By Yvonne Baker
While you are tucking into your ice cream on the beach this summer, you are actually closer to science than might at first meet the eye. So here – with the help of my Dad, who spent many years developing ice cream – is a quick lowdown on the science of ice cream, with which you can impress your family, amaze your friends and also enjoy yourself checking samples!
Firstly, what is ice cream? Well, scientifically speaking it is a frozen matrix of water, fat (dairy or vegetable), milk proteins, sugars, salt and air, with – interestingly from a physical chemistry side of things – a physical structure including liquid, solid and gas phases. This is because, simultaneously, air cells, ice crystals, fat particles and other components all exist within the continuous liquid phase which binds it all together. Water in the form of ice crystals, fat and air sizes, and relative proportions of these are key to the quality of the ice cream and also determine its storage life.
We have all now seen ice cream being made on a small scale, either in your own kitchen or on various television cooking programmes. The Catalyst article in the National STEM Centre eLibrary even shows you how you can make ice cream without a freezer. However, how is the making of ice cream translated to an industrial scale? – after all, it would take a lot of domestic ice cream makers to feed Brighton Beach on a sunny day!
Well, this is where what chemical engineers call ‘unit operations’ come in – these are bits of a process which, when linked together in the right combination, transform raw materials into a finished product. For ice cream production, key unit operations include:
- Formulation and preparation – a typical ice cream formulation includes 10% fat, 12% non-fat milk solids,15% sucrose , 0.5% stabiliser/emulsifier (such as locust bean gum/glycerol monostearate) with water making up the remainder. Importantly, not all of the water in ice cream production is ever completely frozen, so stabilisers are used to achieve texture, meltdown and the storage characteristics required by the end customer.
- Pasteurisation – as with milk, this involves exposing the mix to a high temperature for a short time (eg 80’C for 25secs) to ensure it is ok to freeze and safe to eat.
- Homogenisation – emulsifiers are included in the ice cream mix to help destabilise the fat globules during homogenisation where the ice cream mix is forced through a small orifice at high pressure (2000-2500 psi). This reduces the fat globule size (to a few microns) to give a dry stiff texture to ice cream when extruded from a freezer. This then allows the manufacture of the different product formats e.g. cones, tubs, stick products and logs.
- Ageing/addition of flavours – the mix is cooled below 4C for ageing, standing for a few hours before further processing. This is an essential step to allow the physical interactions to occur to produce the desired end product, such as crystallisation of fat particles. Flavours are added at this stage to avoid heat damage.
- Freezing and product formation – the aged mix is pumped through a refrigerated cylinder which has a rotating part with sharp blade. The frozen mix is scraped from the cylinder surface at the same time as air is incorporated, and through mixing occurs. The ice cream is extruded at around -5C into the required product form.
- Hardening and storage – once formed into the desired shape, the ice creams are frozen further (hardened) to around -20’C for storage and distribution. Ideally ice cream products should be kept at the lower (hardened) temperature throughout storage and distribution; this prevents partial melting of the ice crystals, which will form larger crystals when they recrystallize so impacting ‘mouthfeel’. However, in reality, mix formulations and processing techniques have been developed to minimise the adverse effects of recrystallization. Interestingly, soft ice cream outlets (like vans) avoid this problem by buying in sterilised ice cream mix and freezing it on demand; the disadvantage is that storage times are much shorter, and these products must therefore be consumed within a short time.
After all these processes, the ice creams are at last ready to be enjoyed. So, next time you decide to indulge in a Magnum, Cornetto or even a Twister (one of the many ice creams my Dad had a hand in developing as part of his working life), give a quick thought to the many engineers, scientists and others who have helped bring you this little bit of summer heaven, and reflect on the many uses that science has!