The gelatin, the glucose, the glycerine, and the fat you mean? What purpose does it serve in a batch of rolled fondant? Obviously it keeps it flexible, but then water will keep a mass of powdered sugar flexible too…at least for a while. Why all the fancy stuff?
The first part of the answer you can probably guess for yourself. Powdered sugar is made up of sugar crystals. Crowd them together in a paste and they’ll want to form even bigger sugar crystals, the urge is just too strong. You don’t want your fondant icing forming an impenetrable force field around your cake of course, so steps must be taken to keep those little sugar LEGO’s from connecting with one another. Gelatin is a good start. Those long, ropey molecules will wrap themselves around some of the crystals, preventing them from getting a tight grip on one another (they’ll also give the fondant a little “body”). All those little ball-bearing-like glucose molecules will also help in that regard, flowing around and in between, keeping the crystals separated.
But then what’s that glycerine all about? That’s kind of a weird thing, isn’t it? Mmm, yes and no. Glycerine, technically glycerol, is what’s known as a sugar alcohol. That means it’s sweet-tasting, but it also means that like common grain alcohol it’s a solvent. Thus it not only helps keep sugar crystals from forming, it breaks them down a little as well. Yet glycerine does far more than that in a fondant. Glycerine is what’s known in chemical circles as a humectant, a word which I’m betting at least a few of you have seen before. Anyone? Anyone? Yes you with the spiral perm. Correct! On shampoo and conditioner bottles. Humectants are moisture-retaining (hygroscopic) substances, which is to say they are molecules that bond with and trap water molecules. They’re great for keeping hair moist and skin moist, which is why glycerine is also found in a wide variety of cosmetics.
What does a humectant do in a fondant? Obviously it traps moisture, but then sugar does that as well. Sugar is in fact one of the best known hygroscopic substances in the kitchen, sometimes for good and sometimes for ill. On the one hand sugar helps keep a muffin moist, on the other it will absorb so much moisture from air that a thin and caramelly tuile cookie will go limp in a matter of minutes on a humid day. Which means what exactly Joe? I’ve got a conference call I’m supposed to be on! Alright already! The thing of it is that you don’t want that tuile scenario happening in your rolled fondant. You don’t want the sugar to absorb moisture from the air, since water is also a solvent and will eventually make your fondant sticky, then limp, and in a worst case scenario runny. Glycerine keeps a rolled fondant firm by trapping and holding the moisture before the sugar crystals can latch onto it. Happy now?