alpha hydroxy acid found in things like apples and grapes. It was first isolated in apple juice, and we find a lot of it in unripe fruits, which is what gives those fruits the tartness we associate with lack of ripeness.
Alpha hydroxy acids are found in three categories according to the number of hydroxy groups. AHAs like glycolic acid are monocarboxylic acids, AHAs like malic and tartaric acid are dicarboxylic acids, and AHAs like citric acid are tricarboxylic acids. All three groups will behave like AHAs - making skin seem smoother, reducing the look of fine lines, possibly making pores seem smaller - but the latter two groups tend to be quite harsh on our skin, leading to side effects like rashes or sensitivity.
How do we use it? As a pH adjuster and nothing else. The Cosmetics Ingredient Review has determined that malic acid is only safe as a pH adjuster, and that there is "insufficient data to support other uses", such as use as an AHA. It is considered "safe with qualifications", and as such, we shouldn't be using malic acid in our products as anything but a pH adjuster. (If you'd like to see the official reports, click here (PubMed) or click here (CosmeticsInfo.Org) for more information.)
Join me tomorrow for more fun with fruit extracts - banana!