Soluble tannins

Finally, the difference between the total soluble matter and the soluble non-tannins gives the soluble tannins or the tannin uncombined with the hide. 

Hagerman, A. E. and C. T. Robbins, Implications of soluble tannin-protein complexes for tannin analysis and plant defense mechanisms, J. Chem. Ecol., 13, 1243-1259 (1987). 

Astringency is a common phenomenon in many unripe fruits, but generally it disappears when the fruit ripens on the tree. There are some exceptions to this, including many varieties of persimmon and date and some of banana, that remain astringent even when horticulturally mature. Astringency is a result of high contents of soluble tannins in the pulp. Removal of astringency is necessary for the fruit to be edible. 

The strongly astringent taste of persimmon fruits arises from soluble tannins that accumulate in large specialized cells called tannin cells. In this chapter, some chemical properties of persimmon tannins, an outline of simple and reliable methods for estimation of the degree of astringency and the amount of tannins, and some aspects of the physiological changes in the tannin contents of persimmon fruits will be described. 

Fig. 2. Differences in separation pattern of soluble tannins from Fuyu (non-astringent) and Hiratanenashi (astringent) persimmon. (Yonemori and Matsushima 1984)...

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

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. 

During natural or artificial removal of astringency, soluble tannin can change, probably by polymerization or conjugation, into an insoluble form, which is not... 

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. 

Figures 8 and 9 show the results for Hiratanenashi and Jiro fruits, respectively. The growth curves of the fruits were double-sigmoidal in shape. The fruits of Hiratanenashi and Jiro reached harvest maturity in late October or the middle of November, respectively. In Hiratanenashi, the soluble tannin concentration in the flesh gradually declined during fruit development, and was about 1.0% when the fruit reached full maturity. In Jiro, the soluble tannin concentration decreased rapidly, starting in the middle of June, and the fruit became non-astringent in September. The amount of soluble tannins per fruit increased as the fruit developed and reached a maximum at the end of the second growth stage (slow enlargement phase) in both cultivars. The amount of soluble tannins that accumulated in Jiro fruit was much less than that in Hiratanenashi fruit.

Fig. 8. Changes in fruit diameter, peel colour, and soluble tannins of the flesh during fruit development in Hiratanenashi...

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. 

Okuda, T., Yoshida, T., Hatano, T., Ikeda, Y, Shingu, T. Inoue, T. (1986). Isolation of water-soluble tannins hy centrifugal partition chromatography, and hiomimetic s3mthesis of elaeocarpusin. Chemical and Pharmaceutical Bulletin, 34, 4075—4082. 

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... 

Nozilillo C, de Bezada M 1984 Browning of lentil seeds, concomitant loss of viability, and the possible role of soluble tannins in both phenomena. Can J Plant Sci 64 815-824... 

Another ascorbic acid derivative is a water-soluble tannin called elaeocarpusin, which was isolated from the leaves of the evergreen tropical tree Elaeocarpus sylvestris (Elaeocarpaceae). Elaeocarpusin is derived by condensation of corilagin (see Section 8.3.5.1.2, Figure 8-294) with dehydroascorbic acid. 

An abihty to bind proteins and form insoluble or soluble tannin-protein complexes. 

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