Determination of sulfite remaining in fruits

In order to estimate amounts of remaining sulfite in fruits after treatment with SP (sodium sulfite solution adjusted its pH to 5.5 with ortho phosphoric acid), colorimetric method with rosanilin∙HCl-formaldehyde reagent was investigated. Into 10ml sample solution, 1 ml 0.04% rosanilin dissolved in 6% HCl and 1 ml 0.2% formaldehyde were added to develop red-violet color. Intensity of color was estimated by determining optical density of the solution at 580 mμ. Color development is rapid and reaches to constant value in a few minutes. However, this should be carried out at least at room temperature, since color intensity was decreased at 37℃. Usually SP had been employed for treatment of fruits with sulfite. Phosphoric acid had not notable influence on color development. Thus, as indicated in Fig. 3, proportionality of optical density at 580 mμ to concentration of sulfite was established. In addition, it was observed that apple juice does not disturb the estimation of sulfite, except that color intensity of sulfite added into apple juice is decreased as judged from standard curve with Na_2SO_3 solution. This decrease might be attributed to the reaction of sulfite with some native substances in apple, as supposed from protection of brownning of apple with SP. Thus, rosanilin∙HCl-formaldehyde reagent was proved to be useful for the determination of sulfite in fruit and juice of apple. On the other hand, juice of astringent variety of kaki (Japanese persimmon), even without sulfite, developed blue-violet color having the absorption maximum at 590 mμ and produced precipitate. However, the juice of sweet variety of kaki as well as tannic acid developed red-violet color as with sulfite, but not produced precipitate. Owing to these coloration, it was concluded that kaki disturbed the colorimetric method of estimating sulfite with rosanilin∙HCl-formaldehyde reagent. In addition, it was observed that blue-violet coloration and precipitation were brought about only with flesh astringent kaki. When fruit of astringent variety was naturally matured, the color developed was red-violet as with sweet variety. Similar red-violet coloration was also recognized with astringent kaki which had been treated with alcohol or dried to remove astringency. Then, polarography of sulfite was investigated to find out the suitable method estimating sulfite in kaki. A mixture of 6 or 3 ml 10^-3M Na_2SO_3, 5 ml McIlvaine buffer (10^-1M citric acid～2×10^-1M Na_2HPO_4) of pH 3 and 4 ml or 7 ml H_2O was employed for estimation of polarogram and the typical one of sulfite having half-wave potential at -0.7 volt was determined. Furthermore, it was observed that wave height has the direct relationship with concentration of sulfite. As shown in Fig. 10, the standard calibration curve of sulfite was obtained from wave height of polarograms with various concentrations of sulfite. When the mixture was acidified with phosphoric acid in stead of McIlvaine buffer, polarogram of sulfite was also determined. However, a maximum wave was detected in the polarogram, although it has half-wave potential at -0.671～-0.70 volt. Using the condition just established, at pH 3 with McIlvaine buffer, the distinct reduction wave of sulfite in juice of kaki was estimated, while definite polarogram could not be determined when it was estimated at pH 6 or 8. Some weak reduction waves of constituents in kaki were checked, whereas no waves were recognized in polarogram of apple juice. At any rate, it was established that juice of fruits of kaki and apple does not disturb the polarographic estimation of sulfite which should be carried out at pH 3 adjusted with McIlvaine buffer.