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Why is there so much citric acid in citrus fruits? And how did it evolve i.e. what did it come from? Is it a by-product of the ripening process? Why have citrus fruit evolved a particularly high amount of citric acid compared to other fruits? Was it due to particular selection pressure for acidic fruits or was there a genetic bottle neck that caused this such as human cultivation?

I've been trying to search for this but google scholar isn't throwing up anything so I'd really appreciate some help.

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  • $\begingroup$ This is a related question but I don't think the answer there has addressed the evolutionary aspect of acid accumulation. $\endgroup$
    – WYSIWYG
    Commented Jan 26, 2016 at 19:21
  • $\begingroup$ Thank you for the link anyway, the answer may give me some useful information. $\endgroup$
    – Phototroph
    Commented Jan 26, 2016 at 19:32
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    $\begingroup$ It's been a difficult search, honestly. I'd like to think the citric acid preserves the fruit in it's native growing condition; lemon trees originate in Asia, for example (high humidity, lots of bugs, etc.). I've come up largely empty-handed! $\endgroup$
    – CKM
    Commented Jan 28, 2016 at 21:08

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Actually citric acid is not only available in citrus fruit but also in pretty much any living thing. it is an important intermediate in Krebs cycle, but I guess the main reason that it is available in high concentrations in citrus fruits is because citrate is an inhibitory compound for phosphofructokinase, that means it stops the glycolysis and hence it makes it hard for bacteria to use the fructose available in the fruit body.

Source: Stryer, Lubert; Berg, Jeremy; Tymoczko, John (2003). "Section 16.2: The Glycolytic Pathway Is Tightly Controlled". Biochemistry (5. ed., international ed., 3. printing ed.). New York: Freeman. ISBN 978-0716746843.

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    $\begingroup$ Then why haven't more fruits evolved this? I'm aware that citric acid is found in all living things, I apologise if the wording is misleading. My question more why does citric acid occur in such high concentrations in citrus fruits compared to other fruits. $\endgroup$
    – Phototroph
    Commented Jan 29, 2016 at 19:04
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    $\begingroup$ Actually, I think it is more a question of diversity. most of the fruits have their own preservatives, in citrus fruits case it's citric acid.in some fruits it's high sugar concentration.in some other it's vitamin C. it's a matter of diversity and the reason that other fruits went down another path is probably a matter of chance and environment and previous terms. I hope I had answered conveniently. $\endgroup$
    – Shayan Kabiri
    Commented Jan 29, 2016 at 19:26
  • $\begingroup$ @Phototroph I think by asking "Then why haven't more fruits evolved this?" you are making your question opinion-based. You might want to read biology.stackexchange.com/questions/35532/… for help. In short, evolution is not meant to make organisms better, but just to become different from others. Its natural selection by which better ones survive while worse ones can't. $\endgroup$
    – another 'Homo sapien'
    Commented Apr 10, 2016 at 12:03
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    $\begingroup$ I realise how evolution works, apologies if my wording doesn't make it clear. Haven't contributed further to this because Shayan's comment more or less answers my question (it's a question of diversity), but have left this open to see if anyone else has anything further to contribute $\endgroup$
    – Phototroph
    Commented Apr 11, 2016 at 19:44

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