why do lemon juice keeps apple slices from turning brown? thanks.

Answer 1

I found this quote in an article about citric and ascorbic acid, both of which are present in lemon juice:

<< A third group of preservatives targets enzymes in the food itself that continue to metabolize after harvest. The enzyme phenolase, for example, goes to work as soon as an apple or potato is cut. It browns the exposed surface. Acids such as citric acid and ascorbic acid (vitamin C) inhibit phenolase by making the pH uncomfortably low for the enzyme.>>

Answer 2

You can find the quote in this link:
http://pubs.acs.org/cen/science/8045/8045sci2.html

It is from a publication of the American Chemical Society