Soluble tannins persimmon

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)...

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. 

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. 

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. 

Fig. 10A3 Changes in soluble and total tannins during post-harvest treatments for removal of astringenqr in Hiratanenashi persimmon. A Ethanol vapour treatment. B Carbon dioxide gas treatment...

Swain T, Hillis WE (1959) The phenolic constituents of Prunus domestica. I. The quantitative analysis of phenolic constituents. J Sci Food Agric 10 63-68 Taira S, Shimomura M, Oba S, Watanabe S (1990) Changes in soluble and total tannin content during fruit development and their relation to removal of astringency in Japanese persimmon Diospyros kaki Thunb.). Abstr XXIII Int Horticultural Congr, 27 Aug-1 Sept 1990, Firenze, vol 2, no 3279... 

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