Perhaps the title should be: Why don't all fruits containing phenol residues go brown quickly when left in the fridge?

Bananas go brown over time because of the oxidation of phenol residues.

Bananas go brown quicker in the fridge than left at room temperature. This seems to contradict the first point since the reaction should be slower at colder temperatures. Dialogue from the naked scientists briefly explains why my line of thinking might be wrong.

Emily: Well it’s a good question and the answer is that they will brown faster in the fridge. It’s mainly due to the formation of ice crystals, so if you put your banana in the fridge, the ice crystals grow, and they actually rupture the cells of the banana skin. This releases an enzyme called polyphenol oxidase and as the name suggests it acts to oxygenate phenols, which have a ring-like structure, into quinones, and these quinones can then all join together or polymerise and produce a black, brown, or red pigment called polyphenol and this is what gives it the brown colour. So, if you have your banana in the fridge, this will occur and you'll get a brown banana. But another interesting thing about bananas is that if you have them in the fruit bowl, they'll release ethene and this will make the other fruits in the bowl ripen faster because it’s a ripening hormone.

Chris: So, the question is though, if this is a chemical reaction making this brown pigment, if you slow down the reaction by lowering the temperature, then it should happen more slowly, therefore, going in the fridge should make the bananas go black slower?

Emily: I can see why you're thinking that but actually, it’s more the rupturing of the cells. So if you have a banana on your table, the cells are going to be intact, the enzymes contained, and the reaction is not happening at all whereas if you put in the fridge, the enzyme is released and the reaction can happen – although yes, it might happen slower.

At the risk of channeling people into a yes or no answer, could someone clarify for me if the following statement is true (maybe with a peer reviewed source which I've had no luck finding): bananas go brown quickly in the fridge because cell lysis caused by ice crystal formation at cold temperatures in the fridge, and this releases Polyphenol oxidase (PPO) freely causing more browning.

This leads me to another seeming paradox: Why doesn't this rapid browning in cool temperatures happen for other fruits and vegetables? What is special about bananas?

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    $\begingroup$ That is a good question indeed, but funding bodies are likely to be industrial, and they generally don't care much about writing papers, and often choose to go for patents instead. $\endgroup$ – AliceD Nov 20 '15 at 3:24
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    $\begingroup$ @AliceD... I am going to flag you for using the There is no money in basic research argument. I don't know that I buy Emily's argument completely. Most fridges will not go below 4°C. I have actually observed this phenomena in my freezer and seen the brown bananas (Makes really good banana bread that way), but I just don't see ice crystals forming before you get to 0°C. There has to be something else going on. Maybe the enzyme is more stable at colder temperatures? Seems counter intuitive, but maybe it is an adaptive response to frost. $\endgroup$ – AMR Nov 20 '15 at 3:38
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    $\begingroup$ Maybe it is the confined space, Ethene concentration is allowed to build up to higher concentrations in the enclosed space of the fridge. Try this experiment. Seal a banana in an airtight plastic bag at room temp in the dark, maybe with a slightly damp sponge to simulate the humidity in the fridge, leave one out, put one in the fridge, and one in the freezer and compare after 24 hours. $\endgroup$ – AMR Nov 20 '15 at 4:32
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    $\begingroup$ @AMR- I may have been unclear. What I meant was that I think chances are slim basic scientists have bothered with it. Food industries may have. $\endgroup$ – AliceD Nov 20 '15 at 5:15
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    $\begingroup$ @AliceD NIH is sponsoring programs for scientist to learn to market their science to industry... The US thought we lost a lot of ground when we defunded stem cell research... Wait until they see the kind of science we start to do when we are expected to sell it. $\endgroup$ – AMR Nov 20 '15 at 8:46

What's interesting with this one is we don't really know very well the mechanism behind what's called chilling injury. It happens to a range of fruits, like bananas, peaches, avocado, or apples. The belief is that the chilling alters membrane permeability to storage vacuoles inside the plant cells. Try On Food and Cooking, pp.269, and Puig et al. (2015) for references here.

Now the membrane permeability to the storage vacuole changes (it gets leaky), and inside these vacuoles are phenolic compounds. In the cytoplasm of the plant cell, there's an active enzyme called polyphenol oxidase (PO). The reactions it catalyzes end up with a compound called melanin, a brown/black pigment:

enter image description here

Fig A. An overview of what a PO might do.

So the phenolic compounds leak out and PO goes to work catalyzing reactions that start building up the dark pigments associated with browning. Being said, I can't at this moment find a good paper that says "look, this is how the entire process happens," but I'll be attempting to read a little deeper into the matter.

Keywords: flesh browning, enzymatic browning, chilling injury, polyphenol oxidase, storage quality, storage conditions


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