Ice cream seems like a simple concept. Take some dairy, add some sugar and flavors, and freeze.
But to get a perfectly creamy, smooth textured frozen treat, we need more than a low temperature-it requires a careful interplay of chemistry and three states of matter: solid, liquid and gas.
What’s in the box?
Commercial ice cream includes many ingredients: air, water, milk fat, so-called milk solids (mostly milk proteins and lactose), sweeteners, stabilizers, emulsifiers and flavors. Ingredients are mixed and pasteurized for food safety.
Homemade ice creams usually use milk, heavy cream, sugar and flavors, such as fruit, berries, or chocolate. The exact amount varies from recipe to recipe, but the processing steps are similar.
Milk contains everything a baby cow needs to grow and develop—water, fat, carbohydrates, protein, minerals and vitamins. These substances react in different ways when they are frozen.
First, the crystals
As the ice cream mixture is cooled, small clusters of water molecules gather to form tiny ice crystals. The size of the ice crystals is responsible for the ice cream’s mouthfeel—the smaller the crystals, the smoother the feel.
If the crystallization is not well controlled, these crystals can be very large. Ice cream makers (commercial or for home use) secure the tiny ice crystals by stirring or churning the liquid as it freezes. This prevents the movement of water molecules and prevents the crystals from growing.
The mixing process also includes air, which is the secret ingredient to give the ice cream a lighter texture.
Next, the fat
Milk fat is present as globules surrounded by proteins. These proteins bind fat and water, helping to keep fats suspended. (Milk appears white because light scatters these fat globules.)
These milk fat molecules have different properties at different temperatures. At room temperature they are semi-solids (like butter), and about two-thirds solid at 0 ℃.
Fat globules can stick together—so you get a layer of cream on top of unprocessed milk. A process called homogenization forces the milk through a small opening under very high pressure, breaking up the large fat globules into smaller ones. This process produces many tiny fat globules—as many as a trillion per liter. Homogenized milk ensures that the mixture freezes evenly, and the separated fats do not stick to the mixing machinery.
Freezing the fat globules causes them to clump together, with surrounding proteins acting as bridges to other fat molecules and the ice crystals. These fats melt in your mouth, giving a creamy feel and taste.
Then, the sugar
Sugar and other dissolved milk components are also important to the final texture of the ice cream. The presence of sugars in water lowers the freezing temperature of the mixture to below 0 ℃.
Here’s why that’s important. As ice crystals begin to form, the concentration of sugars and other dissolved materials in the unfrozen liquid increases, further lowering its freezing point. By the time most of the ice crystals have formed, the resulting liquid is highly concentrated in sugars.
This concentrated liquid, known as “serum”, bridges between the ice crystals, solid fat globules and air bubbles. The serum remains a liquid well below 0℃ and adds enough flexibility to the mixture so that the ice cream can still be scooped or shaped.
In this way, the unique chemical properties of water, fat, protein and sugar combine with air to give the solid, liquid and gaseous mixture we know and love.
It’s not all ‘ice cream’
So-called “ice cream” is actually governed by a food standards code. That’s why not all frozen desserts can legally be called ice cream, because they don’t have enough milk fat.
There are many variations on the standard ice cream recipe. Gelato uses more sugar, contains less air, and usually contains less fat and other solids. Sorbets eliminate dairy and often contain a lot of sugar, but historically have used egg or gelatin as a protein source.
Regardless of the exact recipe, the basic ice crystal formation, fat stabilization, and serum phase separation steps are the same.
Product names like “soft serve”, “dairy dessert”, or “ice confection” are usually an indication that the ingredient list includes vegetable fats rather than more expensive milk fats.
Soft serve products are also formed by agitation as the mixture freezes, but tend to contain less air than the ice cream you buy in a tub, due to the constant agitation inside the dispensing machine.
Ice poles, ice blocks, freezies, or freeze pops (depending on your local phraseology) and other “water ice” are frozen inside a mold or plastic tube. The shape of the mold limits the ability to stir the mixture, so the freezing process is usually done “quietly”, which means resting. Ice crystallization is poorly controlled, and you may experience large crystals growing (technically “seeds”) from the popsicle stick.
Mankind has enjoyed ice cream for centuries. It is an incredibly versatile food with endless variations of flavors, additives, and toppings combined with memories of joy, comfort, indulgence and nostalgia. And lots of chemistry, too.
Provided by The Conversation
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Citation: How does ice cream work? A chemist explains why you can’t freeze ice cream and expect results (2023, July 24) retrieved on 24 July 2023 from https://phys.org/news/2023-07-ice-cream-chemist-results.html
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