Inhibitory mechanism of sodium hexametaphosphate on enzymatic browning in yellow alkaline noodles.

[Display omitted] • SHMP inhibited the browning of yellow alkaline noodles. • SHMP inhibited PPO activity by chelating Cu2+ in the active center of PPO. • SHMP inhibited browning by hampering auto-oxidization of quinone. • SHMP reduced brown polymers by decreasing the pH value and converting melanin into light-colored substances. The effect and mechanism of sodium hexametaphosphate (SHMP) on polyphenol oxidase (PPO) enzymatic browning in yellow alkaline noodles (YAN) were investigated. The browning degree and PPO activity in YAN or PPO solutions decreased with the SHMP concentrations increased. Variations in pH values (pH 7–8.5) adjusted by HCl or acetic acid slightly affected the PPO activity, but SHMP inhibited PPO activity more pronounced. The inhibition of SHMP on PPO activity was irreversible. SHMP formed coordinate covalent bonds with Cu2+ to make PPO inactive. HPLC analysis revealed that SHMP reduced the browning products and led to the color of PPO-catechol systems being lightened. Furthermore, SHMP inhibited browning by hampering the auto-oxidization of intermediate products due to the change in pH value. Besides, the HPLC chromatogram, UV–vis spectrum, and mass spectrometry revealed that SHMP could convert melanin (m / z 248.97, 723.5, and 836.58) to light-colored substances (m / z 137.11). [ABSTRACT FROM AUTHOR]