Ice cream is accurately described as a complex, frozen colloidal system that includes a foam, an emulsion, and a suspension. The transformation relies almost entirely on physical changes and specific principles of physical chemistry.
The Primary Process: Physical Change vs. Chemical Change
A physical change involves an alteration of a substance’s form or state without changing its fundamental chemical composition. A chemical change, conversely, results in the formation of entirely new substances by rearranging atoms and breaking or forming chemical bonds.
The core process of turning a liquid ice cream base into a frozen dessert is a physical change. Freezing the water content simply changes its physical state from liquid to solid ice crystals, but the water molecules themselves remain \(\text{H}_2\text{O}\). Incorporating air into the mixture during churning is also a physical process, dispersing gas bubbles into the liquid without any chemical transformation. The mixture’s chemical identity remains consistent, only its physical structure is altered through cooling and mechanical agitation.
How Solutes Affect Freezing Temperature
The ability of ice cream to remain soft and scoopable, even below the freezing point of pure water, is due to freezing point depression. This is a colligative property, meaning it depends solely on the concentration of dissolved particles, or solutes, in the solvent. The primary solutes in ice cream are sugars, such as sucrose, glucose, and lactose, dissolved in the water phase.
These dissolved sugar molecules interfere with the ability of water molecules to align and form the structured lattice of solid ice. A higher concentration of solutes requires a much lower temperature for the mixture to fully solidify. This effect ensures that a significant portion of the water, typically 50 to 60 percent, remains unfrozen and liquid at standard serving temperatures, preventing the ice cream from becoming a solid block of ice.
The traditional method of using a salt and ice mixture in a hand-crank machine also demonstrates colligative properties. Adding salt to the ice lowers the freezing point of the water in the cooling bath, allowing the surrounding brine to drop to temperatures as low as \(-6^\circ\text{C}\). This colder environment extracts heat from the mixture quickly, which is necessary to freeze it before large, undesirable ice crystals can form.
Creating the Structure: Emulsions, Foams, and Stability
Ice cream is a sophisticated colloidal system, consisting of multiple phases dispersed within one another. The final product is a combination of two colloidal systems: an emulsion and a foam. This complex structure is responsible for the characteristic smooth texture and mouthfeel.
The ice cream base starts as an emulsion, a stable dispersion of tiny milk fat globules suspended within the continuous aqueous phase. Proteins from the milk act as emulsifiers, coating the fat globules to stabilize the initial mixture. Adding specific emulsifiers, such as mono- and diglycerides, helps to slightly destabilize the fat globules, which is a necessary step for structure formation.
Structure Formation
Mechanical agitation during churning partially coalesces the fat globules, creating a network that surrounds and stabilizes the whipped-in air bubbles, forming a foam. The volume of air incorporated, known as overrun, significantly impacts the texture, with more air resulting in a lighter product.
Simultaneously, the water phase begins to freeze into small ice crystals, creating a suspension of solid particles that contributes to the overall firmness. Stabilizers like various gums or gelatin are often included to bind excess water. This helps prevent the small ice crystals from growing into larger, grainy ones during storage.