The Science Behind Ice Crystals in Ice Cream: Why Milk Matters

Ice crystals forming in ice cream can be a disappointing consequence of several factors, such as temperature differences, water content, and the physical properties of the mixture. Whether you're trying to churn homemade ice cream or simply enjoying a shop-bought treat, understanding why ice crystals form can help you appreciate the nuanced science behind this beloved dessert.

Understanding the Formation of Ice Crystals in Ice Cream

When you pour milk onto ice cream, it can trigger the formation of ice crystals due to several interacting factors.

Temperature Differences

Ice cream is typically much colder than milk, averaging around -18°C (0°F). When milk is poured onto ice cream, the heat from the milk can cause some of the ice cream to melt. This melting increases the water content in the mixture, creating a more conducive environment for ice crystals to form.

Water Content

Ice cream contains a blend of air, fat, and dissolved sugars, which help to create its smooth texture. Milk, on the other hand, has a high water content. When you add milk to ice cream, you're introducing more liquid water. At low temperatures, this excess water can begin to refreeze, leading to the formation of ice crystals.

Nucleation Sites

Ice crystal formation often begins at nucleation sites—tiny imperfections or particles within the mixture. The addition of milk may introduce these sites, allowing water molecules to begin organizing into a solid structure. These sites can serve as the initial points where ice crystals start to form.

Concentration of Solutes

The melting of ice cream and its subsequent mixing with milk can alter the concentration of solutes, such as sugars and fats. This change in solute concentration can affect the freezing point of the mixture. As the mixture seeks a new equilibrium, further crystallization may occur, leading to the formation of ice crystals.

Agitation

Pouring milk onto ice cream introduces physical agitation, which can encourage the formation of ice crystals. The stirring action helps water molecules come into contact with colder surfaces, promoting the crystallization process.

In summary, the interaction between the cold ice cream and the warmer milk, combined with the changes in water content and physical agitation, leads to the formation of ice crystals throughout the mixture. This process highlights the delicate balance of temperature, composition, and physical properties that contribute to the texture and quality of ice cream.

Additional Insights: The Role of Butter Fat and Churning Techniques

Ice cream's high content of butter fat plays a crucial role in its smooth texture. The water in milk, however, has a higher freezing point than the fat in ice cream. When churning ice cream, salt is often added to the ice surrounding the churn drum to lower the temperature of the ice water below 0°C (32°F). This helps the fat in the ice cream mixture to solidify and semi-freeze within the drum, ensuring a smooth and creamy texture.

If you try churning ice cream with half and half or heavy cream, you'll notice that ice crystals do not form as readily. This is because these dairy products have a higher fat content, which resists the formation of ice crystals more effectively.

I have been churning homemade ice cream for over 55 years, and through trial and error, I have learned that the key to a perfect ice cream is understanding the science behind it. By carefully controlling temperature, composition, and agitation, you can achieve the delightful texture and smooth mouthfeel that ice cream is known for.