Phenols are defined as aromatic alcohols, that is, having a hydroxyl group (OH) directly attached to the aryl ring. A monophenol contains one hydroxyl group; polyphenols contains more than one group. In addition, the group includes tocopherols and flavonoids in plants or their derivatives, tyrosine and DOPA derivatives in animal products and additives such as propyl gallate in foods1. Such a diverse group generates interest from many aspects: antioxidant, astringency, bitterness, color, browning reactions and protein constituents are some examples.
Phenols can be considered as the major antioxidants in both plant derived products and food (there are some exceptions such as carotene). The process of oxidation in biological systems affects both our health and quality of life. Antioxidants work by various mechanisms such as a) decreasing localized oxygen concentrations, b) preventing chain initiations by scavenging initiating radicals, c) binding catalyst to prevent initiating radical generation, d) decomposing peroxides before they can be reconverted to initiating radicals and e) chain breaking to prevent further reactions of active radicals2.
With phenols involved in so many processes, being able to estimate the amount of these compounds present can be helpful. This is no easy task, due to the diversity of the group. The method of choice that is used today is based on one published by Otto Folin and Vintila Ciocalteu in 19273. It is a UV-VIS method, based on colorimetric measurements. Total phenol content determined by Folin-Ciocalteu method has historically been used in the wine industry, but has been extended to many other samples such as fruit juices, plant tissue and wood components. The samples tested have been referenced against many compounds including tannic acid, gallic acid, catechin and tyrosine. The most widely used reference standard today is gallic acid, and results are reported in milligrams gallic acid equivalents or GAE per liter.
At NOW we use Folin-Ciocalteu reagent to analyze for total polyphenols in materials like Yerba Mate, Green Tea, Grapeseed Extract and Mangosteen. This is a tried and true method that has withstood the test of time, and is one of the more reliable means of quantitating this group of compounds, ensuring that our products meet label claims.
1 V.L. Singleton, R.Orthofer and R.M. Lamuela-Ravenos,) Methods in Enzymology, 299, Pg 152 (1999)
2 H.J.D Dorman, A. Peltoketo, R. Hiltunen and M.J.Tikkanen, Food Chemistry, 83, Pg 255 (2003)
3 O. Folin and V. Ciocalteu, J. Biol. Chem., 73, Pg 627 (1927)