Folin-Denis assay

Determination of Total Phenolics Folin-Denis Assay... 

The Folin-Denis assay is used as a procedure for the quantification of total phenolics in plant materials, food, and beverages. Reduction of phosphomolybdic-phosphotungstic acid (Folin-Denis reagent) to a blue-colored complex in an alkaline solution occurs in the presence of phenolic compounds (Folin and Denis 1912). 

According to Joslyn and Goldstein (1964), the Folin-Denis method, published in 1912, was first intended for the measurement of tannins in wine and whiskey, and then used for the assay of tannins in fruits (Swain and Hillis 1959 Craft 1961). Various methods for phenolic compound analysis have been reported (Joslyn and Goldstein 1964 Hartley 1987 Karchesy 1989 Hagerman 1989), but the Folin-Denis method is still very useful for the measurement of phenolic compounds in fruits. 

As mentioned before, the amount of soluble tannin that causes astringency in persimmon fruits is usually estimated visually by the tannin print method and can be measured quantitatively by the Folin-Denis method. There is also a protein precipitation method for the measurement of soluble tannins (Hagerman and Butler 1978). In that method, the soluble tannin content is assayed by the addition of the sample to a standard solution of protein and the isolation of insoluble tannin-protein complexes. The complexes are dissolved in alkaline solution, to which ferric chloride is added. The absorbance of the solution at 510 nm is measured. 

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