I was making roux last night from lard and flour and as I stood there stirring, listening to God Music and drinking a Harp, I wondered what was going on in the pot in terms of heat transfer.
First, there would have been almost no temperature variations within the roux. My stirring made sure of that. So the roux was of even temperature with two boundaries - one of the pot and the other of the air. The pot, being cast iron, wasn't quite even in temperature, but probably close enough where it contacted the roux. Same with the air.
Consulting an engineering table, I discovered that lard is almost twice as easy to heat up as water. It takes 1 calorie per gram of water to raise it 1 degree C. It takes 0.54 calories per gram of lard. I'm way rusty on my heat transfer calculations, but to me that says the lard would have been closer to the temperature of the pot than the temperature of the air as compared to what a pot of water would have been.
I'm discounting the effects of the flour suspended in the lard, which is probably a tragic mistake. Flour needs only 0.38 calories per gram to go up one degree Celsius. That suggests roux - an even mix of flour and lard - would be closer still to the temperature of the pot.
Maybe Ohioan at Heart or Mut could lend a hand here. They're a lot closer to the worlds of science and engineering than I am right now.
4 comments:
The lower heat capacity means it heats faster, given a certain amount of heat in. I'd suspect that the rate of evaporation would also be much slower (since it has a much higher molecular weight), this would lower the heat loss to the air.
I'd expect the flour to further increase these effects.
So all other things being equal, I would expect it to heat faster and probably reach a higher temperature.
Lard is a triglyceride. From the looks of things, it's C30 H62 O9. That's substantially heavier than H2 O.
This is the sort of thing I'd be hesitant to try to calculate, what with the sheer number of differences between the two liquids. While the heat capacity is lower for the lard/flour mixture, the dramatically higher viscosity is going to make for a much thicker boundary layer when you stir it, which is going to slow down the heat conduction quite a lot. Especially if it starts to cook the flour enough to make it stick to the sides of the pot. And as Ohioan says, the lard can get a lot hotter than water before it boils.
If I wanted to know which one heated faster, or which one got hotter, I'd stick a thermometer in it and measure directly. There's just too many ways to screw up the calculations, and the experiment is so easy that it could produce all the data in less time than it takes to work it all out from first principles.
Tim, I thought of that, but the roux was too shallow to trust the thermometer, hence the noodling over the thermodynamics of the situation.
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