Emulsifiers

Brent Murray, professor of food colloids at the University of Leeds, believes understanding ice cream chemistry begins with realising that it’s an emulsion and a foam mixed together. ‘Your basic ice cream mix is just milk, cream, sugar and flavourings, which gets homogenised to form an emulsion of fat droplets in water, which wouldn’t ordinarily mix,’ he says. ‘There are natural emulsifiers present in the form of milk proteins, which surround the globules of fat and aid this process.

‘Some artisan ice creams leave it there, but most commercial recipes need to add extra emulsifier, like glycerol monostearate [GMS],’ Brent continues. ‘The confusing thing is that the role of this extra emulsifier isn’t to stabilise the emulsion even further, but rather to destabilise it a bit.’

Emulsifiers like GMS replace some of the milk protein molecules that surround each fat globule. ‘These small molecules, with lower molecular weight, will always outcompete the larger milk protein molecules, partially displacing them, and this can make the droplets slightly less stable. They stick together a bit more and that, in turn, helps the mixture to stabilise and retain air bubbles of small enough size,’ says Brent.

Air bubbles

As the mixture is stirred and frozen in the ice cream machine, it is also whipped to aerate it. Most ice creams have a significant amount of air trapped in them – up to 125% of the volume of the ingredients in some supermarket ‘value’ products, though less in gourmet-style gelatos.

‘To get the right creaminess and stability, the air bubbles in ice cream need to be about 20 µm in size, and as uniform as possible,’ continues Brent. ‘An air bubble that size in water would dissolve quickly – perhaps in a couple of minutes – because air is quite soluble in water. A combination of natural foaming agents (proteins again), the aggregated fat droplets, plus freezing as fast as possible to increase viscosity, helps to trap the bubbles at that small size.’You can use one type of freezer- Curverd Glass Type Ice Cream Chest Freezer.