It is well known that brownning of rinded apple is brought about by the action of phenol oxidase upon some phenolic substances, precursors of colored substances, in fruit. Some procedures of prevention for brownning of apple were so far studied through inhibiting the activity of enzymes with several inhibitors or by losing touch with air. However, another procedure will be possible. If natural substrates of phenol oxidase are chemically modified, they can not be oxidized, preventing the formation of brown color. SP (a mixture of sodium sulfite and ortho phosphoric acid adjusted to pH 5.5) was examined to prevent brownning of rinded apple, since it was established to counteract with tetrodotoxin by reducing OH group in a molecule. Fujii's method for estimation of tyrosine with a-nitroso-p-naphthol was modified. The procedure was as follows. Into 3 to 5 ml of sample solution, 5 ml of saturated α-nitroso-β-naphthol solution, 2 ml of 2.5 N HNO_3, 3 ml of 20% HCl and H_2O were added to a total volume of 15 ml. The mixture was heated at 90℃ for exact 2 minutes and rapidly cooled with running tap water. A colored solution was filled up to 20 ml with H_2O. The optical density was estimated at 517 mμ using a Hitachi spectrophotometer EPU-2 with cells of 10mm light pass. As shown in Fig. 1, optical density is proportional to the amounts of tyrosine until 0.9 mg with higher accuracy compared with Fujii's method. However, it was found that the estimation of tyrosine with α-nitroso-β-naphthol was disturbed by SP. This interference was attributed to the action of SP upon the reagent, as presumed from an observation that the maximum peak of spectrum of α-nitroso-β-naphthol at 380 mμ was diminished by addition of SP. An apple was rinded and cut into a several pieces. They were immersed in 2×10^-2 M SP for 10 minutes at room temperature and washed with water. Two pieces of apple which had been immersed into H_2O or 2×10^-2 M NaCl became brown within a hour, while SP-treated piece of apple did not discolor, as shown in Fig. 3. However, non-discolored pieces were obtained, if surface of pieces was rinded thin. Finally new surface of all three pieces became again brown on standing for a while. These observations indicate that brownning of rinded apple and its prevention by SP take place on surface of apple. A piece of rinded apple was quickly homogenized with H2O in a Waring blender for 30 seconds and filtered by suction. Apple juice obtained became brown soon. However, non-discolored juice was obtained if apple was extracted with SP. Using this colorless juice with SP as control for colorimetry, a rate of brownning of apple juice could be quantitatively estimated by Hitachi electric colorimeter with blue filter. Thus, it was found that the intensity of color of apple juice increased proportionally to time for 30 minutes and reached to approximately constant value after 60 minutes. By means of the procedure established, some bleaching of brown juice of apple with SP was estimated. When brown apple juice was mixed with SP, its optical density was decreased, although extinctive alteration of color could not be established by viewing the juice. Furthermore, bleaching of juice by SP was not complete. Brownning of apple juice was prevented by SP in a final concentration of 2~4×10^-4M under the condition examined. On the other hand, prevention of discoloration of rinded apple fruit was attained by immersing it into 1 M SP for a few minutes or into 1~2×10^-2M SP for 5 minutes, even it was washed with water after immersion. Prolonged immersion of apple rendered to reduce the concentration of SP employed. Using our colorimetric method for estimating sulfite, remaining amounts of SP in juice or fruit was examined. SP was decreased within apple juice or more quickly on surface of apple. Amounts of SP remained was reduced by decreasing immersion time or concentration of SP and by washing fruit. Whole apple was immersed into SP and amounts of SP within a fruit was also estimated. It was found that most of SP were estimated in outer part of apple and the minimal amounts were detected in middle part. At any rate, amounts of SP remainded in juice or fruit was established to be less than limited amounts permitted of addition into food by the Low in Japan, 30 mg per kg, after treatment with SP for prevention of brownning of apple.