Different authors in same food use different standards to analyse TPC and TFC. Why is Gallic acid and Catechin frequently used as standard, respectively for TPC and TFC? Other phenolics used as standards for TPC and TFC analysis include retinol, vitamin C, ferulic acid e.t.c. Some people say the major phenolic compound of the food in question has to be used as the standard but this is not easy to know as one has to do a complete HPLC identification and quantification of the possible phenolic compounds before deciding.
Gallic acid esters, such as tannins, catechin gallates and aliphatic gallates are potent antioxidants in vitro. However, gallic acid itself also appears to have antioxidant, anticarcinogenic and antiangiogenic activity in vitro. It is used as a standard for determining the phenol content of various analytes by the Folin-Ciocalteau assay; results are reported in gallic acid equivalents.
Dear Habtu Shumoy
In the in vitro assays, the sample is reacted with with some agents like FCR. the oxidation of the dye results with a complex whose concentrations are measured in terms of absobance. Phenolic or flavonoid contents are determined by their reducing capacity in comparison to standard phenolics or flavonoids. These contents are not actually gallic acid or quercetin, etc. These assays are actually the preliminary study where we get an idea of the type of phytocontituents may be possible. If there is a high content of these contents, we can estimate by HPLC or HPTLC with some major compounds or isolate the fraction which has maximum contents.
Thank you @Jatinder! it is a nice explanation. in our lab there was a time that we used qercetine instead of catechin as a standard for TFC analysis and the result were higher than TPC. TFC > TPC (gallic acid as standard for TPC). can you tell me how to explain this. Normally TFC is only part of TPC so it shouldn't be higher than TPC?
Any of the known standards could be used, but it is important to mention the particular standard used for the sake of reproducibility.
Hi Habtu,
Thank you for raising very important question. It is probably a question of many people who are working on phytonutrients.
I will humbly open up a follow-up question for fellow researcher why the in vitro antioxidant assay will not produce consistent result among them. I was expecting that the antioxidant activity of plant extract will give uniform result as measured by different assays (DPPH, FRAP, and ABTS). However, in one of my experiment, I found that the value that was increasing uniformly with DPPH assay, showed irregular trend when measured by FRAP and ABTS. why such an irregularity is happening??
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