Soluble tannins measurement

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

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. 

The average polymer stereochemistry may also be deduced from measurement of the specific rotation of a solution of the polymer in water (25) at 578 nm. The measurement may also be performed in methanol, methanol-water (1 1, v/v), or acetone-water (1 1, v/v), which are useful supplementary solvents for less soluble tannins. The value of [alsvg for a particular polymer is similar for any of the above solvents (L. J. Porter, unpublished observations). At 578 nm the specific rotation of pure freeze-dried epicatechin or epigallocatechin homopolymers (which... 

Quantitative analysis of tanning products (raw materials and extracts) requires firstly rational sampling and then suitable preparation of the sample and solution, and includes mainly determinations of the total soluble matters, the tannins and non-tannins, water and insoluble substances.1 Other determinations sometimes made are those of the ash, sugar and sulphurous anhydride, and in industrial practice the specific gravity and colour of the solutions are often measured. 

The ability of tannins to precipitate water-soluble proteins is the main activity by which various naturally occurring polyphenols are defined as tannins. Several methods for measurement of tannins based on these activities have been developed (Bate-Smith 1973 Porter and Woodruffe 1984 Hagerman 1989). Methods for the assay of relative astringency (RA) and relative affinity to methylene blue (RMB) have been used frequently in pharmacological studies for the measurement of the tannin content of plant extracts (Okuda et al. 1985). 

Almost all of the tannin in the fruit of astringent Hiratanenashi seems to be in soluble form (Taira et al. 1990). The amount of insoluble tannins in non-astringent Jiro fruits could not be measured, due to difficulty in the extraction of the insoluble tannin (Taira et al., unpubl.). 

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