Folin-Denis method

Persimmon tannin in soluble form reacts with ferric chloride, forming tannin-Fe ion complexes, which are blue-black. We can estimate the degree of astringency in persimmon fruits by colour development during the reaction. 

5 ml of 1 N phenol reagent (Folin-Ciocalteu reagent), shake well 

1% of soluble tannin on a fresh weight basis. Fruits containing less than this amount are almost non-astringent when eaten (Kato 1984 Ben-Arie et al. 1991). 

The major drawback of the Folin-Denis method is that the reagent reacts with other phenolic constituents such as xanthine, proteins and amino acids. However, persimmon flesh usually contains small amounts of such substances, so the values obtained by the method probably reflect the amount of soluble tannins present. 

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Folin-Denis method Reduction of complex polymeric ions formed from phosphomolybdic and phospholungslic heteropoly acids to complex molybdenum-tungsten blue. detection wavelength 725 - 770 nm recommended for uniformity 765 nm complexes and reagent are unstable in alkaline solution, formation of precipitates, controlled sequence and timing of the addition of reagents (reproducibility ), deviation from Beer-Lambert law (high phenol contents), reaction is stoichiometrically predictable 105,106,110... 

Folin- Ciocalteu method See Folin-Denis. see Folin-Denis reagent commercially available alterations longer heating, addition of HCI and U2SO4 effects compared to Folin-Denis method no precipitates. absorption band is sharper and more symmetrical, more sensitive, slightly less color with other reductants 47,49,110, Ml... 

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. 

Fig. 4. Procedure for the measurement of soluble tannins (Folin-Denis method)...

Fig. 5. Calibration curves of gallic acid and (+)-catechin measured by the Folin-Denis method...

Analyze by the Folin-Denis method (see Fig. 4) measurement of total tannins... 

The hydrolysis of insoluble tannin with HCl solution can be used to measure the amount of total tannin (Manabe 1982). After insoluble tannins are hydrolyzed as shown in Fig. 6, total tannins can be measured by the Folin-Denis method. My group used this method in studying both PCNA and PVA fruit (Taira et al. 1990). 

Seasonal changes in fruit diameter, colour development of the peel (estimated from colour charts) and soluble tannins of persimmon fruits of the cultivars Hiratanenashi (PVA, a parthenocarpic type Fig. 7A) and Jiro (PCNA Fig. 7B) were studied ca.every 10 days at Tsuruoka City in the north of Japan. Soluble tannins in the fruit flesh were measured by the Folin-Denis method. In Tsuruoka, both cultivars usually bloom, starting early in June. 

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. 

Although several qualification and quantitation methods for tannins have been developed, only a few are specific towards hydrolyzable or condensed tannins. Among colorimetric methods, the Folin-Denis or Folin Ciocalteu reagents (for total phenolics), and vanillin-HCl and w-BuOH-HCl methods are widely used for quantification of condensed tannins [80]. 

The colorimetric method based on the reagents of Folin and Denis or of Folin and Ciocalteu has been generally preferred over other methods to determine total phenols in complex natural materials such as wines and fruits (1, 2, 3, 4, 5). This method is relatively simple, convenient, reliable, generally applicable, and it is accepted as an official analysis in several countries for total phenols in wines and a number of other products. Although it is a preferred method, it can be even better than is commonly recognized. 

Folin, O., and Denis, W., 1912b, Tyrosine in proteins as determined by a new colorimetric method, J. Biol. Chem. 12 245-251. 

A colorimetric method for determining mz/o-inositol has been described.104 The color is developed with the phosphomolybdotungstic acid reagent of Folin and Denis, after oxidation of the inositol with bromine. Enzymic methods, based on the inositol dehydrogenases of Acetobacter suboxydans44 and Aerobacter aerogenes,106 have also been used. These methods could be employed with any cyclitol readily attacked by the respective enzyme systems (see pp. 144 and 147). 

A colorimetric method for determination of uric acid was introduced by Folin and Denis (F5) this initial method, which used phospho-tungstate, was modified by a series of investigators (B16, B32, F4). Brown... 

Despite the overall limitations of these procedures in producing quantitative analyses, certain amino acids that contain appropriate functional groups were readily determined at considerably more sensitive levels by the use of colorimetric methods. Folin and Denis (1912) introduced such an approach to the measurement of tyrosine and tryptophan, and procedures for the determination of arginine (Sakaguchi, 1925), histidine (Koessler and Hanke, 1919 Kapeller-Adler, 1933), and phenylalanine (Kapeller-Adler, 1932) were subsequently reported. The highly reactive thiol group of cysteine has also... 

A further colorimetric determination of stilboestrol depends on the reduction of Folin and Denis reagent by the phenolic groups of the stilboestrol molecule the method is applicable to tablets and the following details are modifications of those originally published by Tubis and Bloom. ... 

A determination of vanillin in vanilla extracts is by Folin and Denis colorimetric method on which the details given below are based. The method is applicable in the presence of coumarin, which has often been used as an adulterant. 

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