Fall is here, and you know what that means!! Apple pie, apple cobbler, baked apples, caramel-coated apples…mmm. At the store this week I couldn’t resist a bag of bright green Granny Smiths – I’m a sucker for the pucker. Apple crisp was my recipe of choice. I tossed this batch with a light syrup of lemon juice and agave nectar, included some frozen rhubarb for a little color, stirred in a handful of crunchy walnuts, and topped with a sweet and buttery oats crumble. Tasted great with vanilla ice cream after dinner, and was even better this morning with a side of Greek yogurt.
No matter the apple recipe, almost always on the ingredient list is lemon juice. We all know if you coat sliced apples in lemon juice and they won’t brown. Why? Enzymatic browning. The enzyme polyphenol oxidase (PPO) is the culprit responsible for initiating browning in apples. Whether sliced, diced, or bitten, PPO is activated when the fruit cell wall is damaged and the tissue exposed to oxygen. You know PPO is at work when you spot that unappetizing brown color. Treating apples with lemon juice (or vinegar, if you dare) controls PPO activity by lowering the tissue pH and inhibiting enzyme activity. (My college textbooks remind me that this enzymatic browning reaction is “highly specific to ortho-diphenolic compounds” like catechol. But that ramble seemed excessive for this particular post. If you’re interested in more detail, post your question and I’ll respond.)
A Food Science tid-bite: PPO doesn’t always display an undesireable effect in food. In fact, the reaction is used in tea and coffee production to give that characteristic dark brown color.
Source: Genevieve L. Christen, ed and J. Scott Smith, ed. Food Chemistry: Principles and Applications. Science Technology System, 2000.