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Butterfat and Ice Cream

Contrary to what you may have heard, fat is good.

One of the biggest factors in ice cream’s texture is the amount of “butterfat” (the fatty part of milk) that it contains.  If there’s not enough butterfat, the ice cream tends to taste icy and not very smooth—which is fine if you’re making a sherbet or a sorbet, but not great if you’re aiming for your own version of Ben & Jerry’s.  On the other hand, if there’s too much butterfat, it starts to taste greasy like lard, and it sticks to the inside of your mouth.  Or in other words, it has bad “mouth feel”.

Another big factor that controls the texture is the amount of air in the ice cream.  Companies with big fancy machines can control the amount of air they introduce into the ice cream, and adjust the mouth feel that way as well.  But since our home machines add a fairly fixed amount of air each time, our main method of controlling the texture is by controlling the butterfat content.

The percentages of butterfat listed above are by weight, not by volume.  So if you took a gallon of 16% super-premium ice cream and pumped it full of enough air to make it into two gallons, it’d still be 16% butterfat by weight, but it’d taste entirely different (and terrible, probably).  When you eat a super-cheap grocery store ice cream that tastes like you’re eating flavored air, that’s pretty much what it is.  They’ve put too much air into it, so they can sell more of it without actually adding more dairy ingredients.  Of course, the opposite can happen as well.  If there’s not enough air in the ice cream, it tastes too dense, almost like a frozen block of cream.  There’s a fine line between too much or too little air, and too much or too little butterfat.

The increase in volume of ice cream from adding air is referred to as overrun, and is calculated as a percentage of the original product. So if you started with a gallon of ice cream base and added the same amount of air to double it, it’d have a 100% overrun, which also happens to be the maximum allowed in the U.S. Some countries allow a maximum of 120% overrun.

Gelato and frozen custard are usually sold fresh, meaning they’re not stored like ice cream in a deep-freeze until they harden.  This is partly because the butterfat content is low enough that the water in the mix would freeze into larger crystals, making it taste icy.

Most ice cream recipes I’ve seen use heavy whipping cream, light cream, milk, or half-and-half.  The recipe I mentioned for the Sweet Cream Base [4] uses 2 cups heavy whipping cream and 1 cup whole milk.  Most of the butterfat comes from the cream, and the milk is mostly just to increase the volume.  You could even use skim milk if you wanted, and then just increase the cream a bit to compensate for the lost butterfat.  Or use half-and-half with less cream.  That particular 2-to-1 ratio is just the one Ben & Jerry’s picked because it’s easy to make and it comes out to the amount of butterfat they wanted, which if I calculated correctly [5], is about 19%.

Whole milk is what recipes usually mean when they say just “milk”.  It has basically the same butterfat as raw milk straight from the cow.  If you let raw milk sit for a while, it separates into milk and cream, which rises to the top.  If you “skim” off the cream, you’re left with skim milk.  If you add more cream to it, you get the creams listed above, with heavy whipping cream being pretty close to pure cream.  If you mix the cream and milk together real well and basically smash all the little fat globules together, it’s homogenized, and won’t separate anymore.  If you heat the milk to a specific temperature for a specific amount of time, it’s pasteurized, and won’t contain bacteria like salmonella [6] anymore.

By the way, ever wondered what 2% milk is two percent of?  Well, it’s not 2% of the butterfat in whole milk, because that’d be 2% of 3.5%, or in other words, white water.  :-)  It’s 2% total butterfat, just like whole milk is 3.5% total butterfat.  2% milk is about 5 grams of fat per cup (240 ml) compared to 8 grams per cup in whole milk.  Drink up.