[neilep (OP)]

Bananas get brown spots really easily don't they ?..even quicker if you squish one !!

Why's that then ?...and is it the fruit affecting the skin of the skin affecting the fruit ?

[Karen W.]

Thats a great question, I have heard the answer before, but do not recall the answer!!

[ukmicky]

I know the anwser, its because your leaving them too long before you eat them. Try it you'll see.

[neilep (OP)]

I know the anwser, its because your leaving them too long before you eat them. Try it you'll see.

LOL...and...DOH !!!

...an there I was thinking it was the Banana Fairies !!

[elegantlywasted]

A little bit of research has told me....

Physiological & Physical Disorders
Chilling injury. Symptoms include surface discoloration, dull or smokey anal color, subepidermal tissues reveal dark-brown streaks, failure to ripen, and, in severe cases, flesh browning. Chilling injury results from exposing bananas to temperatures below 13°C (56°F) for a few hours to a few days, depending on cultivar, maturity, and temperature. For example, moderate chilling injury will result from exposing mature-green bananas to one hour at 10°C (50°F), 5 hours at 11.7°C (53°F), 24 hours at 12.2°C (54°F), or 72 hours at 12.8°C (55°F). Chilled fruits are more sensitive to mechanical injury. 

Skin abrasions. Abrasions result from skin scuffing against other fruits or surfaces of handling equipment or shipping boxes. When exposed to low (<90%) relative humidity conditions, water loss from scuffed areas is accelerated and their color turns brown to black. 

Impact bruising. Dropping of bananas may induce browning of the flesh without damage to the skin. 

This is From the masters of bananas at http://postharvest.ucdavis.edu/Produce/ProduceFacts/Fruit/banana.shtml

I would suggest checking out the entire site, its got some interesting info on ripening bananas

[Karen W.]

I knew about cold temps as I put them in frezzer to save for bananbread, they turn totally black, but they peel away with a perfect fruit  for baking bread.. Bruising yes I know that one two, as well as over ripening.. I did not think about water loss. But as I read that I realize it is similar to a pumpkiin in that if you do not damage the outer skin of a pumpkin it will keep for upwards on a year without turning to mush, kept in a even temperature etc. If you even nick the skin, the deterioation process begins and it decomposes rather quickly then.. I take it it is similar to the banana in that respect.

[WylieE]

Enzymes present in fruits, mainly polyphenol oxidase cause the browning in damaged fruit.  Normally polyphenol oxidase works in plants as a defense against insects.  When activated this enzyme turns phenols in the plant into quinones, these quinones then turn into melanins which have beneficial properties to the plant- antibacterial, anti-fungal, and UV protection (another good reason to eat your fruits and veggies).   Melanins are dark in color and so make the areas where they are present appear brown.

In healthy plant tissue the phenolic compounds are stored in the vacuole, well separated from the polyphenol oxidase enzyme so no activity happens.  However, when a cell is smashed, cut, or otherwise tortured, the separating membranes are ruptured and polyphenol oxidase can access the phenolic compounds and start the process of turning them into quinones and then into melanins.

Fortunately there are some ways to stop this reaction. 
1.  Removal of oxygen-  polyphenol oxidase requires oxygen to work so keeping the oxygen out helps to keep the reaction from occurring.  This can be done in a couple of ways- putting the fruit under water (depending on the type of fruit – I can't imagine that would make a tasty banana) or using antioxidants- a common one is vitamin C, found in high levels in lemon juice, so squirting some lemon juice on cut fruit helps to keep it from turning brown (again, I just can't imagine that this would help a banana- that taste combination doesn't sound too appealing). 

2.  Removal of copper- polyphenol oxidase also requires copper to work.  However, many of the agents that sequester copper probably aren't very safe to be putting on food, since we need those minerals too.

3.  Heat- Heat will kill the enzyme- that's why blanching fruit helps to keep its color, but it destroys some of the nutritional value.

4.  Low temperature (for chilling tolerant plants)- Low temperature will slow down the enzyme and for plants that can stand low temperatures this will delay browning.  However, some plants, such as bananas are chilling sensitive and putting them at low temperature does about the same thing to the membranes as smashing them, so again that won't help with keeping bananas from turning brown- it will actually make it worse (as you already know).

5.  Dehydration- without water the enzyme also slows down, but the dehydration process needs to happen quickly to avoid membrane rupture due to dehydration that will again cause browning.  Dehydration is a good choice when you can't keep fresh fruit around because the nutritional value of dried fruit is fairly close to fresh fruit (you will lose some of the vitamin C).

When possible if you rip plants instead of cutting them (lettuce is the best example, not so easy to rip apples) you are less likely to rupture the cells, because the cell walls are pretty tough- so you get less browning. 

Of course this process isn't always a bad thing, tea, coffee, and cocoa wouldn't be the same without browning.

[chris]

Nice answer. Good work.

[WylieE]

Aww, Shucks.  Thanks.  I just find plant biology apeeling (groan).

[chris]

It's nice to find someone who can "sort the wheat from the chaff", or even "see the wood for the trees" whilst getting down "to grass roots level" in this "field".

[WylieE]

Thanks Dr. Chris,

 I had always hoped my budding interest in plant biology would bear fruit. 

 I guess a good approach is to get to the root of the problem and not get distracted and start "barking up the wrong tree".

 My love of plant science stems from a the fact that most life on earth depends on photosynthesis and that results from someone who is "oustanding in the field" could really branch out and  make a significant contribution to mankind from environmental preservation to "sowing the seeds" of peace.  I imagine an "olive branch" sprouting around the world from self sustainability of natural resources. 

Shoot, I guess I should leaf the punning to the experts. 

Colleen
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(Me outstanding in the field with some  . . (corn?))