注記 |
A rinden fruit of astringent variety of kaki (Japanese persimmon; Diospyros Kaki Thunberg) was immersed into SP (sodium sulfite solution adjusted its pH to 5.5 with ortho phosphoric acid) as with apple fruit and white precipitate was produced on surface of fruit . The precipitate is insoluble in water, methyl alcohol, ethyl alcohol, ether, acetone and acids, while it is soluble only in alkali. A few amount of precipitate was also produced by immersing it in water for longer time, but could not be obtained from a fruit of sweet variety even through SP treatment. In a mixture of 6% tannic acid and 1 M SP (1:1 in volume), white precipitate was also formed. However, it is soluble in alcohol, ether and acetone too. Sodium sulfite can be determined colorimetrically with 0.04% rosanilin in 6% HCl and 0.2% formaldehyde by estimating optical density of red-violet solution at 580 mμ. Color intensity of sulfite, however, was decreased by addition of tannic acid, suggesting that sulfite is decomposed by reaction with tannic acid. Infra-red absorption spectra of above precipitates were estimated and compared with those of tannic acid and kaki-tannin. In the spectrum of the recipitate formed from tannic acid and SP, an apparent shift of the absorption at 1310 cm^-1 to 1360 cm^-1 was observed, although other absorptions at 750, 870, 1020, 1085, 1200, 1450, 1540, 1620 and 1710 cm^-1 were not moved. Thus, it was suggested that tannic acid was precipitated by the action of SP at [aromatic ring]-O. On the other hand, the spectrum of precipitate obtained from astringent kaki was compared with that of soluble kaki-tannin, "diospyrin", estimated by Ito. Regarding the spectra, 7 absorptions were determined at corresponding positions between approximately 1000 and 1600 cm^-1. However, notable decreases in absorptions at 1440 and 1350 cm^-1, the main and the second absorptions of kaki-tannin, were recognized. From these observations, it was presumed that the precipitate may be derived from kaki-tannin by the action of SP at OH groups. Thus, a principle of a procedure to remove astringent taste from kaki was obtained by treatment of rinded fruit with SP which resulted in the formation of insoluble derivative from soluble kaki-tannin. Since precursors of colored materials in apple as well as kaki-tannin belong to polyphenol group, other compounds of this group are supposed to be susceptible to the action of SP. If it is the case, the efficiency of SP in food processing should be extended in addition to prevention of brownning of apple and removing astringency of kaki. Therefore, the action of SP on naringin (5, 7, 4'-trihydroxyflavanone-7-rhamnoside), bitter substance in Natsudaidai or grapefruit, was also briefly examined. A typical polarogram of naringin which has the main half-wave potential at -1.7 volt was obtained when it was estimated in a mixture of 5 ml 10^-1M naringin-ethanol solution, 5 ml McIlvaine buffer (10^-1M citric acid -2×10^-1M Na_2HPO_4) of pH 6 and 5 ml H_2O. On the other hand, the reduction wave having the half-wave potential at -1.45 volt was determined in a mixture of naringin, buffer and Na_2SO_3, while half-wave potentials were estimated at -0.63 and -1.15 volt with Na_2SO_3. When naringin-ethanol solution was adjusted its pH to 6 with ortho phosphoric acid instead of McIlvaine buffer, the distinctive polarogram could not be determined. However, half-wave potential was checked at -1.5 volt on a mixture of naringin-ethanol and Na_2SO_3 and those at -0.40 and -1.27 volt on Na_2SO_3 solution. Polarogram of an aqueous solution of naringin was also estimated with the half-wave potentials at -1.67 volt in the main and at -1.43 volt in the minor. The shift of half-wave potential to -1.35~-1.40 volt and appearance of abnormal wave between -0.8 and -1.15 volt were observed by addition of Na_2SO_3, depending on amounts of sulfite added. Furthermore, polarogram of naringin, Na_2SO_3 or their mixture could be estimated when the solution had been adjusted to pH 4 with phosphoric acid. At any rate, the change of the half-wave potential of naringin through addition of sulfite may be attributed to the reaction between naringin and sulfite.
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