The Chemistry of Ice Cream

In this section, we will see the chemistry behind the recipe of ice cream. In more detail, we will see the chemistry recipe of ice cream, the processing of ice cream, and the transporting ice cream can face problems. Moreover, this section will be concluded by the future possible development of ice cream. To begin with, we will start with some figures related to ice cream.

The annual global production of ice cream is approximately around 5 billion litres and this volume can fill a standard swimming pool (2 metres deep) and has an area of 1 mile square. Besides that, the highest annual consumption of ice cream per capita is USA (23 litres), and then followed by Australia (18 litres), Sweden (14 litres), UK (7.5 litres), and Spain (6 litres).

The basic recipe of ice cream is comprised of 4 main ingredients; ice (hard crystalline solid), fat (soft crystalline solid), sugar (includes flavouring, hard crystalline solid) and air (79% of nitrogen and 21% of oxygen). If you want to write the chemical reaction of ice cream, it can be written as:

but just mixing these components together will not make a good ice cream, it requires processing as well (see the chemistry of chocolate).

One of the important ingredients of is flavouring and the most popular flavour of ice cream in the UK are vanilla, chocolate, and strawberry; the most chemical of flavouring agent for vanilla ice cream is shown below.

Vanillin

In many Asian countries, they prefer savoury ice creams such as green tea or red bean in Japan, sweetcorn in Malaysia, chilli in Indonesia, and sesame seed in Korea.

'Savoury' cream
Above: green tea, red bean, and sweetcorn (left to right)
below: chilli and sesame seed (left to right)

Another technique variation in ice cream processing is cream rationing during and after World War 2 which led to UK population developing a taste for ice cream containing vegetable fat, and vegetable fat is banned as an ice cream ingredient in USA until 1997. Besides that, ice cream is also have legal definitions which is vary depends on the country, for example in the UK:

"ice cream must contain at least 5% fat and 7.5% non-fat milk solids and 'dairy' ice cream must contain no fat other than milk fat".

Meanwhile, in the USA:

"ice cream must contain at leat 10% milk fat and 20% total milk solids and must weigh at least 0.54 kilograms per litre."

As we have seen what ice cream is, we can have a brief history of ice cream and there are 4 key technical advances for modern ice cream manufacture. Firstly, in around 2000 BC, food can be cooled by mixing it with snow and ice. Then followed by dissolution of salts in water to produces a cooling effect at around 400 AD. Thirdly is mixtures of salt with snow or ice cools even further in 1589 and lastly is the invention of mechanical ice cream maker in mid-19th century by Nancy Johnson. Nancy Johnson patented an ice cream maker in 1843 and sold it for $200. Furthermore, the invention of refrigeration in late 19th up to early 20th century led to the development of the modern ice cream industry.

Nancy Johnson's ice cream maker

One of the oldest technique of ice cream processing is by obtaining low temperatures without refrigerators. This technique can be done by employing the colligative property which is freezing depression of salt solutions.

The freezing point at different salt concentration

From the figure above, we can see the freezing point of salt water can be low as -28°C which such low temperatures are essential for making ice cream. Furthermore, a sharp increasing freezing point around 240 grams of salt per kg of water is due to the system turns into a hydrated salt, not a salt water. As we consider freezing point of concentrated sugar solutions as shown in figure below, a sugar concentration of about 1300 g/L depresses freezing point from 0°C to -10°C.

The freezing point at different sugar concentration

Therefore, only around 72% of water is frozen at a serving temperature of -16°C, so non-frozen concentrated sugar solution keeps ice cream 'scoopable'. Besides that, if we consider the relative viscosity of concentrated sugar solutions at -10°C, the viscosity of liquid matrix in ice cream depends on sugar. Hence, sugar solution viscosity is more than 65 times greater than of pure water.

The relative viscosity of sugar solution

Then, we can see the structure and composition of ice cream as we have discussed the physics behind ice cream processing. As it is mentioned earlier, there are 4 main components in ice cream; ice crystals (30-50 μm), air bubbles (50 - 80 μm), aqueous sugar solution, and crystalline fat particles. As you see, in this size the size particles is determined the taste of ice cream (the same way as chocolate).

