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. 

    一个苹果被切开或被碰伤时，进入苹果受伤组织的氧就会与其中的一些化合物发生反应，使之变成褐色。当氧存在于苹果组织的细胞中时，叶绿体（含有叶绿素和进行光合作用的地方）中的多酚氧化酶，会迅速地将天然存在于苹果组织中的酚类化合物氧化为邻醌。然后，这些无色化合物就会自我组合为聚合物，或与氨基酸或蛋白质发生反应，在苹果组织内生成一些深色的次级产物。

    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.) 

    我们可通过减轻多酚氧化酶的氧化作用或减少能够附着这种酶的基质数量，来抑制这种酶的褐色化反应。给新鲜苹果切片涂上糖或糖浆，可减缓氧扩散，并借此放慢褐色化反应速度。柠檬汁或菠萝汁都含有抗氧化剂，也能抑制变色反应。此外，这两种水果汁都属酸性，它们所产生的低pH值也能降低多酚氧化酶的活性。虽然加热会改变苹果的组织结构，但是它能减除多酚氧化酶的活性--如果将苹果放在沸水中煮上四、五分钟，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. 

    酶的褐色化反应并非单单出现在苹果身上。几乎所有的植物组织都含有多酚氧化酶，多酚氧化酶还存在于细菌、动物和真菌类植物身上。事实上，多酚氧化酶的褐色化反应并非总是多余的：茶、咖啡和可可常见的暗色调，就是在产品加工处理期间，通过多酚氧化酶的褐色化反应添加上去的。