Ice cream at its simplest is made of milk, sugar, cream, and some flavoring, such as fruit puree or vanilla. An essential ingredient in addition to those is air, without which ice cream would not be the special treat it is.
In the U.S., ice cream must contain at least 10% milk fat, and at most 50% air, and must weigh at least 4.5 pounds per gallon.
Ice creams termed "premium" and "super premium" have higher fat content (13% to 17%) and lower air content (called "overrun" in the ice cream trade).
Butterfats add rich flavor, smooth texture, body, and good melting properties. The triglycerides in butterfat melt over a wide range of temperatures, so there is always some bit of solid and some liquid butterfat. Some of the butterfat almost turns into butter while the ice cream is churned, adding to the unique texture of ice cream.
The proteins help to incorporate air into the mixture, helping to form small bubbles of air. They modify the texture of the ice cream in other ways as well, making it chewier, and giving it body. The proteins also help to emulsify the fats, keeping the fat globules suspended in the mix.
The proteins coat each fat globule, keeping them from sticking together. However, making the globules stick together in chains and mesh-like structures is important in giving ice cream its texture and ability to hold air, and its ability to stay firm as the ice inside melts. Emulsifiers, such as the lecithin in egg yolks, stick their fatty acid ends into the fat globules, and prevent the proteins from completely coating the fat. This balance between proteins and emulsifiers allows the fat globules to chain and stack, without flowing together.
The milk sugar lowers the freezing point of the water in the ice cream. Adding extra sweeteners, such as sugar and corn syrup, also have this effect. This ensures that a portion of the water never freezes, keeping the ice cream from becoming a solid chunk of ice. Added sweeteners are inexpensive, and make up about 15% of the mix by weight. The use of high-fructose corn syrup will reduce the freezing point further than sugar, resulting in a softer ice cream.
Emulsifiers such as the monoglyceride glycerol monostearate and related diglycerides help to keep the milk fat in suspension, and limit the growth of ice crystals. Other emulsifiers such as lecithin and polysorbate 80 perform similar functions. Emulsifiers have a significant effect on making the fat globules stick together in chains, rather than flowing together in larger globules, or staying separated as tiny ones. This adds to the structure of the ice cream, and affects the texture and the ability to incorporate air into the mixture.
Gums such as guar gum, locust bean gum, xanthan gum, carageenan, and methylcellulose help to prevent ice crystals from forming during freezing and re-freezing after a trip from the grocery store. They also have a "mouth feel" similar to milk fat, so the milk fat is not missed as much in low fat ice creams. Like emulsifiers, they also aid in keeping the air whipped into the mix. Gums keep the ice cream from becoming grainy due to crystals forming from either ice or lactose.
Some ice creams contain sodium citrate to decrease the tendency of fat globules to coalesce, and to decrease protein aggregation. This results in a "wetter" ice cream. The citrates and phosphates are both used for this effect. Calcium and magnesium salts have the opposite effect, making a "dryer" ice cream.