It is common belief that adding citrus juice (or in some claims, any acid, including ascorbic acid/vitamin C, vinegar, tomatoes, etc) to guacamole will reduce browning. Examples of the claim:
You can stop this reaction dead in it's tracks by introducing an acid.
The acid you use can range from tomatoes to vinegars to lemons/limes. Regardless of your preference, it must be added immediately to stave off oxyidzation; the reason guac browns.
Add something acidic!
However, there are also claims to the contrary, that adding acid to guacamole has no effect, or can even have a negative effect:
It's a myth, as is adding something acidic.
What about acid? Many books claim that lime or lemon juice will prevent avocados from browning. That's not what my tests said. In fact, depending on how much I added, some batches of guacamole actually browned faster in the presence of citrus juice—significantly so. By the time I added enough acid to slow the browning down to a reasonable degree, the guacamole was inedibly sour.
Is it true that adding something acidic to guacamole will prevent an enzyme reaction, that causes oxidization, and thus browning?