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.

• 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。