Mushroom polyphenoloxidase (PPO) partly unfolded by dynamic high pressure microfluidization (DHPM) are more sensitive to higher concentration of citric acid than native mushroom PPO, which is useful for inactivation of mus hroom PPO.After DHPM treatment, mushroom PPO can be inactivated by adding less citric acid.The difference in activation and conformation changes of native and partly unfolded mushroom PPO after citric acid treatment was measured with UV spectrophotometer, far-UV circular dichroism and fluorescence emission spectra.