial use. There are six types that are naturally derived from three main sources: sugar cane, milk and fruit.
Glycolic acid has the smallest molecules of all the alpha-hydroxy acids. It is colorless, odorless and water soluble. Glycolic acid is naturally derived from sugar cane, but is synthetically produced by several manufacturers. Glycolic acid is commonly known for its use in cosmetic and haircare products, but it is also used in higher concentrations to clean concrete, industrial equipment, metal, and water.
Lactic acid is actually derived from soured dairy products, as well as fermented vegetables and fruit. Lactic acid is also naturally occurring in humans and animals and is associated with muscle energy usage. It was commonly thought that lactic acid caused muscle fatigue; however, as noted by the New York Times in a May 2006 article, evidence has existed since the 1970s indicating lactic acid may actually provide energy to fatigued muscles. Cosmetically, lactic acid---also known as milk acid---is suggested for consumers with sensitivity to other alpha-hydroxy acids.
Tartaric acid is found in unripe grapes and is the primary active ingredient that gives wine its distinct taste. Commercially, tartaric acid is found as a white powder and is often used in the food industry to give foods a sour taste.
The highest level of naturally occurring citric acid is found in oranges and lemons; however, it also occurs as part of the metabolic process in most living creatures. Citric acid is an antioxidant and also a natural preservative. It is often used in the food industry to add a sour taste to drinks and foods.
Malic acid is found in several fruits and vegetables, but is most associated with apples and cherries. Malic acid also occurs naturally as part of the metabolic process in humans and animals. It is often used to enhance flavors in beverages, candy, confections, and in medicinal products like lozenges.
Mandelic acid is an alpha-hydroxy acid derived from bitter almond. Mandelic acid has antibacterial properties. It has been used medicinally as a treatment for urinary tract infections and as an oral antibiotic. It is also used cosmetically as an anti-aging and acne treatment.