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Thermal resistance goes down as temperature goes up for metal coils. I wanted to know if there is data available on rice and vegetables in particular, which would let us see if the same relationship exists for them.

This question comes from a real life observation. While re-heating rice (without stirring / disturbing it), I observed that if I heat it on high then the bottom layer start getting burnt before heat dissipates throughout. On the other hand, if I start the flame on low and progressively turn it high, then it heats evenly throughout, which makes it seem like it conducts heat better when it's already warm.

I've only conjectured that this thermal-resistance/temperature relationship exists, but if data exists on these materials, we can be sure (either that, or at least we can dismiss this observation as a fluke / try to explain it through some other phenomena).

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The observations in the linked question are unreliable because it is not clear whether electrical or thermal resistance has been measured.

A simpler explanation for your observation is that when you increase the heat supply gradually you are allowing more time for heat to be conducted away from the base of the pan. The base of the pan is very hot for a much shorter time than when the heat setting is high throughout warming. So the bottom layer of rice has less time to get hot enough to burn.

There is a small increase in the thermal conductivity of cooked rice as its temperature is increased, by about $5\%$ per $20K$. However, this is smaller than the variation due to moisture content. See Table 3 in the paper below.

The hotter rice at the bottom of the pan loses moisture more quickly due to evaporation, resulting in its thermal conductivity dropping by about $35\%$ over the same $20K$ increase in temperature. This drying-out effect could explain the burning at the bottom of the pan.

Effect of cooking and drying on the thermal conductivity of rice. M. N. Ramesh. International Journal of Food Properties. 02 Sep 2009

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  • $\begingroup$ I haven't looked at the linked PDF file, but I suspect that a significant proportion of the heating of the upper layers is not due to direct conduction through the rice but from the warm water vapour percolating up from the lower layers. I eat, and re-heat, a lot of rice. ;) $\endgroup$
    – PM 2Ring
    Apr 1, 2020 at 0:55
  • $\begingroup$ @PM2Ring Yes. Very likely. Perhaps this is why stir-fried rice is so popular. $\endgroup$ Apr 1, 2020 at 0:57

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