Flavanols availability

The rolling or leaf maceration step is carried out in order to disrupt cell structure and allow contact between tea flavanols and tea polyphenol oxidase. The physical condition of the leaf mass must also facilitate oxygen availability. 

Finally, reactions of flavonoid and nonflavonoid precursors are affected by other parameters like pH, temperature, presence of metal catalysts, etc. In particular, pH values determine the relative nucleophilic and electrophilic characters of both anthocyanins and flavanols. Studies performed in model solutions showed that acetaldehyde-mediated condensation is faster at pH 2.2 than at pH 4 and limited by the rate of aldehyde protonation. The formation of flavanol-anthocyanin adducts was also limited by the rate of proanthocyanidin cleavage, which was shown to take place at pH 3.2, but not at pH 3.8. Nucleophilic addition of anthocyanins was faster at pH 3.4 than at pH 1.7, but still took place at pH values much lower than those encountered in wine, as evidenced by the formation of anthocyanin-caffeoyltartaric acid adducts, methylmethine anthocyanin-flavanol adducts,and flavanol-anthocyanin adducts. The formation of pyranoanthocyanins requiring the flavylium cation was faster under more acidic conditions, as expected, but took place in the whole wine pH range. Thus, the availability of either the flavylium or the hemiketal form does not seem to limit any of the anthocyanin reactions. 

A dual-electrode liquid chromatography-electrochemistry (LCEC) system used in the detection and identification of flavanols and procyanidins in wines and grape seeds is a valuable tool (30). Voltammetric behavior of phenolic compounds by LCEC could provide information that cannot be obtained using HPLC with UV detection, for which the identification is usually based on a comparison of the retention time with that of standard compounds, especially for the identification of catechins and procyanidins with a small amount of sample available (30). Figure 10 shows the procyanidins commonly found in wines. 

Compared with anthocyanins, the information available on stability of flavonols, flavanols and phenolic acids is limited. As these compounds until recently had only minor interests as food components, investigations were mostly performed for the purpose of biological screening and identification, less for studying stability during processing and storage. However, the mere definition of flavonoids and phenolic acids as antioxidants indicates that the compounds react easily and should be handled as unstable compounds. 

In general, by HPLC and UV-Vis detection it is possible to analyze some anthocyanin derivatives in wine, such as vitisins, several pyranoantho-cyanins and flavanol-ethyl-anthocyanin derivatives. On the other hand, due to the absence of standards commercially available, identification and quantification of these compounds is difficult. 

Langer S, Marshall LJ, Day AJ, Morgan MRA (2011) Flavanols and methylxanthines in commercially available dark chocolate a study of the correlation with nonfat cocoa solids. 

Flavanols are present in nature in a great diversity of plants. However, the data available in the literature about their cmicentrations in plants and foodstuffs regard... 

When discussing the biological activity of flavonoids in general, and flavanols in particular, there are some major factors to be considered bioavailabDity from food, matrix effects, absorption and metabolism in the gastrointestinal tract, tissue and cellular distribution after absorption, possible interactions and/or accumulation, which are the chemical form(s) biologically available to the cell/tissue and their potential metabolism at cellular level [3, 42, 69, 70]. 

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