Oxidation/food chemistry
May 12, 2022 3:25 PM   Subscribe

Will chopped potatoes turn brown faster in air, or submerged in cool water? And why?

Bonus: is it the same for apples? Most fresh/raw plant foods? Are there examples that go opposite ways?

I know about other ways to delay or prevent it etc., mostly interested in this as written. Thanks!
posted by SaltySalticid to Food & Drink (11 answers total)
 
They will turn brown much faster in air. As your title says, this is oxidation, which is a reaction with oxygen. There's much less oxygen available for the reaction in water.
posted by ssg at 5:03 PM on May 12 [6 favorites]


SSG has it. I'm trying and failing to think of anything where it would work in reverse. Putting things in water might cause other problems to happen faster, but not oxidation.

For non-food stuff, there are obvious examples, like sodium metal. That's because it can be oxidized by water, not just oxygen. I don't see a way to analogize that insight to any food substance, or at least not one that any carbon based life form would want to eat!
posted by mark k at 5:23 PM on May 12 [1 favorite]


And as far as I know, all fruit and vegetables that turn brown when cut do so due to oxidation (caused by the same enzyme), so I don't see any reason that any of them would turn brown faster in water than in air.
posted by ssg at 5:52 PM on May 12 [1 favorite]


Response by poster: Ok thanks all. Follow up then: why does iron rust faster when wetted? Or am I somehow wrong about that too‽ I've certainly found tools that rusted when I forgot them out in the rain, while the dried tools inside have not.

If it also oxidizes from water in addition to oxygen, then what is it that property that some metals have and food stuff doesn't? (I'm embarrassingly ignorant of basic chemistry like this, even though I know a lot of other science, thanks for bearing with me)
posted by SaltySalticid at 6:03 PM on May 12


Rust was the obvious counter-example I was thinking of too.
posted by LionIndex at 6:08 PM on May 12 [1 favorite]


It's all oxidation, but browning of fruit has to do with melanin formation, so a pigment. In contrast, rust requires three separate things to form: iron + water + oxygen = hydrated iron oxide. Tarnish (on silver) and verdigris (on copper) are also caused by oxidation.

I thought this was a decent overview.
posted by gemmy at 6:29 PM on May 12 [3 favorites]


rust requires three separate things to form: iron + water + oxygen = hydrated iron oxide.

Oxygen is in water itself.

I was guessing that for whatever reason, oxygen atoms prefer: to bond with iron (or sodium, potassium, etc) rather than their two hydrogen buddies in water, but oxygen in air prefers its current partners (other oxygen atoms or carbon probably); while oxygen in air prefers whatever's in a veggie (quite possibly carbon!) over its current partner, while oxygen in water is quite happy with hydrogen, otherwise plants would have a really hard time staying hydrated. But why is that?
posted by LionIndex at 6:48 PM on May 12


Response by poster: Ok thanks, gemmythat clears up what's happening in the foods.

It seems food oxidation is a very specific thing with PPO and melanins, while metal oxidation is more of a direct property of elemental metals thing, but it too has lots of differences between different metals and compounds. Some of them go faster with water, some don't, it probably requires serious learning about redox reactions to understand why silver tarnishes differently than iron rusts or aluminum forms its hard oxide shell. Wikipedia page on Tarnish tells me that tarnish oxidation is self-limiting (unlike rust) and seems to imply that it does not involve water, and may require or be greatly accelerated by hydrogen sulfide. But then that metal oxide may react with water…

Is that basically right?
posted by SaltySalticid at 6:58 PM on May 12


Per google, rust is apparently a specific type of iron oxide that includes water and gives it that flaky quality.

I know that the reason aluminum doesn't "rust" is not that it doesn't oxidize, but rather that the oxide remains attached to the solid and you end up with a few atoms' worth of oxide layered on top that prevents the bulk of the metal from the even being exposed to oxygen. The physical properties of the oxide go a long way to determining what we'll actually observe macroscopically.
posted by mark k at 7:22 PM on May 12


Best answer: Rust actually happens in water and requires water, which is why in very dry environments, things take forever to rust — there just isn't much water in the air, so nothing rusts. Basically, a little bit of iron dissolves into water and reacts with dissolved oxygen from the air and the water itself to form rust. Here is a more detailed explanation.

Oxidation is actually a very broad term for whenever something loses electrons in a chemical reaction. It was originally used to refer to losing electrons to oxygen (oxygen is very eager to accept electrons because of the structure of its atoms), but now refers to any loss of electrons. Many, many reactions involve oxidation, not just reactions that produce metal oxides, browning or things of that nature. Here is an explanation.

What's happening when potatoes oxidize is far more complex than the kind of oxidation that happens to iron to produce rust. Rust is a pretty simple electrochemical reaction, potatoes browning is a much more complex biochemical one. They aren't really the same thing at all.
posted by ssg at 7:38 PM on May 12 [6 favorites]


Response by poster: Right, my big error was not properly considering the vast breadth of oxidation reactions, and failing to note that iron oxide rust is also a decent outlier compared to the other metals.
Thanks again all!
posted by SaltySalticid at 9:23 AM on May 13


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