My research indicates that adding lemon juice helps to stop the
process of browning (polyphen oloxidase). It is also recommended that
you should heat the juice to at least 160F.
?To keep your apple juice from turning brown, simply push a lemon
through your juicer right before you begin juicing your apple.?
?To prevent cut apples from turning brown, a result of oxidation, toss
them with citrus juice.?
?Give cut apples a quick "bath" in orange juice to preserve their
color and keep them from turning brown.?
?Acid inactivates polyphenoloxidase, so you can slow the browning (but
not stop it completely) by dipping raw sliced and/or peeled apples
into a solution of lemon juice and water or vinegar and water or by
mixing them with citrus fruits in a fruit salad. [...] Ninety-eight
percent of all juices, including apple juices, sold in the United
States are pasteurized to stop all natural enzyme action that would
otherwise turn sugars to alcohols, eventually producing the mildly
alcohol beverage known as apple cider (non alcoholic cider is plain
apple juice). Pasteurization also protects juices from potentially
harmful bacterial and mold contamination.?
?I you want to drink the juice now without making cider, pasteurize it
by heating to at least 160 degrees Fahrenheit. Then, pour juice into
clean glass jars or bottles and refrigerate.?
You may be interested in this New Scientist Q and A on this topic.
?The browning reaction can be blocked by one of two agents, both of
which are present in lemon juice. The first is vitamin C, a biological
antioxidant that is oxidised to colourless products instead of the
apple's phenolics. The second agents are organic acids, especially
citric acid, which make the pH lower than the oxidases' optimum level
and thus slow the browning.?
I hope this answers your question. If it does not, or the answer is
unclear, then please ask for clarification of this research before
rating the answer. I shall respond to the clarification request as
soon as I receive it.