Effect of pH on the browning of the model system, "apple enzyme-chlorogenic acid", was investigated by estimating the absorption spectrum or the variation of the optical density of the reaction mixture at various pH. Above pH 5, the browning of the system was brought about and the optical density in the visible range of the wavelength was increased. However, the spectra without absorption peak were determined, except that the spectrum at pH 8 has a shoulder at 500 mμ. The maximum increase of the optical density was observed at 390 mμ, 386 mμ and 382 mμ at pH 5～6, 7 and 8 respectively. On the other hand, in the ultraviolet range of the wavelength, the peculiar spectra were observed according to pH of the reaction mixture. However, the uniform variation of the optical density was established, which indicates the maximum increase at 263 mμ and the maximum decrease at 325 mμ. Then, the time course of the browning of the system at various pH was also estimated. The browning was the highest at pH 5～6 for 4 hours, while it was higher at alkaline range after 24 hours; the highest browning being at pH 8. When sodium chloride was added to the system, browning was the highest at pH 6 for a short time, although no browning was observed at pH 4 differing from the system of "apple enzyme-catechol." In this case too, the higher browning was observed at alkaline range after 24 hours. In connection with the browning of apple, O_2 uptake by polyphenol oxidase as well as the browning was comparatively estimated in the presence of anti-browning substance such as sodium sulfite, cysteine or sodium chloride. Although the browning was retarded by sodium sulfite or cysteine, the enzyme was inhibited by the former, but not by the latter. Notable effect was not shown by sodium chloride on the browning and the enzymatic activity. Thus, as suggested by the preceding study, it was demonstrated that the course of the enzymatic browning of chlorogenic acid was divided into two steps, the enzymatic oxidation of chlorogenic acid and the non-enzymatic polymerization of the oxidized product to melanin.
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