Earlier this year, the US Department of Agriculture (USDA) approved genetically modified apples - "Arctic Grannies" - that do not express at high levels the enzyme PPO when they are sliced open and so, purportedly do not turn brown, these Grannies stay whiter than white or greener than green, depending on your perspective.
Now, our dog is a malophile, loves apples. Well actually, she is a yellow Labrador retriever and will eat almost anything. This week, however, when our backs are turned she has taken to plucking the low-hanging fruit from the apple tree in our garden. My wife caught the little malophile red-pawed and harvested for the humans in the family the few apples that were left. She then proceeded to juice them and, not surprisingly, the liquor took on the familiar brown hue of apple exposed to the air and having undergone enzymatic browning through the action of that aforementioned enzyme PPO (polyphenol oxidase).
It's not an attractive color, so, on a whim and with a little chemical savvy she added some ascorbic acid (vitamin C) along with the sugar to the pan in which she was simmering the brew. The addition of the vitamin C visually reversed the browning and the pan carried on gently bubbling with fifty shades of green...not even a hint of beige to be seen. As a lowly chemist, I'd always assumed that the oxidative and enzymatic degradation of apples to that brown color was an irreversible biochemical process. Obviously, you can use lemon or lime juice to stave it off (ascorbic, anti-scurvy, acid, you see?) but once it has gone over to the brown side, I'd assumed (in fact, we both assumed) that that was it.
A quick web search gave a few vague hints. A research paper from the journal HortScience entitled "Enzymatic Browning, Polyphenol Oxidase Activity, and Polyphenols in Four Apple Cultivars: Dynamics during Fruit Development" from a team at the University of Santa Catarina in Brazil who had this to say:
"Enzymatic browning is one of the most important reactions that occur in fruits and vegetables, usually resulting in negative effects on color, taste, flavor, and nutritional value. The reaction is a consequence of phenolic compounds' oxidation by polyphenol oxidase (PPO), which triggers the generation of dark pigments. This is particularly relevant for apples, which are rich in polyphenols and highly susceptible to enzymatic browning."
As plant secondary metabolites, phenolic compounds produce colors, astringency, flavor, and have nutritional qualities in fruits and vegetables. Now, I should have known, and maybe I did in my chemical youth, that these compounds are perhaps acting as indicators of oxidation state and thus, as with many other indicator compounds will exist in equilibrium and thus oscillate between colors (or colorless states) depending on the concentrations of other chemicals (acids, alkalis) present in the mix. But, I wasn't sure. Further web searching revealed that various people who put sliced apple in their kids' school lunchboxes discovered that a sprinkling of vitamin C protected the slices from browning, and at least one "soccer mom" discovered that she could reverse the browning, even after the fruity snack had been ignored in the lunchbox for a whole day...making it fine for eating as an enforced, after-school snack for "Junior"...
There are countless web pages with tips for malophiles with the knowledge to keep their beloved fruit of life from turning brown. And, of course you could opt for those GM apples that apparently never turn brown, once the regulators and the market perish the thought of leaving them on the shelf, as it were.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".