Ice Cream Basics
Before we get into the recipes, it’s good to have an understanding of ice cream physics and chemistry. As simple as it seems when you eat it, ice cream is pretty complex when you look at it under a microscope. Understanding the properties of ice cream at a molecular level will help you make great ice cream. I got to attend ice cream school, but you don’t have to.
Solid, Liquid or Gas?
When I teach ice cream making to middle school students, I make them learn some ice cream science before we get started. It’s only fair, right? Even if it’s after school, they should learn a thing or two. I love asking the students what phase of matter ice cream is. I’ll get all three answers, and the kids will each be certain of their answer.
It’s a trick question. Ice cream is all three. It contains all phases of matter, except plasma (that would be some hot ice cream!). Ice cream contains the following, when it is frozen for human consumption:
1) Solids: Ice crystals, fat molecules, proteins (called caseins)
2) Liquids: water, held in solution by sugar
3) Gases: Air molecules trapped in the matrix, as a foam
It’s important to understand this when working on an ice cream flavor, especially one with inclusions (the chunks or ribbons included in some flavors). These physical properties will affect the ice cream flavor, both right when you make it and over time, as it is held in freezers.
By federal law, ice cream has to have certain properties. It must be pasteurized before it is frozen. And it must be frozen under agitation. So fudge pops that are immediately frozen in a glycol bath aren’t ice cream. And it has to be less than 50% air by weight. And it must have at least 10% butterfat and a certain amount of milk proteins. The government is looking out for us. Otherwise, ice cream manufacturers might pump a lot of air into their products and short us on what makes good ice cream - the fat!
SOLIDS
Let’s start with the solids though. When you make ice cream at home, you can play with these. One of the biggest variables is the amount of butterfat in your mixture. Butterfat lends creaminess and richness to ice cream. But it can also inhibit flavors. Too little butterfat and your ice cream will taste thin and can have an icy mouthfeel. Too much butterfat and your flavors will be muted. The fat in the cream can even coat the roof of your mouth.
You can also control the iciness in your product. Ice crystals can be your nemesis. The larger the ice crystals in your ice cream right when you make it, the more they will grow after you hold it in your freezer for a while, especially in a freezer attached to a fridge. Each time your ice cream thaws a little and then freezes again, the ice crystals in it will grow. This is why your ice cream will be inedible after a month in a freezer with an aggressive defrost cycle.
To fight this agglomeration, you have to use stabilizers. These can be natural products like xantham gum or guar gum, to name a few. These stabilizers act like bodyguards in ice cream, standing in the way between two ice crystals and preventing them from locking each other in a headhold. Stabilizers can help keep your ice cream smooth and creamy. Don’t be afraid of them, even if they have unappetizing names like locust bean gum (which is actually from the same tree that we get carob from - why can’t they call it carob gum?).
It’s also helpful to think of solids when you want to add sweet ribbons or other similar inclusions. Without something in your syrup or jam to prevent it, the ice in your mix will cause certain inclusions to harden or be brittle. A little butter in your caramel ribbon will keep it chewy, for instance.
LIQUIDS
People are often shocked to hear that ice cream is also a liquid. Of course it’s a liquid when it melts, but why is it a liquid when it’s frozen?
It comes down to the magic of sugar. Along with salt and alcohol, sugar lowers the freezing point of water. So you can have a mixture that is 20 degrees Fahrenheit, but still liquid, with enough sugar or salt. If you didn’t have sugar in your ice cream, it would be a solid block of ice.
I remember the first time I used a commercial ice cream maker. We had just received it and plugged it in. I was anxious to give it a try. So I poured water in the chamber and pressed the on button. What followed was the most godawful sound. The loud knocking went on and on and I thought I had broken my new $10,000 machine. I turned it off in time and discovered that the inside of the machine was chock full of ice. Without sugar in the water, it had immediately frozen, and caused quite a disturbance inside the chamber.
You can adjust the amount of liquid in your mix by changing the sugar content. Add too much sugar and your ice cream will be soupy. Add too little and your ice cream texture will be stiff. You can also affect these properties with salt and alcohol. A little alcohol can make a big impact, so use it sparingly, or cook off most of the alcohol before you add it to your mix.
It’s also important to realize that the liquids in ice cream will affect the texture of anything you want to include in the ice cream. It will make certain ingredients soggy and break down.
Once I wanted to make chocolate potato chip ice cream. I love dipping potato chips in liquid chocolate. So I made our chocolate ice cream one day and threw some potato chips into the mix. On the first day, it tasted brilliant, with the classic salty and sweet combination. But after a day or two, the potato chips turned mushy. The water in the ice cream mix had broken the potato chips down into soggy, unappealing flakes.
So when you want to include something crisp in your ice cream, you need to protect it from the water. That’s why pecans are buttered. If you don’t coat them in fat, they will get soggy over time. You can also enrobe ingredients in chocolate or other fatty barriers. Think of tin roof sundae ice cream, with its chocolate-covered peanuts.
GASES
It’s hard for a home ice cream maker to have much of an impact here. Commercial machines can spin much faster than home ones, and can whip more air into an ice cream. Ice cream factories can even inject air into the mix as it spins.
Your home ice cream maker will automatically fold some air into the mix. If you want to try to add more air, you can whip the cream into your mix before adding other ingredients. This will add some air, though much of the added air will dissipate as your machine churns.
Air is important in ice cream for mouthfeel. It can fool your mouth into thinking the ice cream is creamy, when you have little butterfat. It also makes ice cream a lot cheaper to make. If a factory can make ice cream half air, then their ingredient costs are way cheaper! So if you see inexpensive blocks of ice cream in the store, know that these will have much less ice cream in them than the slow-churned Ben & Jerry’s.
SALT
Finally a word about the importance of salt in ice cream. I firmly believe that salt, when used properly, enhances other flavors and brings out their full expression. You'll find many ice cream recipes and scoop shops out there that omit salt. As a proponent of bold flavors, I always add salt to my ice cream, specifically Diamond Crystal kosher salt. The recipes below all include a similar amount of salt. If you use Morton's, you should double the amount of salt in the recipes below.
Ice Cream Basics
First, some ice cream facts.