Lots of boxed dishes say this, and I’m wondering whether it is a function of temperature reduction or is there something chemical going on? Or is that redundant? Are heat and chemistry related? If it’s temperature, would the sauce thicken quicker if I just added dry ice?
I always thought it was because as it stands, it’s releasing steam. Since the steam is made up of moist air, the moisture content of the sauce is reduced, thereby thickening it.
But I’m not a scientist and only got a C+ in Chemistry, so what the hell do I know.
It is a good question. I have speed thickened sauce by putting the pan in the freezer. However, sauce also can thicken when it is still on the same heat so it may be both.
Technically, I don’t think that thickening, that’s reducing.
Alton Brown says it’s because the starches in the sauce take time to absorb water from their surroundings. Or something like that. When he whips out those props, sometimes I get a little glassy-eyed.
It could also be that just cooling off will thicken some sauces, especially if they contain fat. Some fats can be very runny when hot, but almost solid at room temp or slightly lower.
Sauces can be thickened in a couple of ways. If you’ve every made homemade stock, it will be liquid on the stove, and solidify to a block of gelatin when cool. Starches will also melt when heated and congeal when cold. So when you thicken pudding with cornstarch it will be liquid (although thick) on the stove, and only turn pudding-y when cooled.
I think it is “chemical” rather than temperature.
In my case it’s always tapioca.
Instructions: Mix with 2 cups milk, bring to a boil, remove from heat, stir twice over 15 minutes, server warm or cold.
You should try this yourself, since I’m just recalling things, I wasn’t doing an experiment.
But, when I was in a hurry I put it into the freezer. It turned into cold soup, not pudding.
That’s Jell-O Tapioca Dessert mix, not plain tapioca flour.
I presume pudding would give you the same answer you seek.
I’m guessing that it’s a lot like setting cement. There’s evaporation going on, but it’s more than that. If you don’t allow the correct amount of time for it to create bonds, say by pouring the cement when it’s too cold out, you will end up with loose lumps in water instead of a solid block.
polymer chemist here: 
I think the sort of thing we are talking about is physical in origin. Thick sauces are usually due to dissolved polymers (starches or proteins). The initial boiling is usually just to get the polymer to dissolve. As the solution cools the molecules move slower, causing an increase in viscosity, which if great enough can cause it to set (although some setting may be due to phase change)
physics reason: temperature is the measure of movement of the atoms of the object. ice atoms dont move much, but water atoms move a lot. when the sauce cools, there is less energy in the sauce so the atoms dont move as much. evaporating water takes energy with it quickly, but the sauce will come to room temperature after a while (obviously) and it will be cooled that way.
Now that the GQ crowd has answered, I’m moving this to Cafe Society, where most cooking threads, even those concerning theory, are usually better started.
samclem GQ moderator
All these answers make sense to me. I think for some boxed pasta and sauce products the pasta may still be absorbing liquid as it cools as well.
I’m thinking most people don’t let their Hamburger Helper cool to room temperature before eating it. The starch theory sounds the most plausible.
That’d be my guess, though I’m not a polymer chemist. (This things are just too big for me. I prefer my organic molecules with a molecular weight of no more than about 1000.)
I’ve always assumed it had more to do with starches absorbing water and expanding rather than any sort of evaporation. In fact, I’d be shocked if it were primarily evaporation at work, as it usually takes quite a long time to reduce something at a full boil to any sort of thickness, much less having the residual heat off the fire evaporate off enough liquid.
Very simply, in the case of augratin potaoes, Noodle concoctions, and rice packets, it is a function of the starch in dry mix packets combined with the natural starches of the particular grain.
…or potato.