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:
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You can stop this reaction dead in it's tracks by introducing an acid.
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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.
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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:
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It's a myth, as is adding something acidic.
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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?