When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. O-quinones then produce the well documented brown color by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers. 

    yigepingguobeiqiekaihuobeipengshangshi
，jinrupingguoshoushangzuzhideyangjiuhuiyuqizhongdeyixiehuahewufashengfanying，shizhibianchenghese。dangyangcunzaiyupingguozuzhidexibaozhongshi
，yelvti（含有葉綠素和進行光合作用的地方）中zhong的de多duo酚fen氧yang化hua酶mei，會hui迅xun速su地di將jiang天tian然ran存cun在zai於yu蘋ping果guo組zu織zhi中zhong的de酚fen類lei化hua合he物wu氧yang化hua為wei鄰lin醌kun。然ran後hou，這zhe些xie無wu色se化hua合he物wu就jiu會hui自zi我wo組zu合he為wei聚ju合he物wu，或huo與yu氨an基ji酸suan或huo蛋dan白bai質zhi發fa生sheng反fan應ying
，在zai蘋ping果guo組zu織zhi內nei生sheng成cheng一yi些xie深shen色se的de次ci級ji產chan物wu。

    In the home kitchen enzymatic browning can be prevented by either reducing PPO oxidation activity or lowering the amount of substrate to which the enzyme can bind. Coating freshly cut apples in sugar or syrup can reduce oxygen diffusion and thus slow the browning reaction. Lemon or pineapple juices, both of which naturally contain antioxidants, can be used to coat apple slices and slow enzymatic browning. In addition, both fruit juices are acidic and the lower pH that they bring about causes PPO to become less active. Heating can also be used to inactivate PPO enzymes; apples can be blanched in boiling water for four to five minutes to nearly eliminate PPO activity. (Be warned that cooking will affect the texture of the product.) 

    womenketongguojianqingduofenyanghuameideyanghuazuoyonghuojianshaonenggoufuzhezhezhongmeidejizhishuliang

，laiyizhizhezhongmeidehesehuafanying。geixinxianpingguoqiepiantushangtanghuotangjiang
，kejianhuanyangkuosan，bingjiecifangmanhesehuafanyingsudu。ningmengzhihuoboluozhidouhanyoukangyanghuaji，yenengyizhibiansefanying。ciwai，zheliangzhongshuiguozhidoushusuanxing，tamensuochanshengdedipH值也能降低多酚氧化酶的活性。雖然加熱會改變蘋果的組織結構，但是它能減除多酚氧化酶的活性--如果將蘋果放在沸水中煮上四
、五分鍾
，差不多就能消除多酚氧化酶的活性。

    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. 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. 

    酶的褐色化反應並非單單出現在蘋果身上。幾乎所有的植物組織都含有多酚氧化酶，多酚氧化酶還存在於細菌、動物和真菌類植物身上。事實上
，多酚氧化酶的褐色化反應並非總是多餘的：茶、咖啡和可可常見的暗色調，就是在產品加工處理期間，通過多酚氧化酶的褐色化反應添加上去的。