Why do apples brown? Get in the know with PPO.

why does lemon juice stop apples from browning?

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.  (Note…we ate those Granny Smiths before I took a pic. The above are orchard apples. 😉 )

Why add lemon juice in apple recipes?

No matter the apple recipe, almost always on the ingredient list is lemon juice. (Or vinegar…bleh…swap that out for fresh lemon juice!)

We all know if you coat sliced apples in lemon juice they won’t brown. Why? Enzymatic browning.

Brown Apples and Polyphenol Oxidase

The enzyme polyphenol oxidase (PPO) is the culprit responsible for initiating browning in apples. (Go ahead…pull that one out at parties. Your food geek friends will come running!)

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.

Okay…so WHY does lemon juice stop apples from browning?

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…(Yea…maybe that one isn’t a party-pleaser…)

A Food Science tid-bite: PPO doesn’t always display an undesireable effect in food as it does with brown apples. You will also find the reaction used in tea and coffee production to give that characteristic dark brown color…and coffee is always good. 🙂   


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