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I've heard this claim before, but every time I dig deeper I am unable to find a reason for why cucumbers would be significantly colder than their surroundings. Is there any validity to this claim? If so, why does this phenomenon occur?

Example: How Stuff Works: Cucumbers: Natural Food

All that water also gives cucumbers their unique refreshing quality, especially on a hot summer day. The phrase "cool as a cucumber" is actually a scientific fact: The inside temperature of a cucumber can be up to 20 degrees cooler than the outside air [source: CDC].

Note: The linked CDC page does not (no longer?) mention cucumbers. Presumably 20 degrees refers to an interval of 20 degrees Fahrenheit (an interval of about 11 degrees Celsius).

Flimzy
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user1787270
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    Is the "20 degrees" part of the claim? Asking whether cucumbers are regularly cooler than their surrounding and asking whether they are constantly 20 degrees cooler are kind of different claims and also different to investigate. – Alenanno Apr 09 '14 at 23:38
  • @Alenanno: I provided a notable source of the 20 degrees. – Oddthinking Apr 10 '14 at 01:22
  • I found a copy of the CDC article at the Internet Archive, but it doesn't give any reference or attribution for the claim. A number of other sites reference some 1970 research study as the source of the 20 degree number, but I can't find the actual study itself. – Johnny Apr 10 '14 at 06:16
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    "The inside temperature of a cucumber can be up to 20 degrees cooler than the outside air" - sure, put it on fire and the surface of a cucumber will be way hotter than the insides. True for many other veges :) Never take claims with "up to" seriously. 0 falls within "up to 20". Same applies for "can be", which doesn't mean usually, and doesn't mean more than once. – sashkello Apr 10 '14 at 10:54
  • Re-iteration: On the sun, the surface of cucumber can heat up a lot, while it will stay cool inside for a while (until it catches up, but after some time). This is obviously true not only for cucumber. Surfaces on the direct sunshine heat up to 30-40C easily and inside is initially same as air temperature before sunrise, so 11C difference is not that impressive if you think about it. Also nobody claims that it will stay that way. – sashkello Apr 10 '14 at 10:59
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    @sashkello We've had the "up to" argument before: http://meta.skeptics.stackexchange.com/questions/2285/rejected-edit-review. Conclusion: don't read it so literally. It's reasonable to assume the claim is that the 20 degree difference is actually reached, and has happened more than once. –  Apr 10 '14 at 17:23
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    @Articuno: It's still a meaningless claim. The ambient air temperature in a field will rise by more than 20 degrees after sunrise. If you open a cucumber after sunrise+20 degrees, you'll likely find the internal temperature to be lower. That's not surprising. If you stick your finger into the dirt, you'll feel a similar drop in temperature. The sun-surface example is an exaggeration, but quite valid. – Flimzy Apr 10 '14 at 19:13
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    @Flimzy That's not meaningless. Rather, that sounds like you have the answer. Why not write it up? –  Apr 10 '14 at 19:16
  • @Flimzy My example of surface on the sun is not really an exaggeration. Air temperature goes up quite slowly, but with direct sunlight surface can be significantly hotter than air. I believe, this is the situation when such difference in temperature happens... And it is not something special - sun goes up every morning and being exposed to direct sunlight is not uncommon. ;) Good answer otherwise. – sashkello Apr 10 '14 at 23:10
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    What would make for a much more interesting question is: Are cucumbers exceptionally cooler than other vegetables of similar dimensions? – Flimzy Apr 10 '14 at 23:21

2 Answers2

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Cucumbers, like all matter, are made up of, uh... matter.

And all matter has properties of thermal conductivity which dictate how quickly heat will move through the matter. The thermal conductivity of a cucumber is incidentally reported at 0.62 W/m K. This means that it will always take some time for heat to penetrate from the outside of the cucumber to the center of the cucumber.

If the outside air temperature rises quickly enough, then the inside of the cucumber will, at some point, be 20 degrees lower than the outside air temperature--same as with any other matter.

