I’ve forgotten most of my high school classes, but I vividly remember making peanut brittle in chemistry class. This was practical. I spent the next few weeks “studying chemistry” by making every type of candy imaginable. My enthusiasm leveled out, but not before I connected chemistry with all sorts of cooking. Almost every technique in cooking—browning, baking, caramelizing—relies on chemistry. Making ice cream is a prime example.
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Components of Ice Cream
In order to make ice cream, you need four basic components: ice, air, fat, and sugar. While these ingredients may not seem like they’ll make ice cream, chemistry steps in and does the hard work. Here’s more information about the four ingredients and how they fit into ice cream.
Ice crystals make up 30 percent of the finished ice cream. Where does the ice come from? Milk is 90 percent water, and cream is 30 percent water. The smaller the ice crystals, the smoother the ice cream. When you let ice cream melt and re-freeze, the ice crystals get bigger, and you lose the smooth texture.
When you use an ice cream maker, paddles stir the ice cream mixture and incorporate tiny air bubbles. These air bubbles make ice cream light and fluffy. You can adjust the creaminess of your ice cream by changing the speed of the mixing: the faster the paddle moves, the more air bubbles form. If you want a dense, creamy ice cream, then turn the paddle slowly, but if you want a light and fluffy ice cream, turn the paddle faster. The shape of the paddle in the ice cream maker can also affect how much air gets incorporated into the mixture.
Ah, fat. It makes things taste so good. Most of the fat comes from the cream and egg yolks, but a little may come from the milk, depending on what type you use. The fat is what gives ice cream its smooth texture and locks in the flavor so it’s evenly dispersed. It also coats each and every pocket of air in the ice cream, helping give it a smooth texture.
If you don’t use an emulsifying agent, then the fat will not mix with the water. Water is polarized: one side of the molecule is positive and the other negative, but fat is nonpolar, so it isn’t attracted to either end of the water molecule. To make it more complicated, milk fat globules are coated with protein, so you need an emulsifier that will replace those proteins in the chemical structure. Egg yolks work because they contain lecithin. The lecithin moves between the fat molecules and binds all the ingredients together.
The sugar solution in ice cream is complicated. It combines three different solutions: water in a sugar solution, sugars in sugars solution, and nonfat milk solids in sugar solution.
Water in the Sugar Solution
Most of the water from the milk and cream freezes, but some stay in liquid form even after the ice cream hardens. This is because a high concentration of sugar lowers the freezing point. As more of the water freezes, the water that remains liquid gets sweeter. Soft serve ice cream recipes call for such immense concentrations of sugar that the ice cream never hardens and freezes.
Sugars in the Sugar Solution
While many people think that the only thing sugar does for ice cream is make it sweet, that actually isn’t true. In addition to adding sweetness to the ice cream, sugar also gives the ice cream more body and helps keep it soft. This is because sugar reduces the freezing point of water, so ice cream stays soft even when plain water would freeze solid.
Non-Fat Milk Solids in the Sugar Solution
Milk solids non-fat (MSNF) are what’s left of the milk after all the fat and water are removed. MSNF is mostly protein, but there are also some salts and minerals. Fat globules use milk solids to trap air bubbles and stabilize the chemical structure of the ice cream. MSNF also gives ice cream its milky flavor.
An emulsion is a mixture of two liquids that generally won’t blend together, like oil and vinegar salad dressing. If you don’t shake before you pour the dressing on your salad, then you’ll just get a lot of oil on your salad. And after shaking, the oil and vinegar immediately begin to separate again. However, if the salad dressing had an emulsifier mixed in, the oil wouldn’t separate from the vinegar.
Cream naturally separates from milk. After the farmer milks the cow, the cream will rise to the top. While Little House on the Prairie days have passed, milk is still the same. Since the best ice cream is smooth and creamy, you need something to stop the separation. This is why many ice cream recipes use eggs—the egg yolks bind the milk and cream together.
Ice cream balances three states of matter: the solids are ice and cream, a sugary solution makes up the liquid, and air bubbles are the gas. These same ingredients give ice cream its texture. The cream makes it smooth, and ice cream’s softness and meltiness are a result of millions of tiny air bubbles. So how do you get all this together? Again, it’s thanks to emulsifiers.
