Molly asks via Twitter,
One of the most important considerations for the texture of your ice cream is how much fat you have in it. It’s not the only thing, of course. How quickly you freeze it, and if you properly agitate the mixture while you’re freezing it, are both terribly important. But fat is vital because it creates a great deal of the body of the ice cream.
There’s a continuum of iced treats that starts on one end with proper, Italian gelato, which has a fat content so high your heart would seize up just reading it, to the other end, which contains your slush drinks (Slurpee, Slushie, Squishee, Margarita, and so on). The process to make them are generally the same. You create a base of some sort, put them into a container that will both bring the temperature down quickly and stir the concoction at the same time, and wait.
In the case of the slush drink, you keep churning until it’s time to serve. The ice and sugar will just freeze into a solid mass if you let it. In the case of ice cream, there’s a lot more going on inside, which allows you to put it into a freezer once you’ve hit the right point where it maintains its texture indefinitely.
When you are churning ice cream, ice crystals form very quickly on the outer edge of the mixture, near the churning bowl’s surface. The medium in the bowl freezes at a much lower temperature than water, so it is able to absorb the heat from the ice cream base very quickly. The faster the mixture freezes, the smaller the ice crystals. The agitation from the churn helps that along.
The fat jumps in next. The fat coats the ice crystals in much the same way that the melted marshmallows coat puffed rice when making treats. Everything can still flow around for as long as things are kept moving, but if you were to set the mixture into the freezer and let it rest, then it solidifies. You don’t want that to happen yet, because you haven’t worked in the air.
Churning ice cream is basically the same process as making whipped cream: you are working air in-between the fat molecules in order to increase the volume. However, because the mixture is colder, the whipping doesn’t have to be as energetic. Air is folded in at a leisurely rate, but it can’t escape because the mixture is so thick. The inclusion of air is called the overflow, and it’s one of the ways you know your ice cream is ready; when it increases significantly in volume, you can stop the churning to check the consistency. If the consistency is the same as soft-serve ice cream (which is what you have before you cure it in the freezer), then you’re ready to eat or freeze it, as is your preference.
You can eliminate some of the need for milk fat by doing what some of the industrial houses do, which is to add in some gelatin. I do not recommend this. If you’re going to do that, you could easily skip the trouble of making your own and just buy something down at the grocery store. Harold McGee says in On Food and Cooking that you want about 60% water, 15% sugar, and 10-20% fat.
To make life easier, I recommend using Fine Cooking’s handy Ice Cream Recipe Generator, which gives you several options for customizing your own ice cream. After using it for a bit, you’ll even get the hang of it and you’ll be able to make your own variations without any help.
As far as keeping out the fat, my recommendation is to make it with a good amount of cream, but cut down on the portion size. I know, that hardly seems right, but I’d rather have a small amount of really good ice cream than a big bowl of mediocre ice cream.