No, I'm not concerned about the propellant.
I only the use canned air in short bursts and not for drying the entire thing completely. I do a quick spurt to chase a water droplet into an area where I can reach it with a paper towel.
The white foamy spot you're describing is the result of what happens when the volume of the air in the can decreases, thereby promoting a temperature change and pressure change. The equation physicists use to determine pressure and temperature is as follows:
PV=nRT where Pressure * Volume = n * R (a constant) * Temperature
I won't bore you with the rest (your eyes probably glazed over already <img src="">
Basically, in order for the pressure to remain the same to shoot out the air, then the temperature is going to decrease as the volume in the can decreases.
To answer Fred's question about the temperature of the water: generally the bugs we've got are not thermophillic, so the heat of the water in the hot pot, combined with the physical action of wiping it off is sufficient to remove traces of microorganisms. The temperature at which water boils is 212 F (100 C). Most bacteria are killed off at temps of 140F (60C), so I'm confident that the heat of the water in the hot pot is hot enough.
My electric kettle has an indicator that falls down when the water comes to a rolling boil. Generally, I assume this to be 100 C. If I wanted to be extra sure that it was hot enough to be effective, then I would boil a mixture of 4 parts water to 1 part salt, ( 1c water and 1/4 cup salt) and that would raise the boiling point a couple of degrees.
If you really want to be aggressive, you can just let them soak in vodka (but why waste perfectly good vodka?) or just follow up once they're dry by wiping it down with an alcohol prep pad.