Are you wondering why an apple gets the color brown when it's being sliced? The introduction of oxygen into the wounded plant tissue occurs when an apple is chopped (or bruised) When oxygen is available in cells, enzymes called polyphenol oxidase in the chloroplasts of the cell rapidly oxidize phenolic chemicals that are naturally present in apple tissues to produce o-quinones. These o-quinones are colorless precursors to brown-colored secondary products. O-quinones then produce the well-documented color brown by interacting to form compounds with amino acids or proteins, or by self-assembling them into polymers. 
Either way, this process results in the formation of brown pigment. One of the questions that are frequently asked in conjunction with yours is, "Why do some apples appear to brown faster than others? PPO is present in almost all plant tissues; nevertheless, the amount of PPO activity and the quantity of substrate in this case, the phenolic compounds—can vary significantly between different species of fruits (say, Granny Smith versus Red Delicious). Furthermore, the level of PPO in tissue might change based on the growth conditions and the age of the fruit. One method that is utilized by the food business in order to forestall enzymatic browning is the selection of fruit kinds that are less prone to discoloration, either as a result of having a lower PPO activity or a lower substrate concentration. However, it's possible that the "culinary scientist" at home won't find this strategy to be very useful. Either decreasing the quantity of PPO oxidation activity or decreasing the amount of substrate to which the enzyme may bind is two ways to stop enzymatic browning in a home kitchen. The browning reaction can be slowed by coating freshly sliced apples with sugar or syrup, which reduces oxygen diffusion and so slows the reaction. Coating apple slices with lemon or pineapple juice, both of which naturally contain antioxidants, will decrease the enzymatic browning process and make the apples last longer. In addition, both of these fruit juices are acidic, and the decrease in pH that is brought about by them causes PPO activity to decrease. Heating is another method that can be used to deactivate PPO enzymes. For example, apples can be blanched in boiling water for four to five minutes to almost completely abolish PPO activity. 
The enzymatic browning process is not exclusive to apples. PPO is a mixture of monophenol oxidase and catechol oxidase enzymes, and it is found in almost all of the tissues of plants. In addition to plants, microbes, mammals, and fungi all contain PPO. In point of fact, browning caused by PPO is not always an unpleasant reaction. In fact, the brown color that is so recognizable in beverages like tea, coffee, and cocoa is produced by enzymatic browning caused by PPO when the products are being processed. The browning caused by enzymes is not something that is exclusive to apples. When cut, pears, bananas, and eggplants likewise rapidly turn brown after being bruised or otherwise damaged. Browning caused by enzymes is also responsible for the desired dark color that develops in prunes, coffee, black tea, and chocolate. If you want to prevent apple slices from turning brown, you should store them in the refrigerator. This will slow down the chemical reaction. You might also coat them with juice from lemons or pineapples. These juices contain acids, which slow down the reaction, and antioxidants, which prevent the reaction from occurring. If you don't mind the additional sugar, coating the slices with honey, caramel, or sugar syrup is an efficient way to prevent them from being exposed to oxygen. Scientists are currently striving to genetically design apples that do not produce the PPO enzyme. If they are successful in their efforts, brown apples may one day be a thing of the past. When you bite into an apple or cut it open, air is allowed to go to the center of the fruit. 
When an apple is cut open and exposed to oxygen, enzymes within the apple begin the process of converting naturally occurring chemicals known as polyphenols into a compound called "melanin." Melanin is an iron-containing compound that gives the flesh of the apple its characteristic rusty brown color. Because the reaction takes place so quickly, a sliced apple can begin to turn brown in just a few minutes after it has been exposed to it. In the beginning of this year, the government of the United States gave its approval to a genetically engineered apple type that is incapable of producing the enzyme that causes apples to become brown. Although it has not yet made its way over the Atlantic, these new varieties of apples may be diced and sliced without compromising their natural, crisp appearance. If you want to eat a crisp, red apple, go ahead and take as many bites as you like out of it. Just don't leave it out in the open for more than an hour, or you'll be left with a soggy, brown mess instead. It would appear that apples include a self-destruct switch that is triggered the instant you cut into them with a knife or take that first bite, and this switch causes the apple to immediately rot and become inedible. It will eventually get brown and mushy as the process continues. It could appear to be some kind of magic to a three-year-old child who thinks that Hogwarts is real (and, for the record, many adults also believe this), but in truth, it's just science. In point of fact, this is a prototypical illustration of what the more mature members of society refer to as a "oxidation response". There are two ways to solve the issue of apples turning brown: one is to remove oxygen from the reaction, and the other is to interfere with the activity of the enzymes that produce PPO. 
Eliminating oxygen from the body is a simple process; all that is required is to sever the connection between the apple that has been partially digested and its source of oxygen. This can be accomplished in a variety of ways, the most common of which include coating the apple in sugary syrup, lemon juice, or the juice of any other citrus fruit. This will create a physical barrier between the exposed cells in the flesh and the oxygen in the surrounding environment, which will result in a decrease in the amount of oxygen that is able to diffuse through the apple's cells. Some individuals just place their recently peeled and cored apples in a container full of water in order to deprive the apples of their oxygen supply. It will most surely work, but you will need to keep them submerged in water for a relatively short period of time in order for it to be effective. In point of fact, covering an apple with lemon juice changes not only the amount of oxygen that is available to the fruit but also the way that its enzymes work. The pH of the area around the apple is lowered by the lemon juice, which inhibits the PPO's ability to do its job. It is possible to prevent apples from turning brown by applying heat to them in one of two ways: either by frying them or by short blanching them in water.
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