Does cooking time affect heat retention?

June 8, 2014 3:11 AM

Years ago, a friend of mine mentioned something he'd read about the effect of cooking time on a food's cooling time. The example he gave is a pot of chili. If, for example, you were to heat the chili to a certain temp, and then continue to apply heat just enough to keep it at that temperature overnight, it would take longer to cool than if you heated it to that temperature and then immediately allowed it to cool. Is there any truth to this? Can you please point me to articles/resources that can prove/disprove this theory?
posted by phreckles to Science & Nature (13 answers total)

If you were to simplify it - heat two masses of water in sealed vessels to 80C, leave one turned on overnight, the cooling times would be the same.

With a real world pot of chili, if you've evaporated off water, the remaining stuff might have a higher heat capacity, and then cool more slowly than stuff with water. I guess.

Double checking, probably not. The kidney beans and beef cool off more quickly than water. That is, a 1 kilo lump of kidney bean needs to lose less thermal energy to drop temperature 1 degree Celsius.

Also the stuff you were cooking would have undergone chemical changes, more commonly known as "cooking". So it'd be a slightly different mix of chemicals arranged diffently, perhaps with different heat capacities.

Can you please point me to articles/resources that can prove/disprove this theory?
One resource would be your kitchen ...
posted by sebastienbailard at 3:35 AM on June 8, 2014 [3 favorites]

When you say 'heat the chili to a certain temp' you are really saying 'turn on the pot until the pot thermostat reports it is at a certain temperature'. Depending on the pot design, at that point in time, only the one part of the pot where the thermostat is located may actually be at that temperature, the rest of the pot and maybe a lot of the chili has not yet actually reached that temperature.

However, if you keep the pot on overnight, the heat energy can transfer through the rest of the pot and chili, while the pot thermostat turns itself on and off, keeping everything at the same temperature.

So, turning the pot off as soon as the thermostat says it's at a certain temp, means that not as much heat energy has been transferred overall and therefore the chili takes less time to cool.
posted by atlantica at 3:41 AM on June 8, 2014 [7 favorites]

Oh, and the difference between heat and temperature.
posted by atlantica at 3:42 AM on June 8, 2014 [1 favorite]

One consideration is that if you boil a dry bean in water, you break it up into lots of subcomponents, which can jiggle around in the water. For a lump of starch in water versus it broken into two separate smaller lumps - the system with two smaller lumps have more degrees of freedom to store energy in which means a different (larger?) heat capacity.

I think.
posted by sebastienbailard at 3:43 AM on June 8, 2014

I wonder if it's a matter of appearances being deceptive. Chili, being stodgy, doesn't convect well. It can be boiling at the bottom, near the heat of the pan - and therefore bubbling and giving off steam - and lukewarm at the top. When you stir it, the boiling stops for a while till some location somewhere gets back up to temperature. That's true of almost anything you cook excepting thin liquids (which is probably not 'food' as your friend is imagining it).

So, if you heat it up till it appears to boil, then turn it off, it will cool quicker than if you heat it longer, till it's properly boiling, and *then* turn it off - because most of it isn't boiling in the former case.
posted by How much is that froggie in the window at 3:46 AM on June 8, 2014 [3 favorites]

Heating the food for longer might also increase the temperature of the whole kitchen, which would result in a slower cooling rate. This would depend on the type of hob, I guess.
posted by daisyk at 3:59 AM on June 8, 2014

Assuming that the fast-cooked chilli is heated to a uniform temperature before being switched off, and that no mass is lost from either pot through evaporation, I think that the absorption of heat by cooking reactions could explain the effect. By morning the slow-cooked chilli would have exhausted a certain amount of its potential to absorb heat by breaking chemical bonds, but the fast-cooked chilli would still have those bonds and would cool faster as they were broken.

I'm not sure how large the effect would be, though. A quick google search for '"heat of reaction" cooking' found this article about cooking potatoes, which gives the heat of reaction of potato starch at 23kJ/kg. That sounds like enought to make a few degrees' difference; the actual size of the effect would depend on the composition of the chilli and the temperature it reached (with higher temperatures likely to produce a greater effect).
posted by A Thousand Baited Hooks at 4:29 AM on June 8, 2014

Probably unlikely but maybe your friend was thinking about the Mpemba effect
posted by night_train at 5:03 AM on June 8, 2014

It's not true. I can see how it would SEEM true on a run through, because a just-heated pot of chili isn't, in fact, all the way heated. It convects so poorly that a pot that seems like you just got it up to temperature, probably isn't all the way up - not heated uniformly through every little bean. Also, consider the pot itself - a big dutch oven that's been sitting and heating in the oven for hours is also hot all the way through, and store that heat in every part of itself and give it back to the chili, while a pot sitting on the stove briefly is just not going to be as hot - certainly the lid won't be, and the air in the pot won't be, etc. But if you controlled the experiment to account for these variables you'd find the cooling rate was the same.
posted by fingersandtoes at 6:15 AM on June 8, 2014 [3 favorites]

Not true. You can do some contortions to imagine where taking longer to heat something up changes it fundamentally, but that isn't the question. The way I hear the question is "by heating something longer will it stay hot longer" and the answer, assuming the identical masses of atoms in identical pots of chili have been excited to identical points, is no. Doesn't matter how you get to the same point, and if you aren't at the same point, well, things are different.
posted by dirtdirt at 7:53 AM on June 8, 2014 [1 favorite]

Perhaps he's conflating the rule about resting times for meat? Generally the longer something cooks the longer it should rest before slicing/serving. Even braises benefit from being allowed to cool to room temp in their liquid and then gentle reheating.
posted by JPD at 8:26 AM on June 8, 2014

Here's why, essentially. None of those factors (delta-T, the material, or surface area) are different. Assuming the chili was heated up to isothermal both times (not just hot on the outside with a cold inside), it would cool the same.

We do intentionally limit heatup and cooldown rates of thick metal pieces like reactor vessels, which makes it take a long time. That has to do with internal stress in the metal and giving it time to heat or cool evenly, not for heat transfer performance.

You don't say whether you want it to cool faster or slower, but your biggest control factor there is going to be the pan design. Spread out over a cookie sheet is going to cool a lot faster than a perfect insulated sphere (or crock pot, close enough.)

On the other hand, he's right in a tricky bar-bet kind of way. Say your quick heating method heats the chili in 1 hour and it cools in one hour. You get heated, then cooled chili in 2 hours.

If the slow heating method takes 24 hours, you get heated then cooled chili in 25 hours. Which is, in fact, a longer process overall.
posted by ctmf at 9:46 PM on June 8, 2014

Response by poster: Thank you, all!
posted by phreckles at 12:31 AM on June 9, 2014