Determination of free diferulic, disinapic and dicoumaric acids in plants and foods.

Hydroxycinnamates are common phenolic compounds of plants and plant foods, often found in substantial quantities. Due to their high in vitro antioxidant activity they can easily be oxidized under oxidative conditions. In this study, we found that in vitro oxidation of coumaric, ferulic and sinapic acids resulted mainly in dimeric compounds. We hypothesized that these dimers are present in plants and plant foods not only in their bound form but also as free acids that can be extracted from non-hydrolyzed samples. By applying sensitive UHPLC-MS/MS method, we were able to identify and quantify four free hydroxycinnamic acid dimers for the first time, namely 8-8'-disinapic, 8-5'-diferulic, 8-O-4'-diferulic and 8-3'-dicoumaric acids, in wheat sprouts, Chinese cabbage, millet sprouts, light beer and parsley. Concentrations of dicinnamates in plant tissues ranged from 0.05 to 2.8 μg g(-1) DW and the monomer:dimer ratio ranged from 2 to 850.