Ice cream seems like a simple concept: take cream or milk and make it cold enough to freeze. But in reality this frozen treat we know and love is one of the most complex food products available. There’s a lot of chemistry going on in a single scoop.
Milk is a complex mixture of water, sugars, fats, proteins, vitamins and minerals, but even more complex is the way that these chemical components are arranged. Water is the main component, with sugars (about 5g per 100 mL), some proteins, and minerals like calcium, magnesium, potassium, zinc and phosphate dissolved into it. Larger assemblies of proteins such as casein are present as colloids – microscopic particles that are not dissolved, but are suspended in the water. These particles scatter light, which is why milk looks white.
The fats are present as tiny globules suspended in the water as an emulsion, supported by additional proteins that coat the fat globule surface to help them stay suspended in the water, with vitamins dissolved into the fats. The globules can easily join together – which is why you get cream on top of milk – but homogenising milk makes these globules very small, giving an astounding 2.4 trillion fat globules in every litre of milk.
Sugars such as sucrose or glucose, or corn syrup, are almost always an ingredient of ice cream, not only to provide sweetness, but also for texture. The other important sugar is lactose, which can be up to 20% of the carbohydrate in the finished ice cream. However, too much lactose can give a sandy texture, which you may have experienced in a low-end ice cream that has too much milk solids-not-fat added.
There are also a greater number of large, floppy molecules that are added to ice cream mixes as stabilisers. These include starches, vegetable gums and extracts from seaweed such as carrageenan and alginate. Their primary role is to bind with water, which influences ice formation and gives a smooth texture, while also helping with stability.
The often forgotten ingredient of ice cream is air. It is incorporated during the freezing process by agitating or whipping the mixture to generate a foam – small bubbles suspended in the ice cream matrix. The air content is known as the “overrun”, and is one reason why ice cream is sold by the litre instead of by the kilogram. Some cheaper options have an overrun of 100%, meaning the volume of air in the mixture is equal to the volume of liquid – half the tub is air! More expensive products typically have a lower overrun.
As the water in milk and cream are chilled, small clusters of molecules are assembled, forming small nuclei which are the start of hexagonal ice crystals. These are surrounded by water, which can either form more nuclei, or can add to the slowly growing crystals. From a thermodynamic perspective, it’s better for water molecules to join with the already formed crystal, making larger crystals the likely outcome. But in ice-cream making, these large crystals are undesirable. A mechanical mixer (small at home, but enormous in the factory) continuously agitates the mix to ensure the ice crystals remain small, as well as incorporating plenty of air bubbles.
Under suboptimal refrigeration conditions, the ice crystals can also go through a recrystallisation, where they are partially melted and the liquid water molecules join with other crystals to give undesirable larger crystals, which can also disrupt the other two components of the mixture – the fats and air. (This effect is familiar to anyone who has left their ice-cream container on the dinner table for too long …)
While the ice crystals are forming, the various sugars, minerals and proteins that were dissolved in the water get more and more concentrated into a liquid phase, known as the serum. This serum is the liquid glue that binds our three phases together, trapping air and fat particles in the process.
The milk fats also have a unique response to the cooling process: they’re liquids at body temperature, semi-solids at room temperature (think a block of butter), and about two-thirds solid when at 0°C. This is related to the chemical structure of the milk fats. The mixing and freezing disrupts the structure of the fat globules, causing them to partially glob together. The proteins of the milk can lose their structure during this processing, and they stick to the surface of the partially crystalline fat globules, providing a bridge between the liquid serum, fat particles and air bubbles. Not only do the fats contribute to the structure of ice cream, but they carry the flavours and also affect its texture.
Milk fats may be substituted with animal or vegetable fats. The vegetable fats used are typically saturated coconut or palm oils, but animal fats have been used in the past, which is perhaps the source of the urban legend that a certain fast-food chain uses lard in their frozen desserts.
The delicious finished product of freezing and mixing is a stabilised foam made from unfrozen serum phase, pockets of air and microscopic ice crystals partially surrounded by coalesced fat globules. And this is just the tip of the waffle cone, with the specific properties of gelato, sorbet, low-fat, non-fat, water ices, dot ice creams and all their myriad flavours and textures arising from their unique ingredient profiles and freezing processes.
Next time you enjoy an ice cream treat, take a moment to consider the frozen scientific wonder it is.
This excerpt is republished online from Cosmos Magazine Issue 93, on sale now.
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