Why are lemons sour?

[Dave the Cook]

The prevailing theory regarding fruits says that they want to be eaten -- it's a mechanism they've developed to ensure the dispersion of seeds. That's why fruits -- tomatoes, apples, cherries, peaches and the like -- are at their sweetest and prettiest when their seeds have matured.

So what's the case for lemons, limes and other sour fruits? People have engineered them for greater sweetness, but in their natural state, they never become what you could call attractively sweet (they don't even develop additional sugar off the tree). Are there tropical animals that have a preference for sourness? Or does the aromatic peel attract insects or microbes that remove the protective skin? Or is the point to protect the seeds until the fruit rots and the seeds just propagate where they fall?

[Chris Amirault]

While we ponder this question, can we also ponder what, exactly, "sour" means? I am having a hard time learning about how sourness works as a flavor in relation to brix, ph, and so on, and knowing that may help explain the role of sourness in the evolution of the lemon.

[Lior]

fascinating subject. Made me wonder if animals have the same tastes as humans and what tastes do they prefer? DO they taste umami?!

ALso, what about fruits that don't really have a part to eat? Like the Yemenite citron I used to make truffles-it had absolutely not a drop of juice-true to its kind. Whay would such a fruit exist? Perhaps there are creatures that like the rind?

I can go on and mention how it is odd that fruits differ so from each other-some have no peel at all, some have seeds on the outside, some are mostly seeds, some have thorns on the peel, thick peel thin peel, it is perhaps catered to certain creatures...

[slkinsey]

Plenty of fruits are not particuarly pleasant for humans to eat, and plenty of them aren't all that sweet. The olive, for example, is a kind of stone fruit. Tufted dandilion and thistle seeds are fruits. Radish, carrot and beet seeds are fruits. Chili peppers are fruits. None of these things is naturally attractive for humans to eat. And some of them (e.g, thistle seeds) aren't necessarily "meant" to be eaten. But, for many of them, there is an animal that likes to eat it. Birds, for example, seem entirely impervious to capsaicin and can eat as much as they like with no ill effect.

With citrus fruits there is also the problem that these plants are very easily hybridable and have been extensively modified by humans over the millennia. Whether there is such a thing as a truly "wild" or "natural" lemon. . . probably not. So it's hard to say whether the progenitors of the sour fruits we know as lemons and limes were actually sour before we started monkeying around with them.

[slkinsey]

While we ponder this question, can we also ponder what, exactly, "sour" means? I am having a hard time learning about how sourness works as a flavor in relation to brix, ph, and so on, and knowing that may help explain the role of sourness in the evolution of the lemon.

"Sour" is the perception (and thus a psychological phenomenon) proceeding from the sensation of acidity. The sensation can be either inhibited or amplified by things such as sugar, temperature, etc. -- and, of course, the perception of the sensation can be affected by various factors as well.

By the way... I know that people throw around "brix" like it's a scientific way to talk about "sweetness" or "amount of dissolved sugar," but that's not really what it means. "Brix" is the measure of all the dissolved solids in a liquid. One could have a very high brix liquid that contained little or no sugar or sweetness.

Edited February 19, 2010 by slkinsey (log)

[Florida]

Not all fruit evolved to be eaten. Not only are lemons sour, but have you ever picked one off the tree? The trees are incredibly spiney (as are all citrus trees) and, while I am no botanist, I can deduct these trees likely developed sharp spines as a means to keep potential foragers away from their fruit.

[Adam Balic]

Lemons, like most of the citrus we eat are hybrids. Some of these are natural, but even so their success and dispersal has been due to human action. Lemosn are sour becasue people like them that way, otherwise lemons would just be some change hybrid growing and being ignored like thousands of other fruit trees. Its likely that many of the ancestral citrus were actually quite sour and bitter as this is the state of most of the extant species (like the native citrus in Australia), many animals have no problem at all with eating these.

[scubadoo97]

Our citrus rats have no problems with sour or thorns for that matter.

[PBHokie]

The prevailing theory regarding fruits says that they want to be eaten

I honestly don't know, but I question whether this is actually the case or if it's even a “prevailing theory”. Only the hungriest of wild animals would choose to eat a wild apple and if there is a creature that favors peaches, I doubt very seriously if it also ingests the stone. Also, as mentioned by others, many fruits have mechanisms (namely thorns) which clearly indicate they do NOT want to be eaten.