“ice cream” only contains two simple words “ice” and “cream”. But it is actually quite a complex and multi-faceted composition. It should be solid, but soft. On a molecular level, ice cream is made up of three basic components: ice crystals, concentrated sweetened cream (include liquid water, milk fat globules, milk proteins, and sugar), and air.
1. Ice Crystals
The ice crystals are created when the water-content in the base starts to freeze, they put the “ice” in “ice cream”, giving solidity and body. The size of the crystals determines whether the ice cream is fine and smooth or coarse and grainy, and also determines the perceived temperature as it hits your tongue: grainy ice cream will feel colder than a more smooth-textured mixture. (Why is it so? Because the larger crystals require more heat to melt. That heat comes from your mouth, and thus large ice crystals cool your mouth more, making the ice cream feel colder overall.) So to make the ice cream smoother, we need to keep Ice Crystals small. Though ice crystals account for much of the ice cream’s solidity, they only make up a fraction of its volume.
2. Concentrated Sweetened Cream(include liquid water, milk fat globules, milk proteins, and sugar)
The concentrated sweetened cream is what is left of the mix when ice crystals form. Because of all the dissolved sugar, about a fifth of the water in the mix remains unfrozen even at 0 °F ( -18 °C). The result is a very thick fluid that is about equal portions of liquid water, milk fat globules (both liquid and semi-crystalline), milk proteins (casein micelle and whey), and sugar. This fluid coats each of the many millions of ice crystals, and sticks them together. Semi-crystallized globules of milk fat knock together when the ice cream is churned, and form long, pearl-like strands that wrap around the air bubbles, yielding a stable foam, and giving ice cream most of its volume.
Air cells are trapped in the ice cream mix when it is churned. They interrupt and weaken the matrix of ice crystals and cream, making that matrix lighter and easier to scoop and yielding a melty, luscious mouthfeel. The more air bubbles there are and the smaller they are, the smoother the ice cream will be. Large companies have machines that can incorporate lots of air. The air cells also inflate the volume of the original mix. The increase is called “overrun”, and in a fluffy ice cream can be as much as 100%, that is, the final ice cream volume is half mix and half air. The lower the overrun, the denser the ice cream.