Ice cream is a mainstay of summer – for many, a trip to the beach would be incomplete without one. Despite its seeming simplicity, ice cream is a prime example of some fairly complex chemistry. This graphic takes a look at some of the ingredients that go into ice cream, and the important role they play in creating the finished product. There’s a lot to talk about – whilst the graphic gives an overview, read on for some in-depth ice cream science!
Initially, it might be hard to believe that ice cream could be all that complicated. After all, it’s essentially composed of three basic ingredients: milk, cream, and sugar. How complex can the mixing of three ingredients really be? As it turns out, the answer is: very! Simply mixing the ingredients together, then freezing them, isn’t enough to make a good ice cream. To understand why this is, we’re going to need to talk about each of the component ingredients in turn, and what they bring to the table.
Ice cream is a type of emulsion, a combination of fat and water that usually wouldn’t mix together without separating. However, in an emulsion, the very small droplets of fat are dispersed through the water, avoiding this separation. The manner in which this is accomplished is a result of the chemical properties of molecules in the emulsion.
The fat droplets in ice cream come from the cream used to make it. Fats are largely composed of a class of molecules called triglycerides, with very small amounts (less than 2%) of other molecules such as phospholipids and diglycerides. The triglycerides are made up of a glycerol molecule combined with three fatty acid molecules, as shown in the graphic. The melting temperature of the fats used in ice cream is quite important, as fats that melt at temperatures that are too high give a waxy feel in the mouth, whilst it’s difficult to make stable ice cream with those that melt at too low a temperature. Luckily, dairy fat falls just in the right range! As it happens, you can also make ice cream with palm oil and coconut oil, as their melting temperatures are similar.
As we’ve already said, we’d expect the fat in ice cream to separate from the water. The reason it doesn’t can be put down to some of the other ingredients. Milk proteins from milk or cream play a role. During ice cream’s manufacturing process, the fat is forced through a small valve under high pressure to break it into small droplets. The milk proteins stick to the surface of these fat droplets, creating a thin membrane. This membrane of protein molecules helps prevent the fat droplets from coalescing back into bigger droplets, as the proteins coating individual fat droplets repel each other when they come into close contact.
We’ve yet to mention emulsifiers, but they’re also an important part of the ice cream mix. Usually, these are molecules that make it easier to create emulsions – one end of them is soluble in water, whilst the other is soluble in fats and oils. Emulsifiers surround droplets of fat and oil, and allow them to mix with water, rather than forming separate layers.
In ice cream, the role of emulsifiers seems slightly in opposition to their name – they’re actually present to help de-emulsify some of the fat. They do this by replacing some of the milk proteins on the surface of the fat droplets. This leads to a thinner membrane surrounding the droplets, which in turn means they’re more likely to coalesce and cluster during whipping. We need some of the fat in ice cream to be de-emulsified, because it plays an important role in trapping air.
When ice cream is made, it is simultaneously aerated and frozen. Most ice creams will have a significant volume of air contained within them, and this is what the fat, protein and emulsifier combination is vital for. It’s actually very hard to incorporate as much air into products where fat and protein aren’t present – for example, sorbets. Better quality ice creams tend to have a lower air content, and hence a higher density. A higher air content ice cream also melts more quickly.
Freezing adds another important element to the ice cream: the ice itself. Modern factories commonly use liquid ammonia to produce the low temperatures required, though before this was available, mixtures of water and salt would have been used. Adding salt to water can lower its melting point to as low as -21.1˚C, whereas liquid ammonia is used at around -30˚C. The colder the refrigerant being used is, the quicker the ice cream can be made.
Ice cream is made in a barrel with rotating scraper blades. When the ice cream touches the sides of the barrel, it freezes, but then is immediately scraped off by the scraper blades. The very small ice crystals produced are dispersed throughout the mixture. We want the ice crystals to be as small as possible, because the smaller they are, the smoother the ice cream will be.
Sugar is an ingredient we haven’t mentioned yet, but it’s also a key one. As well as sweetening the ice cream, it helps lower the freezing point of water, reducing the amount of ice produced in the freezing process. By changing the amounts and types of sugars used, we can affect the hardness of the ice cream, as softer ice cream contains less ice. Sugars also affect the viscosity of the liquid syrup in which the fat droplets and air bubbles are suspended.
Stabilisers, too, affect the viscosity of the liquid. They’re water-soluble molecules that are commonly derived from plants, and play a number of roles. A commonly used example is sodium alginate, which is derived from brown seaweed, as is another stabiliser, carrageenan (less frequently used due to its cost). Stabilisers also help reduce the melting rate of ice cream, and give it a smoother texture.
Perhaps the most important ingredient is the flavour of the ice cream. This can, depending on the flavour desired, be added naturally, for example by adding vanilla. It can also be accomplished via the use of artificial flavours. Sticking with vanilla as our example, synthetic vanillin can be added to replicate the flavour. Other flavours can be replicated in a similar manner, though they tend to require a more complex mix of molecule to achieve an authentic flavour. Natural dyes such as anthocyanins can then be added to ensure the ice cream is the correct colour.
Some other ingredients are slightly more surprising. Skatole is a molecule that’s found in faeces – and also in ice cream in very small amounts. It’s an odd molecule, that, at high concentrations, smells exactly how you’d expect it to considering it’s found in faeces, but smells floral at very low concentrations. In some ice creams, it’s added as a flavour enhancer.
Finally, it’s worth taking a minute to address a spot of ice cream misinformation. You may well have come across the claims that ice creams sometimes contain ‘beaver butt’. To give it its more correct, non-sensationalising term, we’re talking about castoreum, a secretion from a beaver’s castor sacs (not its anal glands, as some claim, though they are in close proximity). Castoreum is generally recognised as safe by the FDA, and apparently low concentrations taste like vanilla, so to this point, it seems like a legitimate claim.
However, castoreum is pretty expensive. Apparently, enough castoreum to replace the vanilla in half a gallon of ice cream would cost $120, so it’s unlikely to be a particularly cost-effective way of making ice cream. Additionally, a number of manufacturers, whilst obviously not wanting to divulge their exact recipes, have adamantly stated that they don’t use castoreum in their products – it’s actually more commonly used in perfumes. So, whilst your perfume might contain beaver secretions, your ice cream almost certainly doesn’t!
If your appetite for ice cream science still remains unsated, check out this video on the subject from ACS Reactions.
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References & Further Reading
- Ice cream & chemistry – ACS ChemMatters Online
- Skatole – University of Bristol Molecule of the Month
- Colloidal aspects of ice cream – a review – H Douglas Goff
- The Science of Ice Cream (£) – C Clarke