I’ve tried the lemon juice method but it makes the apple too sour! Adding less lemon juice doesn’t work as well. Any tips here?
Thanks. 
Orange juice or the juice of canned pineapple work too, and aren’t as sour.
If you just want to store cut apples you can store them in water.
In commerical apple processing, various preservatives such as sulfur dioxide are used. The loss of apple products due to browning is in the millions of dollars annually and there is a search for more effective ways to stop apples from turning brown.
Apples turn brown when an enzyme called polyphenol oxidase (PPO) reacts with compounds in the apple to produce oxidized versions of these compounds which are brown in color. The same reaction causes potatoes to brown, and it’s also the enzyme responsible for turning green tea into the black tea that’s more common in Europe and America. (I wonder if green tea, which is an antioxidant as well as a source of polyphenols, would prevent apples from turning brown. It’d probably make them taste like tea, though.)
There is an enzyme in domestic flies that inhibits PPO. Other inhibitors have been found, such as [greek]b[/greek]-fluorocaffeic acid. Using orange or lemon juice, keeping your apples underwater, or using them as soon as possible, seems like a more practical way of preventing them from browning. The brown color is simply an oxidized version of natural compounds in the apple, though. It’s not harmful and doesn’t mean the apple is spoiled at all; it just doesn’t look as nice.
To stop Apple’s what from browning?
And who is Apple?
Ehh. I meant to preview that first. I guess [greek] isn’t the right code.
I meant (beta)-fluorocaffeic acid. I forget the code for Greek characters.
Acidulated water is the go. Put the apples in water containing either a little lemon juice (3 tablespoons per quart) or white wine (1/2 a cup per quart). Be careful that you don’t have local water that contains metal traces as this will darken the apple tannins. Bottled water would be necessary in that case. Some varieties like Golden Delicious and Cortland have lower levels of the evil things Roches describes and are thus not as inclined to brown.
I’ve generally used orange juice when I have it, as it’s not as sour. I’ve also had decent luck the one time I tried crushing and dissolving a vitamin c (ascorbic acid) tablet in water and dunking the apples in it, but I can’t guarantee that it’d work in every case.