In Part 1 we looked at the size of ice crystals and fat droplets, but there is one final important factor to consider. Ice cream makers constantly blend the mixture to ensure smoothness, but why is this necessary?
The Importance of Air Bubbles
Ice cream mix is added to the ice cream maker when the mixture is at about 4 degree Celsius. The first step is to inject air to form large air bubbles, which are eventually broken down into smaller air bubbles as the ice cream is mixed. The fat droplets and milk proteins also bind to the air bubbles to stabilise them, in a similar way that emulsifiers stabilise fat droplets.
The walls of the ice cream maker are cooled down to about -30 degree Celsius, which means when the mix touches it, it instantly forms ice crystals. This newly formed ice cream is scrapped from the side and remixed with the remaining mixture, breaking up the ice crystals. As more and more ice crystals are formed, the viscosity of the solution starts to increase.
The Final Step
You may think once ice cream is made is the end of the journey, but not quite yet. After hardening, the ice cream is ready to be consumed but, after you purchase it, you may want to transport it home. However, this is a highly temperature-sensitive product. If it warms, it easily melts and it it’s kept at very low temperatures, it can alter its microstructure by forming large ice crystals.
The main problem is that ice cream is an unstable emulsion, with a natural tendency to form larger and fewer particles. This is what happens when you buy ice cream, let it melt and attempt to re-freeze it. This never works.
As you take ice cream from the freezer, the temperature immediately starts rising and ice crystals begin to melt. The larger crystals get smaller and small crystals may disappear completely. When you get back home and place the ice cream in the freezer, the fraction that had melted will re-freeze again. Unfortunately, no new ice crystals are formed, but instead surviving crystals simply get bigger. The total amount of ice doesn’t change, but the ice cream microstructure is lost as the average size of crystals increases. If a large amount of ice cream melts, re-freezing may produce a very gritty and lumpy texture.
With this in mind, my recommendation is to eat ice cream straight away to avoid the risk of destroying a complicated microstructure that took years of scientific research to develop!