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. 

    一個蘋果被切開或被碰傷時，進入蘋果受傷組織的氧就會與其中的一些化合物發(fā)生反應，使之變成褐色。當氧存在于蘋果組織的細胞中時，葉綠體（含有葉綠素和進行光合作用的地方）中的多酚氧化酶，會迅速地將天然存在于蘋果組織中的酚類化合物氧化為鄰醌。然后，這些無色化合物就會自我組合為聚合物，或與氨基酸或蛋白質(zhì)發(fā)生反應，在蘋果組織內(nèi)生成一些深色的次級產(chǎn)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.) 

    我們可通過減輕多酚氧化酶的氧化作用或減少能夠附著這種酶的基質(zhì)數(shù)量，來抑制這種酶的褐色化反應。給新鮮蘋果切片涂上糖或糖漿，可減緩氧擴散，并借此放慢褐色化反應速度。檸檬汁或菠蘿汁都含有抗氧化劑，也能抑制變色反應。此外，這兩種水果汁都屬酸性，它們所產(chǎn)生的低pH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(jié)構(gòu)，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、五分鐘，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. 

    酶的褐色化反應并非單單出現(xiàn)在蘋果身上。幾乎所有的植物組織都含有多酚氧化酶，多酚氧化酶還存在于細菌、動物和真菌類植物身上。事實上，多酚氧化酶的褐色化反應并非總是多余的：茶、咖啡和可可常見的暗色調(diào)，就是在產(chǎn)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。