The time for ice cream is here. Hot sunny days ask for nothing else than the cold stuff!
However, have you ever considered what happens during the process of making ice cream? It’s not just a matter of mixing cream or milk, egg yolks and sugar and putting the ingredients in the freezer. You’ll get a frozen desert, but technically it won’t be ice cream. How the various components are prepared will alter the final texture of the product. To achieve a perfect ice lolly it’s important to understand the chemistry (and some physics) of ice cream.
Under the microscope
Under the microscope, ice cream appears as a complex structure of air bubbles (about 50% in volume), ice crystals (30%) and fat droplets (5%), “glued” by a sugary milky solution (15%). Interestingly, ice cream contains all three states of matter at the same time (gas – air bubbles; solid – ice crystals and liquid – milk) and is considered both an oil-in-water emulsion and a foam.
This microstructure has a major impact on the final quality of the ice cream, with small ice crystals and fat droplets, as well as plenty of air bubbles giving a smooth texture, whereas large ice crystals make the ice cream heavy and unpleasant to eat. The key to good ice cream is to understand how to achieve the best microstructure.
Creating a microstructure
As in any recipe, the first step is to determine how much of each ingredient to use. Ice cream usually contains about 50-60% ice (by weight). Too much ice and the product would be too hard, and too little it would melt easily. Crucially, this value is determined by the amount of sugar (sucrose, C12H22O11) added, as this will affect its freezing point.
We all know that pure water freezes at 0 degree Celsius. However, when sugar is added, the freezing point decreases, as the presence of sucrose molecules make it harder to form ice crystals. Cooling a sucrose solution below its freezing point results in the formation of ice crystals, reducing the amount of liquid water and increasing the corresponding concentration of sucrose. This effects further decreases the freezing point, accelerating the formation of more ice crystals.
The other important factor to determine the final texture is the size of the fat droplets. To ensure small droplets are dispersed evenly, the ingredient mix is homogenised to break up the fat globules, increasing its surface area. To maintain this structure, surfactants (such as casein in milk) bind to the homogenised fat droplets, preventing them from re-forming large blobs. As soon as the mixture is cooled below 5 degree Celsius (freezing point for milk fat) it begins to crystallise.
So far, we’ve discussed why the size of ice crystals and fat droplets is vital for quality. In the next post we’ll we look at why the mixture needs to be constantly churned, to ensure dispersal of infinitely small air bubbles.
Read the next part now: The Science of Making Ice Cream – Part 2