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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:

  • Seasoned Advice

    You can stop this reaction dead in it's tracks by introducing an acid.

  • Askville

    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.

  • eHow

    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:

  • Seasoned Advice

    It's a myth, as is adding something acidic.

  • Serious Eats

    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?

  • The same is claimed for bananas! – Sklivvz Jul 7 '12 at 18:04
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I love this question, because, until now, I never doubted lemon juice worked. I love this answer, because it surprised me even more than the question.

Yes... and No! Lemon juice may make things better or worse. It depends on the avocado! Ascorbic acid works though.

In this study, the author looked at different mechanisms to reduce the browning of both plain avocado (in puree form), or a standardised batch of guacamole (which itself, contained lemon juice).

Bates tried to use a photometer to precisely measure the browning, and describes the difficulties found.

Bates started with a "Booth 8" avocado, from Florida.

Lemon juice at 2 and 4% was of no value and actually speeded up browning during the first 4 hours [...], although after 8 hours the control sample read as low [...].

Then, later in the season, replaced it with a "Lula" avocado, also from Florida.

Lula browned much less rapidly. Also, samples acidified with lemon juice browned less rapidly than the control [...]. It is not too surprising that a different browning response was observed since in morphology and flavor these two varieties are quite different.

The author goes on to recommend some other, more successful preservatives based on other experiments described in the paper - including sodium bisulfite and/or ascorbic acid. There are descriptions of some combinations, including lemon juice, where the extra acidity may help improve the effect of the sodium bisulfite.

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