After "what's your favorite ice cream?", the question I get asked the most as an ice cream maker is "what makes gelato different from ice cream?" How does gelato get that soft, elastic texture and slow-to-melt milkiness compared to ice cream's richer, creamier body?

It comes down to three factors: fat, air, and serving temperature. The more complicated answer? Things aren't always clear cut: this is food, not phylogeny, so individual recipes can blur the lines between the two. But there are some basic differences to keep in mind.

All ice cream is mostly water, and as water freezes, it forms hard, crunchy ice crystals. Besides great flavor, the ultimate goal of ice cream making is to keep those crystals as small as possible through added ingredients and technique. Here's how ice cream makers fight crystallization:

• Emulsifying fat into a base (or using already emulsified ingredients, like cream and milk) sticks fat molecules in between water molecules, literally getting in the way of ice as it freezes.
• Sugar also forms a physical barrier to crystallization, just like fat. When dissolved in water, it forms a syrup with a lower freezing point than plain water, and the sweeter a syrup is (i.e. the higher the concentration of sugar), the lower the freezing point becomes. As water starts to freeze in a syrup, the unfrozen water becomes, in effect, a more concentrated syrup. This process continues until you have a bunch of small ice crystals in a sea of syrup so concentrated that it'll never really freeze.
• Air is incorporated into ice cream during the churning process. Just like a light, fluffy angel food cake is easier to cut into than a dense fruit cake, a more aerated ice cream is easier to scoop, and has a fluffier, less dense texture.
• The temperature ice cream is stored at also has an obvious effect: colder ice creams are harder and more solid, while warmer ones are softer, with a looser texture.

There are some other tricks to keep ice cream soft, such as alcohol, starch, protein (in egg and milk), and natural stabilizers like guar gum and carageenan, but the top four above are the big factors at play.

Ice Cream vs. Gelato

Compared to today's American-style ice cream (that's one made with egg yolks, as is basically the new standard in home recipes and commercial products), gelato has less fat in the base and less air churned into it during the freezing process. American ice creams are heavy on the cream, and have a fat content, by American labeling law, of at least 10% (considerably higher in most homemade and many premium versions). Gelato, by comparison, uses more milk than cream, so it doesn't have nearly as much fat. Additionally, it usually—but not always—uses fewer (to the point of none) egg yolks, another source of fat in custard-based ice creams.

American-style ice creams are churned fast and hard to whip in plenty of air (called overrun), which is aided by the high proportion of cream in the base. The most high-end ice creams have an overrun of 25% or so, which means they've increased in volume by 25%; cheaper commercial versions can run from 50% to over 90%, which gives them a light, thin, fast-melting texture that isn't very flavorful (those bites are a quarter to a half air!). Gelato is churned at a much slower speed, which introduces less air into the base—think whipping cream by hand instead of with a stand mixer. That's why it tastes more dense than ice cream—it is.

And what about sugar? Well, sugar levels vary wildly in ice cream andgelato recipes, so there's less of a hard difference there.

If you make ice cream at home, you may be wondering about your ice cream machine: does it churn at ice cream speed or gelato speed? The truth is, most of the consumer models on the market churn at about the same speed, none of which are as fast as the commercial machines used to make American-style ice cream. But you can make both ice cream and gelato in your machine—remember, air is only one of the differences between them.