Persimmon tannin

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. 

Thorough reviews of fruit phenolics have appeared recently (Deshpande et al. 1986 Macheix et al. 1990), so only a brief history of the identification of persimmon tannins will be given here, together with some qualitative differences among the tannins in different fruit types. 

---

Early chemical studies of persimmon tannin were often done with insufficiently purified tannin fractions. It was reported in 1923 that the elemental formula of persimmon tannin might be C14H20O9, and in 1962, it was proposed that the major component of persimmon tannin was leucodelphinidin-3-glucoside. Further studies done by Itoo et al. showed that persimmon tannin might have a more complex... 

Fig. 1. Presumed repeat in the structure of persimmon tannin. (Matsuo and Ito 1977b)...

The basic structure of persimmon tannin seems to be, as proposed by Matsuo and Ito (1977b, 1978), a proanthocyanidin polymer. Some differences in the constituents and chemical properties of persimmon tannin among the four fruit types as classified in terms of astringency have been reported. Nakabayashi (1971) foimd three patterns of low-molecular-weight polyphenols (one in PCNA fruit, one in PVNA fruit and one in PVA and PCA fruit) the components were different. Yonemori et al. (1983) reported differences observed in the amounts of catechin and gallic acid in tannins extracted from immature PCNA fruit and the immature fruit of the three other types. 

The differences in the chemical properties of persimmon tannins between immature PCNA fruit of the cultivar Fuyu and immature PVA fruit of the cultivar Hiratanenashi were further investigated by exclusion chromatography (Yonemori and Matsushima 1984). Tannins from the former cultivar were generally of lower molecular weight than those from the latter (Fig. 2). Fuyu tannins tended to coagulate more slowly in the presence of acetaldehyde vapour than those from Hiratanenashi, indicating that polymerization occurred less readily. 

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. 

Tanaka, T. et al., Chemical evidence for the de-astringency (insolubilisation of tannins) of persimmon fruit. J. Chem. Soc. Perkin Trans. /3013, 1994. 

Matsuo, T. and Itoo, S., A model experiment for de-astringency of persimmon fruit with high carbon dioxide treatment in vitro gelation of kaki-tannin by reacting with acetaldehyde. Agric. Biol Chem. 46, 683, 1982. 

Taira, S., Ono, M., and Matsumoto, N., Reduction of persimmon astringency by complex formation between pectin and tannins. Postharvest Biol Technol 12, 265, 1998. 

Tannins arc found in various parts of the plant, appearing frequently in leaves, and in llie cortical lissues of stems. Tannins may be found in the walls of cells or in the vacuoles often their presence causes the cell to appear dark-colored. Many fruits, such as the persimmon, contain large amounts of tannin, especially before they are ripe. Wound tissues, and especially the hypertrophied tissues known as galls, which result from the... 

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. 

Swain T, Hillis WE (1959) The phenolic constituents of Prunus domestica. 1. The quantitative analysis of phenolic constituents. J Sci Food Agric 10 63-68 Van Buren J (1970) Fruit phenolics. In Hulme AC (ed) The biochemistry of fruits and their products, vol 2. Academic Press, London, pp 269-304 Yonemori K, Matsushima S, Sugiura A (1983) Differences in tannins of non-astringent and astringent type fruits of Japanese persimmon (Diospyros kaki Thunb.). J Jpn Soc Hortic Sci 52 135-144 (in Japanese with English Summary)... 

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

Fig. 3. Changes in results seen in tannin prints of freshly cut surfaces of Hiratanenashi persimmon at five stages of post-harvest treatment with ethanol vapour for removal of astringency. Top Before treatment bottom at the end of treatment + + + strongly astringent — not astringent (by sensory evaluation)...

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. 

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...

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. 

Jiro. The method for the extraction of insoluble tannins from the fruits of nonastringent (PCNA) persimmon needs improvement, if tannin biosynthesis during fruit development is to be studied further. 

Ben-Arie R, Zutkhi Y, Sonego L, Klein J (1991) Modified atmosphere packaging for long-term storage of astringent persimmons. Postharvest Biol Technol 1 169-179 Deshpande SS, Cheryan M, Salunkhe DK (1986) Tannin analysis of food products. CRC Grit Rev Food Sci Nutr 24 401-449... 

Kato K (1984) The condition of tannin and sugar extraction, the relation of tannin concentration to astringency and the behaviour of ethanol during the de-astringency by ethanol in persimmon fruits. J Jpn Soc Hortic Sci 53 127-134 (in Japanese with English Summary)... 

---