The point is that crystal sugar is grainy. You can’t make a smooth icing out of it. In order to create a smooth and glossy icing, you first have to melt those sugar crystals down and disperse their molecules in water. Which is to say, you have to make a syrup. And fondant syrups (as you can see by the recipe below) are highly concentrated, so much so that a change in temperature is all it takes to get the sugar crystallization process started again — though this time it’s the pastry maker who gets to decide how big those crystals get.
Imagine for a second a watery environment containing lots and lots of free-floating sugar molecules. All those molecules, like high school kids at a Catholic school prom, are just dying to latch on to each other. The water molecules are the nuns, dashing around and keeping the sugars at a respectable distance. When those forces are in equal balance, i.e. there are just enough nuns to keep the teenagers apart, a mixture is said to be saturated. The trouble is for the nuns, that balance is easily upset.
For saturation points fluctuate depending on a mixture’s temperature. When the temperature is high (say, the band is playing a fast and frenetic dance tune) it’s relatively easy to keep all the kiddies apart. But lower the temperature (say the band starts to play a sentimental slow dance) and the situation rapidly deteriorates. The solution becomes supersaturated, at which point the slightest provocation will incite mass hook-ups.
This is the situation in the processor bowl when the temperature hits 140. The least disturbance will initiate crystal formation, and unfortunately for the nuns down in there, a food processor blade spins at about 1500 rpm’s.
As the machine is turned on some of the more eager sugar molecules lose no time stacking up on each other and the first few crystals begin to form. As they do so, they leave the areas of syrup surrounding them less saturated, and so less likely to form additional crystals. Thus we have two phenomena happening at once: the crystals that are forming are getting bigger (though not nearly as big as regular sugar crystals), but as they’re doing that the remaining syrup is getting less dense with sugar. Combined with the agitation of the blades, the end result is a mixture of very fine sugar crystals suspended in a solution of water and free-flowing sugar molecules. In other words: a fine-textured and malleable putty otherwise known as fondant.
But then that’s not all that happened in the food processor, since the syrup also became opaque. What’s that about? Simply that individual little sugar molecules do a very poor job of deflecting light rays, which pass straight through the sugar syrup almost unimpeded. Team those sugar molecules up in a bigger crystal, however, and they bounce those light rays right back where they came from. The fondant turns a snowy white.