Rennet refers to a milk-fed calf's fourth stomach and the complex of enzymes that occur naturally in it. Rennin, one such enzyme, is an emulsifier and thickener and it causes milk proteins, such as casein, to coagulate. In calves, rennin assists with digesting the mother's milk. When added to ice cream, rennin improves texture and consistency, preventing lumpy ice crystals from forming.

Cream is a natural emulsion of fat and casein protein, suspended in water. Rennet causes caseins to stick together, forming three-dimensional structures. As these structures form, they trap fat globules within them. To the naked eye, rennet-treated cream looks like a custard. Rennet-treated cream eliminates the need for making an egg custard, a precarious task involving mixing hot cream into room-temperature eggs, or tempering the eggs as for a cream and egg custard-based ice cream.

Renneted milk proteins tend to coagulate at roughly 86 to 88 degrees Fahrenheit, and rennet thrives in temperatures of 85 to 105 F. Ice cream recipes involving rennet require you to heat the sweetened, flavored cream to approximately 110 F, add the rennet and let the renneted lukewarm cream sit undisturbed for about 10 minutes before chilling it. While the cream sits, rennet reacts with milk proteins and subsequent chilling encourages the proteins to coagulate. Failure to follow these steps precisely can sabotage coagulation, preventing the renneted cream from setting correctly.

All milk contains natural anti-clumping agents that prevent casein from clinging together into structures. These anti-clumping agents, called glucomacropeptides, stick to the exterior of casein particles, or casein micelles. Glucomacropeptides are water soluble, and their presence on the surface of casein micelles allows the casein to flow freely through the milk. Rennet removes the anti-clumping agent from casein micelles. Because casein is naturally resistant to water, it begins coagulating at once.

Rennet isn't an essential ingredient to ice cream, only a helpful ingredient. Using rennet is like taking a shortcut to preparing custard. While making custard is one way to make ice cream, it's not the only way. Ice cream is a chilled emulsion, and you can attain this same result by agitating heavy cream over a source of cold.