Why doesn't flour make microwaved water fizz?

Recently the Straight Dope Front Page recycled the column, “Why does Sweet 'n Low make microwaved water fizz so strangely?” Cecil explains the principle at work with admirable succinctness, and notes that the phenomenon is not limited to Sweet 'n Low: “Sugar and salt produce equally vigorous reactions. So will just about any powder except flour, which just sort of lies there.”

So what gives with flour? Are the individual particles too fine to provide suitable nucleation sites, or what? It seems to me that if flour settles out of superheated water, as the column’s wording seems to imply, then by rights it ought to make a reasonably effective substrate for bubble formation. I would have thought there would be some clumping of particles at the very least.

WAG ahead: wheat grains have oil in them. Even fully milled, it strikes me that each particle of flour could be coated with just enough oil to eliminate nucleation sites.

I’m curious, so a bump and a WAG.

The flour absorbs the water before it can become vapor…?

umm… how about you guys just try putting flour into a cup of water. you’ll see that it just stays on the surface. i guess you’ve never baked, because most thing that you pour into a bowl of flower will not absorb at first. it’s also why mixers are very helpful.

i can’t say for sure why this is. it probably has something to do with the oil content, and with surface tension. (ie water likes to stick to itself and it’s got little desire to stick to the flour, so the water would much rather clump up than penetrate.)


The column was actually a bit incorrect in its explanation, although I can’t fault Cecil entirely because he did not ingore the real reason either. What makes kettles boil better than microwaves is not nucleation sites. These are present in your cup about as much as they are in the kettle. Rather, it is only the uneven heating. The thing is that water doesn’t quite boil at 100C. That temperature is technically the one at which the water’s vapor pressure equals atmospheric pressure. (Note: virtually all liquids evaporate. also, virtually all gases condense. the liquid will evaporate at a rate related to its temperature, and in a closed container that will add to the pressure. that vapor will condense as a function of pressure, with higher pressures shoving the vapor into the liquid faster. in a closed container, the liquid and gas will fall into equilibrium at a certain pressure, called the vapor pressure, which is a function of temperature. therefore, at 100C water’s vapor pressure overpowers the crunch of the atmosphere… and it expands indefinately. this is also why water boils cooler at higher altitudes.) However, in a small bubble the water’s cohesion exerts a powerful force in addition to the atmospheric pressure. Therefore, if you’re trying to blow up tiny bubbles you’re going to need considerably more than 100C. In a kettle, the bottom gets superheated to the desired temperature easily, especially in little nooks that resist convection. In a microwave, you’re heating things evenly and to get the nucleation sites superheated, you’ve got to superheat everything. It will happen eventually, however.

Dumping sugar into the water simply throws in a bunch of much larger air bubbles into the mix, which are much easier to blow up than the usual tiny ones. I haven’t done the experiment, but I’m pretty sure that this effect will be momentary and boiling will not continue after the initial dump.