The structure and properties of chocolate

The structure of a substance affects its properties, and this is also true for chocolate. In this experiment, students explore what happens to chocolate when it is melted and allowed to reharden, testing the taste, texture and melting point of the chocolate to determine how these properties have changed as a result of the change in structure.

There are two parts to this activity: taste tests and melting point tests.

For the taste tests, students must not be in a laboratory as they are going to eat. They should be given two pieces of milk chocolate, one from an ordinary chocolate bar and one from a bar of chocolate that has previously been melted and quickly re-set. This should not be a blind trial; they should know which is which. Emphasise to students that the chemical composition is unchanged as the chocolate is heated and cooled within the wrapper.

Once they have eaten the chocolate, they can go into the laboratory. They should be warned not to eat any more of the chocolate which is used during the lesson. For difficult classes it is probably best to have the chocolate broken up into individual pieces prior to the lesson. For students who you can trust to do so sensibly, it is better to have them break up the chocolate themselves so that they can get a better idea about its ‘snap’.

The melting point tests can then be done, graphs drawn, and students given information about the melting points of the various polymorphs present in the chocolate (see Teaching notes) and asked to work out which form is present in each of their samples.

Equipment

Apparatus

• Boiling tubes, x2
• Beaker, 250 cm³ (or a similar-sized container)
• Thermometer, 0–100 °C
• Timer
• Access to a kettle (for boiling water)

Chemicals

• Milk chocolate, 1 square per student and 1 square per pair or group
• Milk chocolate that has been melted and rehardened (same type as above), 1 square per student and 1 square per pair or group

Health, safety and technical notes

• Read our standard health and safety guidance.
• Milk chocolate – for pre-melted chocolate, take a whole chocolate bar (the ones that are fully wrapped in one sealed wrapper are best). Dairy milk generally works well. Put it somewhere warm, such as on a central heating radiator, to melt it. Once it has melted, put it into a refrigerator – not one where chemicals are stored – to harden quickly. Remove once it is set and have at room temperature prior to the lesson. The remaining chocolate should be of the same make and type but simply stored at room temperature.
• For the melting point tests it is easiest if the chocolate is already broken up into pieces small enough to fit in the boiling tubes and placed in labelled containers.

Procedure

Taste tests

Take two pieces of chocolate, one that has been melted and rehardened and one that has not. Note the differences between the two. Try snapping the pieces and see what happens. Then eat the two pieces separately and note any differences in taste and texture.

Melting point tests

1. Put some hot water (at no more than 50 ˚C) into the beaker.
2. Place a few small pieces of chocolate (enough to cover the bulb of a thermometer when melted) into a boiling tube and put in a thermometer.
3. Take the temperature of the chocolate, then put the boiling tube into the hot water and start the timer.
4. Stir continuously with the thermometer and record the temperature of the chocolate every 15–30 seconds for about five minutes. Note any other changes. A results table is useful here.
5. Repeat steps 1–4 with chocolate that has been melted and rehardened.
6. Draw a graph of each set of results and use them to decide the melting point of the samples and if the samples have the same structure.

Teaching notes

This activity is a good introduction to how structure can make a big difference to the properties of a substance. It’s also fun to do, interesting, and the taste tests are always popular!

The taste of the chocolate is partly determined by the recipe used in making the product but there is more to it than that. This is because the taste of chocolate depends on the microscale structure of the chocolate. Chocolate is made up of tiny particles and crystals ranging from 0.01 mm to 0.1 mm in diameter. These govern how the chocolate is perceived by the consumer. To register taste, flavour compounds have to reach the mouth and the nose, but the texture is important too. The overall flavour is a result of both chemical make-up and also how the material melts and breaks up in the mouth.

Chocolate is a mixture of many chemical compounds of which about 400 have been identified. Taste, texture, gloss, ‘snap’ and other properties can be varied according to how the mixture is processed. Manufacturing chocolate is a very complex multi-step process.

Making a chocolate bar begins with mixing the ingredients and grinding them to give a mixture of correctly sized particles. Size is critical to the ‘mouth feel’ of the product, and is typically about 0.02 mm. The next stage is known as ‘conching’ and involves removing volatile compounds and adjusting moisture content and viscosity. This gives the end product its desired flavour. The mixture is melted, sheared (stirred) and cooled in a complex process known as tempering. The temperature and shearing have to be very carefully controlled or the chocolate ends up brittle, crumbly and tasting different. This experiment models the process.

A key ingredient is cocoa butter. It is a fat and it can come in at least six different crystalline forms. This means that the atoms are the same but they are arranged differently. The different arrangements can lead to different properties including melting point, how easily it snaps, strength, glossiness and texture. It’s a bit like Lego bricks. You can use the same bricks to make different structures; some are stronger, and some look better.

The ability of the structure to take on many different crystalline forms is called polymorphism. (‘poly’ means many; ‘morph’ means shape). The details of the polymorphism of chocolate are very complex and this is still an area of active research. One of the six polymorphs – form V – has a far superior taste and texture than the others. It is also the glossiest and snaps well.

The table below shows some of the characteristics of different cocoa butter polymorphs.

Cocoa  butter polymorph	Conditions to make the polymorph	Melting point (°C)
Form I	Rapidly cooling molten chocolate	17.3
Form II	Cooling the molten chocolate at 2°C	23.3
Form III	Solidifying the molten chocolate at 5-10°C(or storing ‘form II’ at 5-10°C)	25.5
Form IV	Solidifying the molten chocolate at 16-21°C(or storing ‘form III’ at 16-21°C)	27.3
Form V	Solidifying the molten chocolate whilst stirring.Needs a special process called ‘tempering’	33.8
Form VI	Storing ‘form V’ at room temperature for four months	36.3

Using their melting point graphs students should be able to work out which forms or polymorphs are present in their two samples, given the melting points of the polymorphs.

The ordinary sample usually has a melting point of around 33–35˚C, showing that form V is probably present. This chocolate has a good ‘mouth feel’ and students may notice a cooling effect on eating it as melting is an endothermic process.

The melted and rehardened sample melts at a much lower temperature and is probably form II or III. The chocolate often tastes stronger in this sample, but it does not snap so well and has a less smooth texture. The cooling effect in the mouth is less pronounced.

This is a genuine experiment as the results are difficult to predict exactly. As can be seen from the table above, the cocoa butter polymorphs will readily change from one form to another at the sort of temperatures experienced by chocolate. The plots which students generate from this may therefore be rather ragged and are rarely as clear to read as those which they may get from stearic acid, for example. In spite of this, the practical is well worth doing as genuine research is always interesting. There is almost always a clear difference between the plots and students enjoy studying a ‘real life’ example of chemistry.