How Does Lemon Juice Prevent Discoloration In Apples?

The acid denatures ("unties") the protein's complex structure.  There is a protein in the apples called polyphenol oxidase that reacts with a particle in the cells of an apple.  When you break the cells (by cutting the apple or dropping it) these two things react together and create a substance called melanin, which is a brown or black pigment.  This is why the apple turns brown.  The acidity of the lemon juice breaks apart the polyphenol oxidase and prevents it from being able to react with the stuff found inside the cell  (called polyphenolic substrates) and as a result melanin can't be produce which equals no brown colour!

It slows the rate of them turning brown because the acid in the lemon juice slows the oxidation rate on the apples....just a little chem! =)

Polyphenol oxidase in the apple is what reacts with the air, oxidizing and turning brown. This chemical is in most plants.

Ascorbic acid in lemons reacts more readily with air, so it basically prevents the reaction from occurring with the polyphenol oxidase.

Also, the lemon lowers the pH at the surface, further inhibiting the polyphenol oxidase from reacting, as it needs a higher pH to react with oxygen.

I don't think it preserves them but it does keep sliced apples from turning brown.

It's the acidic level in the lemon juice that does it. Works on banana's too.
Not sure why though, but it is really good to drop sliced banana's into a bowl of lemon juice before putting them into a banana pudding. Keeps them from turning all brown and yukky looking.

Lemon juice will only keep apples from turning brown after you peel them

Applying lemon juice will cause the enzymes to denature and lose shape which I turn will not allow them to function. Normally they would aid in the decomposition of the apple, but when they are denatured it will slow down the process.