Flimzy
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    If a cucumber expends energy keeping itself cooler (for example, through "sweating" or cells acting as a heat pump), the thermal conductivity isn't the only factor. – Oddthinking Jun 07 '16 at 16:54
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    For example: a cucumber sucks up less water when its roots are cold! [Don't quote me on that; I haven't read the paper thoroughly.] – Oddthinking Jun 07 '16 at 16:58
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    @Oddthinking yes and no. Thermal conductivity is a measured global property. That takes into account structure and other similar effects. Also, a cucumber cannot get cooler on the inside without making the outside hotter. If the outside is constantly at room temperature, it's impossible that it has a heat pump. – Sklivvz Jun 07 '16 at 23:50
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    @Sklivvz: I think you are assuming the fruit is a closed system. Evaporation and roots deep in the soil invalidate that. In any case, I don't actually believe the claim. I am pointing out the answer is theoretical, and it would be much better to have empirical evidence involving thermometers and cucumber plants. – Oddthinking Jun 08 '16 at 01:11
  • @Oddthinking I think the claim refers to taking a cucumber out of a fridge, so, no roots, etc. In my example I am not assuming a closed system. Stuff doesn't cool down in closed systems! Also, I don't think this is theoretical at all: this link in the answer is about a theoretical model of the heating of a cucumber. We allow such theoretical calculations when made by an expert -- and this is actually a peer-reviewed article. – Sklivvz Jun 08 '16 at 09:21
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    Wait, what? They mean a cucumber from the fridge? Buahaha! I wish to withdraw my comments. I interpreted "field cucumber" as "a cucumber in a field", not realising it is the name of a variety (not sold in Australian supermarkets, or if they are, it is under a different name: green, telegraph and Lebanese are the common ones here. – Oddthinking Jun 08 '16 at 14:12
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tl;dr- A cucumber equilibrates to the environment's condition, such that there's no temperature difference. However, cucumbers can resist temperature changes fairly well, such that if you just pull them out of a cool environment (or drop them into a warmer one, e.g. an oven), then they can be much cooler inside than outside.

Ultimately, the claim appears to have come from a trivia fun-fact in a promotional pamphlet.

Temperature differences at equilibrium are impossible by definition

The scientific definition of temperature comes the Zeroth Law of Thermodynamics:

The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third, then they are in thermal equilibrium with each other. Accordingly, thermal equilibrium between systems is a transitive relation.

The gist is that, if you leave things in contact with each other, they'll eventually reach some thermal equilibrium. Then, by definition, those two objects are at the same temperature.

This means that, if something like a cucumber is left in a stable environment, then it'll reach that environment's temperature. No matter what that cucumber does - even if its molecules start bouncing around wildly, or teleport around the place, or start emitting an eerie, supernatural green light - its temperature is defined as that of its environment.

This means that we can't observe a temperature difference at equilibrium because that claim literally doesn't mean anything.

But temperature gradients are possible outside of equilibrium

The above only applies at equilibrium. Obviously, if you drop a cucumber into an oven, the outside will tend to heat faster than the inside, allowing for a significant temperature gradient to exist

If the oven keeps running, then the entire cucumber will eventually reach the same temperature, but it'll take time to get there.

Source of the claim isn't scholarly

The above question cites "Cucumbers: Natural Food", HowStuffWorks:

All that water also gives cucumbers their unique refreshing quality, especially on a hot summer day. The phrase "cool as a cucumber" is actually a scientific fact: The inside temperature of a cucumber can be up to 20 degrees cooler than the outside air [source: CDC]

While this article's link appears to be broken, the claim appears to be from a USDA healthy eating promotional pamphlet, "Summer Food, Summer Moves: Operator's Activity Guide" [PDF] (p.11):

Fruit and Veggie Trivia

[...] Want to cool down? Try some cucumbers! The inside of a cucumber can be 20 degrees cooler than the outside air. So cucumbers are a refreshing way to help make half your plate fruits and veggies.

If you look at the PDF, it's not a technical document; it's full of bright colors and playful photos, apparently meant to help encourage people to eat healthy.

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Nat
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    I don't the the equilibrium argument is necessarily convincing, because it isn't necessarily relevant. Living things often maintain things consistently in non-equilibrium states by expending energy: osmotic regulation in aquatic organisms, temperature gradients in exothermic animals. That doesn't mean that it is possible for a cucumber to maintain a negative temperature gradient, but it is something that needs evidence. – PhillS Jun 29 '17 at 11:26
  • @PhillS Maintaining a temperature gradient requires taking in significant amounts of energy as it's a non-equilibrium process. Since the claim's about harvested cucumbers for snacking, maintaining a temperature gradient would imply that they're resisting equilibrium by drawing on an effectively inexhaustible power source. That would be kinda cool, though - if that were the case, we could use cucumbers to basically make perpetual motion machines, generating energy using a heat pump over the temperature gradient. – Nat Jun 29 '17 at 13:14