The role of polyphenol oxidase and peroxidase in the browning of water caltrop pericarp during heat treatment

► High PPO and POD activities led to browning in water caltrop. ► Enzymatic browning is responsible for discolouration of caltrop. ► Food quality of light-coloured surface can be preserved during postharvest. The mechanism of browning involving enzymatic browning was investigated in the pericarp of water caltrop, an Asian vegetable popular for its taste and medicinal properties. Polyphenol oxidase (PPO) and peroxidase (POD) activities were determined in pericarp at various times and temperatures. Water caltrop consisted of 44.22% moisture content, 37.23% crude fibre, and 2.63% crude protein. PPO and POD activities dropped from 62 and 38units/g sample, respectively, as water temperature was increased from 30 to 80°C. Optimum pH and temperature for PPO activity was at pH 5.0, 25–45°C, and POD activity peaked at 60°C. High PPO and POD activities at 40–50°C resulted in degradation of phenolic compounds, which led to increased aggregation of browning pigments and discolouration (lower L-values) of the pericarp. Enzymatic browning was determined as the major factor in the browning discolouration of heat-treated water caltrop pericarp.