Making ice cream - it's physical chemistry

Simply mixing cream, sugar and egg yolks, with a flavour such as fruit or chocolate, and putting them in the freezer won't give you ice cream. How these ingredients are processed will affect the texture of the product and this comes down to understanding the physical chemistry of ice cream.

Look at ice cream magnified several hundred times in a scanning electron microscope and you will see that it has a complex structure on length scales of 1 μm to 1 mm (Fig 1). There are ice crystals (ca 30 per cent by volume), air bubbles (50 per cent) and fat droplets (5 per cent) from the cream, held together by a viscous sugar solution (15 per cent). Ice cream thus contains all three states of matter simultaneously and is both a foam and an oil-in-water emulsion. The quality of ice cream depends on its microstructure: small ice crystals and air bubbles give the ice cream a smooth, soft texture. If the ice crystals are too large, the ice cream becomes gritty and unpleasant to eat. Creating this microstructure is the key to making good ice cream - and to understand how to do this, you need to know some physical chemistry.