There are many emulsions in cooking: egg yolks, butter, mayonnaise—Italian gelatos even use corn starch or tapioca starch as an emulsifier. Vegans have been exceptionally creative in finding alternative emulsifiers like flax seeds, applesauce, and tofu. Once you know what you need, you make your own choice. The flavor is another consideration. Mustard is an emulsifier, but its not the best ingredient for ice cream unless you’re playing a practical joke on someone.
A quick word about stabilizers. Commercial ice cream producers add stabilizers to their recipes to increase shelf life. If you look at the ingredients list on ice cream containers, you may see a list of chemical-sounding names—those are stabilizers. Without the stabilizers, the ice cream would lose its taste and texture during shipment and storage. Most home cooks don’t use stabilizers because they have no need for them. Homemade ice cream is often made in much smaller quantaties than you’d buy in a store, and it’s intended to be eaten quickly after making it.
Stabilizers do more than extend the storage life of ice cream—they can also reduce the size of the ice crystals or air bubbles. The result is a creamier ice cream that tastes like store-bought but has the ingredients you chose. Some conventional stabilizers used by home cooks are eggs, starches, gelatin, and thickening gums.
The Five Steps of Ice Cream Production
Now that you understand the chemistry that helps ice cream form, you can make your own ice cream. I’ve broken the process down into five basic steps.
Preparing the Mix
After you’ve chosen a recipe, the first step is to mix the ingredients together and heat it up. I know this is counterintuitive, but heating kills any bacteria or Salmonella lurking in the milk or eggs. This is absolutely vital if you are using raw (pre-pasteurized) milk. In addition, heat makes it easier for the sugar to disolve into the milk and cream, which is necessary for a smooth texture.
Heat also starts changing the structure of the milk proteins—chemists call this denaturation because protein changes from its natural state. This will add stabilization later in the process.
Now that your ice cream mixture is hot, you have to cool it back down. There are a few ways you can do this, such as putting the mixture in your refrigerator or putting it in an ice bath. Once it is room temperature, I move it to the freezer overnight. The faster the ice cream cools to room temperature, the smoother and creamier it will be. If you skip cooling process, the finished ice cream may taste grainy or chewy because the sugar, fats, and proteins won’t be able to line up properly, and it’ll feel and taste like that pint of ice cream that got lost in the freezer for ten months before being found.
I didn’t misspell it. “Ageing” is the correct spelling, but I can’t tell you why. As the ice cream mix sits in the freezer, it ages. Ageing involves two chemical alterations that will make more air bubbles in the final freezing process. The first chemical change is to the fat globules. As the mix ages, the fat globules crystalize and make tiny points that will help them stick together in the next step. Think of them as small spears that punch air holes.
The second chemical process is the emulsification. The emulsifying agents push the milk proteins off the fat globules and take their place. By morning, the milk proteins and fats start clumping together. Don’t worry if your mix looks lumpy and unappetizing the next morning—it’s supposed to look like that.
The next step is to start churning and freezing the ice cream. Ice cream usually freezes at 27 degrees Fahrenheit (-3 C), but this varies by the amount and type of sugar. In a freezer bowl ice cream maker, the mix cools, and ice crystals form on the side of the container, and the dasher (blade) scrapes these crystals off and blends them into the mixture. This decreases the temperature more so that more ice crystals are formed.
While the dasher moves ice crystals into the mix, the blades create air pockets. While the dasher evenly distributes the air pockets and crystals, the emulsification process passes through a stage called “partial coalescence”. This just means that the fat globules created overnight in the freezer bounce around until they collide with one another, forming droplets of fat. These droplets are unstable until they wrap around air bubbles. Then, with the help of the emulsifying agent, they stabilize into hardened ice cream.
Churning the hand-crank ice cream maker always seemed to take forever when my brothers and I churned ice cream on my grandparent’s porch. At first, we would fight over turning the crank. An hour later we fought over who had to crank next. I love those memories.
Electric countertop ice cream makers do the churning for you and do it faster. If you really want speed, liquid nitrogen makes ice cream almost instantly. That’s how Dippin’ Dots are made, but you need to be extremely careful when attemping to do it at home.
Congratulations! You made it through a tasty science lesson. Now you are ready to make your own secret recipe for amazing ice cream. Or if you want to include the kids, Scientific American has a simple science project for making ice cream in a bag. It’s a perfect activity for a hot summer afternoon.