How do you keep pears from turning brown?

because of polyphenol oxidase which reacts on pear for sometime.

Polyphenol oxidase (PPO) enzymes, also known as polyphenoloxidases, are able to catalyze the transformation of an array of aromatic compounds that have two adjacent phenolic groups on them. This includes a number of polyphenols in plants that act as antioxidants. These copper-containing enzymes oxidize the phenolic groups to reactive oxygen molecules known as quinones, which continue reacting with each other and other cellular factors to form brown spots known as melanin. This browning causes the deterioration of fruits and vegetables, resulting in large economic losses. Examples include the pear

Enzymatic browningis not unique to pear. PPO - a mixture of monophenol oxidase and catechol oxidase enzymes - is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi. In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.Tentoxin has also been used in recent research to eliminate the polyphenol oxidase activity from seedlings of higher plants. Enzymatic browning is not unique to Apples. PPO - a mixture of monophenol oxidase and catechol oxidase enzymes - is present in nearly all plant tissues, and can also be found in bacteria, animals, and fungi.