The SEM photograph (left) and cartoon (right) of the structure of ice cream

As our teeth come together, the ice, air bubbles and fat particles flow around the teeth. If ice crystals are bigger than 30 - 50 μm, they get caught in gaps between our teeth, producing an unpleasant 'gritty' taste. Hence, the processing of ice cream is the key of making ice cream.

In ice cream making there are 4 important steps that are used. The first step is cream, milk, milk solids, sugars, stabilisers or emulsifiers are mixed together. Then, the mixture is pasteurised and homogenised to get milk fat emulsion. The third step is the mixture is cooled to partially solidify the milk fat. Lastly, ice cream is then whipped and frozen to produce microscopic ice crystals and air bubbles within the mixture and it needs to cool rapidly to around -20°C  to -25°C by using the ice cream freezer as shown below.

Ice cream freezer

By faster cooling, it produces more nucleation, hence smaller ice crystals are formed.

One of the common emulsifier of emulsifier is polysorbate 80 or known as 'Tween 80'. Polysorbate 80 is a non-ionic surfactant which has hydrophobic component (based on oleic acid) and hydrophobic component.

Polysorbate 80

The emulsifier plays role as a fat destabilisation where fat droplets play an important role at the surface of air bubbles to ensure sensation of 'smoothness' in mouth. Milk fat exists as microscopic particles stabilised by a layer of adsorbed milk protein (casein). To make ice cream, milk fat must be partially aggregated into large networks. Polysorbate 80 displace milk proteins and hence aids fat network formation and whipping the mixture introduces air bubbles, promotes aggregation. Partially aggregated fat particles help to stabilise the air bubbles and so provide a firmer texture.

The destabilisation mechanism of milk fat by polysorbate 80

In typical ice cream we can see by the formulation by volume, so it consists of 30% ice crystals, 5% fat droplets, 15% sugar solution, and 50% of air bubbles. The air dispersed as tiny bubbles, so ice cream tastes smooth; and without air, ice cream could not be scooped or chewed. Furthermore, ice cream are usually sold by volume which means more air, more profit.

Ice cream manufacturer comprises of nine main steps: mixing, homogenisation, pasteurisation, freezing, extrusion, hardening, coating, cold storage, and distribution; and in this section we will try to discuss about the distribution problem.

The 9 steps of ice cream manufacturer

As we acknowledge that ice cream is a delicate product and it needs a refrigerated transport, but we need to take the account of air pressure as well. For example, an ice cream factory in the USA have to across Rocky Mountains to deliver the product into consumer and the road pass is located at 2500 m altitude. As the altitude increase the pressure is dropped. Hence, the air volume in ice cream would expand according to Boyle's law. Let the example, the pressure at that height is 0.70 atm and we want to deliver 0.50 litres where the atmospheric pressure is 1 atm, so we will have 0.71 litres in volume at the road pass. Therefore, there are 4 possible technical solutions.

Firstly is to use pressurised transport which is highly expensive. The second option is to leave more space in ice cream tubs and this option is also costly expensive. The third option is to put less air into ice cream which consequently the ice cream is less scoopable. Last option to choose a different delivery route which may longer, so it is more expensive. Hence, there are no good solution in this case.

The shelf-life of ice cream depends on the temperature storage. If it is stored at -20°C to -25°C, typical ice cream shelf-life is 12-18 months. However, temperature fluctuations during storage are problematic because it can cause ice crystal and air bubble growth.

The ice cystal growth and air bubbles growth as the effect of temperature fluctuation

As we acknowledged before, the size of ice crystals and air bubbles determine the quality of ice cream, so good temperature control is essential for a long shelf-life.

Arctic pout fish (Zoarces americanus, left) and the anti-freezing protein of Choristoneura fumiferana

To conclude this section, it is better to see the future of ice cream. Unilever has used genetic modification (GM) technology to make synthetic analogues of naturally-occurring anti-freeze blood proteins found in deep sea species, e.g. Arctic ocean pout fish, and this anti-freeze protein is grown in vats using GM yeast. At only 0.01% concentration, this synthetic protein lowers the temperature at which ice crystals form and also affect their shape and it allows Unilever to make a thick, creamy 'low-fat' ice cream and this such ice cream is already on sale in the USA.