Many fruits and vegetables, e.g. apples, pears, potatoes, mushrooms, etc turn brown when cut open, and exposed
492689
I believe your question is why. So here is the answer. These fruits contain Iron which rusts when exposed in the air to oxygen and water vapor. This rust is reddish brown or brown.
258996
Because it's Brown (and sometimes runny)
99830
the night air is being compared to a knife or sharp instrument - hence cut
227306
They have carotenoid (sp?) in their exo skeleton, (think beta carotene in carrots) so when they are cooked, it turns that reddish color.
30979
Heat during cooking can destroy Vitamin C in food.
1157873
This is what I'm pretty sure what happens: your digestive acids break up the pigments of color and turn it into a brown...waste. Yeah. Especially for plants. The chlorophyl just sorta...you know...disolves. As for why it turns into a brownish color, my only guess is that the digestive acides leave a sort of singe on it...oooor..................it could be that all the colors of the food blend together to form a brownish color (have you ever tried mixing all the colors of the rainbow together using food dye in water? Not pretty) But what do I know? Just ask a gastrologist...or whatever they're called....I guess I forgot...
522216
Food colouring and bleach.
91003
its the oxygen that makes apples turn brown, when you treat them with lemon juice the oxygen cant touch the apple because of the juice layer.
465269
The potatoe was cut before it was ripe. Potatoes are green till ripe.
359353
It is called the energy of activation or activation energy. The bacteria and maybe fungi cannot multiply at the reduced temps so the food will not be attacked by the microbes.Also proteins don't breakdown quickly at lower temps so the food will be fresh when it defrosts
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Witch Hazel and cucumber slices
890781
Apple's a fruit so it contains many sugars,other carbohydrates,minerals,etc.The oxygen in the atmosphere reacts and oxidizes these and so the change in colour.This was for cut surface of course.But for the skin its the pigment which impart the colour at maturity.
418698
Food you eat can also cause I change in smell. Asparagus and Brocolli are notorious. Excessive red meats can do it too.
869922
When you slice through an onion, you break open a number of onion cells. Some of these cells have enzymes inside of them, and when they are sliced open, the enzymes escape. The enzymes then decompose some of the other substances that have escaped from sliced cells. Some of these substances, amino acid sulfoxides, form sulfenic acids, which then quickly rearrange themselves into a volatile gas.  The gas reaches your eyes and reacts with the water that keeps them moist. This changes the chemical's form again, producing, among other things, a mild sulfuric acid, which irritates the eyes. The nerve endings in your eyes are very sensitive and so they pick up on this irritation (this is why our eyes sting when we slice onions). The brain reacts by telling your tear ducts to produce more water, to dilute the irritating acid so the eyes are protected. Your other reaction is probably to rub your eyes, but this will actually make the irritation a lot worse, of course, if you have onion juices all over your hands. \nOddly enough, this volatile compound is also responsible for a lot of the great taste in onions, as well as the pleasant aroma when you cook the vegetable. You'll also get sulfenic acids by cutting up garlic, chives and leeks, among other vegetables, but they don't form the same irritating gas, just a strong smell.  There are all kinds of remedies for dealing with this irritating phenomenon, some more effective than others. As a general rule, move your head as far away from the onion as you can, so the gas will mostly disperse before it reaches your eyes. If you really can't stand the tears, the simplest solution might be to wear goggles. This measure is very effective, but it may seem a bit extreme to those around you, and if your kitchen is steamy, you might not be able to see what you're doing (never a good idea when you're using a knife).   Peeling the onion and then chilling it in the refrigerator before you slice it will minimize the release of gas somewhat, because the change in temperature alters the compounds in the onion. Cooking an onion before you slice it will work also, for the same reason. Another easy solution is to cut the onion under water or run the tap over it as you slice.   Some people say if you hold a lemon, piece of bread or a sugar cube in your mouth, the food will absorb the gas before it reaches your eyes. Breathing with your mouth, instead of your nose, might also help because as you inhale, you suck the gas in and as you exhale, you blow it away. This keeps a lot of the gas from ever reaching your eyes.
795786
It really is dependent upon the enzyme and where it is. You can cook vegetables at high temperatures and not destroy everything. But as a rule if you boil any vitamin then it isn't going to last!
428313
Probably something else.  It takes several hours up to a day before you will know if you have food poisioning.
292514
Mostly bacteria.  Also, fiber, bile acids, etc.