What is happening when peeled apple turns brown?

Answer 1

When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. O-quinones then produce the well documented brown color by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers.

One question that often accompanies yours is, "Why do some apples seem to brown faster than others?"

Well, nearly all plant tissues contain PPO, however, the level of PPO activity and concentration of substrate--here, the phenolic compounds--can vary between varieties of fruits (say, Granny Smith versus Red Delicious). In addition, a tissue's PPO level can vary depending on growing conditions and fruit maturity. One approach the food industry employs to prevent enzymatic browning is to select fruit varieties that are less susceptible to discoloration-either due to lower PPO activity or lower substrate concentration. This approach, however, may not be practical for the home "culinary scientist."

In the home kitchen enzymatic browning can be prevented by either reducing PPO oxidation activity or lowering the amount of substrate to which the enzyme can bind. Coating freshly cut apples in sugar or syrup can reduce oxygen diffusion and thus slow the browning reaction. Lemon or pineapple juices, both of which naturally contain antioxidants, can be used to coat apple slices and slow enzymatic browning. In addition, both fruit juices are acidic and the lower pH that they bring about causes PPO to become less active. Heating can also be used to inactivate PPO enzymes; apples can be blanched in boiling http://wiki.answers.com/topic.cfm?id=waterfor four to five minutes to nearly eliminate PPO activity. (Be warned that cooking will affect the texture of the product.)

Enzymatic browning is not unique to apples. PPO-a mixture of monophenol oxidase and catechol oxidase enzymes-is present in nearly all plant tissues and can also be found in bacteria . In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.

Answer 2

It stinks when that happens, doesn't it? The fruit that Cezanne's still lifes lifted out of the mundane; the fruit that, upon daily consumption, keeps the doctor away -- marred by mean old Mr. Oxidation. There's your culprit.

Breaking the skin damages some of the apple's millions of cell walls, exposing their insides to the air. Oxygen then mixes with "phenols," a type of chemical compound. An enzyme called polyphenol oxidase (PPO) accelerates the process, creating what is called enzymatic browning. The color results from the creation of pigments called melanins. While apple-eaters may not like sinking their teeth into the mushy bruised spots, enzymatic browning does serve a purpose, protecting fruit from insects, bacteria, and fungi.

Even just dropping an apple can break its cell walls, triggering oxidation. One way to prevent browning is to keep the PPO enzyme from mixing with oxygen by placing exposed apples in water. You can also slow the process by immersing the fruit in lemon, orange, or Pineapple Juice, as the vitamin C they contain is an antioxidant. Cooking the fruit will also stave off browning awhile, but using a rusty knife or copper bowl will quicken it. Don't forget: Pears, bananas, peaches, and potatoes also bruise easily. So be gentle.

Answer 3

It is caused by a chemical reaction from the apple part to the air. Since the bag is closed, sealing the air in, the air cannot get out. Therefore, the air turns the cut apple slices brown or yellow.

Hope this helped!:)

Answer 4

Putting salt prevents the apple from turning brown. Apples turn brown in the air because they oxidise in the air.

Answer 5

Well, the oxygen/air makes the apple turn brown. I have to do research for something like this for my science fair project, so if you don't mind putting answers on here, that would be great!

Answer 6

Apples contain an enzyme that reacts with oxygen and iron-containing compounds in the apple. The oxidation reaction basically forms rust on the surface of the apple.