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Why does fruit retain heat when cooked?

Richard ten Brink
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Debra St. John
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    Hi, and welcome to Seasoned Advice. You will probably need to be a bit more specific in your quetion to get the answer you are looking for. What recipe, what circumstances? – Richard ten Brink Mar 10 '15 at 14:10
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    I'd imagine it's hotter because of the amount of water in fruit which is higher than most food stuffs, as appose to drier food – SaturnsEye Mar 11 '15 at 10:12

2 Answers2

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The same phenomenon occurs with tomato sauce on pizza, or vegetables in a casserole: the moist filling feels much, much hotter than surrounding crust or noodles.

In short, this phenomenon is caused by differing thermal properties of the materials involved. The quoted excerpts below (from PhysLink.com) provide some explanation of the physics involved, and I've attempted to extend and simplify it a bit more.

Imagine an apple pie fresh out of the oven:

Despite the temperatures being equal, your tongue is still more likely to get burned by the [pie] filling than the [pie] crust, though. There are 2 principles behind this: thermal conductivity and specific heat capacity.

Thermal conductivity is just the measure of how quickly heat energy travels through a substance.

Pie crust has lower thermal conductivity, since it has less water in it and more pockets of air (even small ones) and the heat energy is not conducted through it as easily.

In contrast, the fruit filling (which is relatively full of water) has a higher thermal conductivity, and can more easily transfer its stored heat to your mouth.

Either hot pie crust or hot fruit filling can burn your tongue, but the hot fruit filling will burn you more quickly.

Specific heat capacity is something like energy density of a substance, and measures how much energy must be contained in a substance for it to have a certain temperature. For example, 100 grams of aluminum at 100 degrees C has more heat [energy] in it than 100 grams of copper at the same temperature. If you dropped both pieces of metal into separate cups of water, the one with the aluminum chunk will get warmer than the other- there's just more energy contained in it. Since the filling is mostly made of water, and water has a very high specific heat, the filling must give off a lot of heat for its temperature to decrease

This has 2 effects: when the pie comes out of the oven, the filling cools down much more slowly, and as a fragment of filling gives up heat to your tongue, it only cools down a tiny bit.

The specific heat capacity is a bit trickier to understand, but it essentially means that the filling (remember, full of water) absorbs more energy to reach the same temperature and therefore has to transfer more heat energy (either to the air or to your mouth) to cool down compared to the crust, with lower heat capacity.

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Erica
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    A higher heat capacity does not exactly mean that the material *"holds on to heat longer"* - rather, it means that more heat energy is stored in the material per degree of temperature change. Cooking oil, for example, has a heat capacity of 2J/gC while water is 4.2J/gC. It takes more than twice as much energy to heat water one degree than it does to heat oil one degree. Mixing 100g of water at 80C with 100g of oil at 20C would end up with 200g of oil+water at 60.6C - the higher heat capacity of water gives it more power to raise the temperature of oil per degree of temperature it gives up. – J... Mar 10 '15 at 18:14
  • @J... feel free to edit, I'd welcome clarification of the wording of that sentence. I find it a challenge to translate thermodynamics into more everyday terminology. – Erica Mar 10 '15 at 18:17
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    I think the quote snippet gets it correct - water simply has more heat in it. Aluminum, as in your example, has a rather high heat capacity, but since it also has a high thermal conductivity it will happily rid itself of this heat quickly, given the chance (burning you very quickly with a lot of heat). I suppose the wording of that one phrase just risks muddling conductivity and capacity. Minutiae, I suppose... heat and temperature are difficult topics. – J... Mar 10 '15 at 18:24
  • @Agos Sometimes the straightforward explanation for something is from chemistry, or biology or some other field. No harm saying it's a simple physical effect when that's the case. (Yes, technically it all reduces to physics, but... http://xkcd.com/1475/) – Cascabel Mar 12 '15 at 19:18
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It's the sugar in the fruit. Sugar tends to hold heat FAR longer than other substances. I can't give you a whole post regarding the chemistry and thermodynamics of it all, but sugar is one of those molecules that tends to hold heat quite well.

Jay
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    The [specific heat capacity of sugar is 1.244 kJ/kg°C](http://www.sugartech.com/heatcapacity/index.php), which is less than [flour (1.59 kJ/kg°C)](http://www.engineeringtoolbox.com/specific-heat-capacity-food-d_295.html) -- and much less than [water (4.18 kJ/kg°C)](http://en.wikipedia.org/wiki/Heat_capacity). It has a more similar thermal conductivity to water, but is again less. – Erica Mar 10 '15 at 19:33
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    You do have to be careful with sugary things (syrup, candy, etc) but it's not that it holds heat better somehow. Water stops at the boiling point (100C) but for example when making candy you can take it up to 150C, so it's actually hotter. On top of that, it's thick and sticky, so if you get some on you it'll stay there and burn you, rather than flowing off. – Cascabel Mar 10 '15 at 20:40