yeast. Group of fungi that ferment sugars. Yeast is a source of beta-glucan, which is a good antioxidant. Yeasts are basically fungi that grow as single cells, producing new cells either by budding or fission (splitting). Because it reproduces well, Saccharomyces cerevisiae is the organism that is most widely used in biotechnology. Nevertheless, some forms of yeast are human pathogens, such as Cryptococcus and Candida albicans.

In relation to skin, there is limited information about how Saccharomyces cerevisiae may provide a benefit. Live yeast-cell derivatives have been shown to stimulate wound healing (Source: Archives of Surgery, May 1990, pages 641–646), but research about this is scant. Much of what is known about yeast’s effects for skin is theoretical, and concerns yeast’s tissue-repair and protective properties (Source: Global Cosmetic Industry, November 2001, pages 12–13) or yeast’s antioxidant properties (Source: Nature Genetics, December 2001, pages 426–434). As a skin-care ingredient yeast has potential, but what its function may be or how it would affect skin is not fully understood.

yerba mate extract. Used for the preparation of the most popular tea-like beverage in South America. It has anti-inflammatory and antioxidant properties (Sources: Fitoterapia, November 2001, pages 774–778; and Life Sciences, June 2002, pages 693–705).

ylang ylang. Fragrant, volatile oil that can also be a skin irritant. See volatile oil.

yogurt. There is no research showing that yogurt is effective when applied topically on skin. Yogurt consumption may negatively impact those with atopic dermatitis (Source: The Journal of Dermatology, February 2003, pages 91-97).

yucca extract. Plant extract that can have anti-inflammatory benefits.