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The question is whether (1) cooking rice in a pot of boiling water for an hour and then leaving the pot (of water and rice) to cool to room temp heats my house the same amount as compared to (2) just boiling a pot of water for an hour and leaving it to cool as compared to (3) simply leaving the stove flame on for an hour. Assume an identical stove setting for all three scenarios of course.

My inclination is that when cooking rice, much of the energy is absorbed by changes to the bond structures of the rice (just as a sweet potato converts to simpler sugars when cooking) and this energy will not be radiated back when you allow it to cool, and thus it will heat house less than (2) or (3).

As for (2) versus (3), my inclination is to say that if all of the water vapor that boils off stays in the house to condense, the two scenarios will heat the house identically.

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3 Answers 3

I Googled in vain for the enthalphy change in the hydrolysis of starch to disaccharides, however a quick survey of dieting web sites suggests both starch and sugar have about the same number of calories (360-380 calories per 100g depending on what web site you believe). This suggests that the enthalpy of hydrolysis of starch to disaccharides is small, or at least small compared to the heat losses from a pot of boiling water.

In your three experiments you're putting in the same amount of energy i.e. burning the same amount of gas. If there is negligable energy taken up hydrolysing starch, and if nothing escapes from your house, then all three experiments will end up heating your house by the same amount.

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Starch is not hydrolyzed to disaccharides when rice is cooked. Proteins are denatured, and water is absorbed, but there is no breaking of polymers, or else the rice will be sweet. –  Ron Maimon May 27 '12 at 22:26
    
I have to say I agree with you. I've eaten a lot of rice and I've never noticed a sweet taste. However I've never boiled it for an hour as Sammy does, and maybe the starch does hydrolyse on that timescale. A pedantic detail, but only sucrose is sweet, so the starch could hydrolyse to other disaccharides or monosaccharides and still not be sweet. –  John Rennie May 28 '12 at 6:02
    
Glucose is also sweet, although less so. I haven't eaten other disaccharides. I don't think it hydrolyzes because starch is pretty stable to cooking (it's used as a stable thickener). The main issue is the absorption of the water and the denaturing of proteins. –  Ron Maimon May 28 '12 at 6:35

They will not be the same in principle, although the energy released or absorbed by the rice is small. The heating of your house will be equal to the amount of heat energy released over the course of the hour plus or minus the energy required to turn dry rice and water into cooked rice with absorbed water.

The process for absorbing water is probably endothermic, the entropy gain of the hydrating polymers seems larger than the entropy loss in going from liquid water to water confined to the rice interior. But the process for denaturing the proteins during cooking is exothermic (see this answer: What is the most energy efficient way to boil an egg?), so it can't be answered without a detailed experiment, since it is hard to say which is bigger a-priori.

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You can analyze it using exergy, but from what you are trying to explain it is going to heat the same as energy can't be destroyed or created just transferred to the surrounding area. Exergy would tell you the efficiency during the process but if you are looking at the result once back at room temp or surrounding temp you will have the same for all processes. To be clear, at some point during the process one of the three will be more efficient in terms of exergy, but the end result at room temp will be equal.

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-1: Except for the energy can be absorbed or emitted by the rice--- this is the whole question. –  Ron Maimon May 28 '12 at 6:39

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