The cool way to shape chocolate

More than 20 years of fundamental research by Cambridge University's Malcolm Mackley on the behaviour of plastics during extrusion has led to an unexpected and remarkable breakthrough in chocolate processing technology.

A complex and difficult substance to work with, chocolate is normally processed by melting and pouring it into a mould. It is then allowed to cool and then removed.

On a chance visit to a chocolate factory, Dr Mackley hit on the idea of cold extruding chocolate. In the experiments, solid chocolate buttons are loaded into a barrel at room temperature. A piston then forces the chocolate through a die at pressures of about 100 Bar, or 100 times the pressure of the Earth's atmosphere at sea level. This force is equal to an adult elephant standing on one's foot. Dr Mackley says that at these pressures the material flows like plasticine. The resulting material is flexible for up to an hour while it recovers its hardness.

Dr Mackley's work is being backed by confectionary giant Nestle which has been quick to spot the enormous potential of the technique to create new weird and wonderful chocolate shapes.

Dr Mackley, a reader at Cambridge's chemical engineering department, is concerned about what he sees as a downgrading of the importance of fundamental research: "This new process would not have been invented without the long-term, measured investigation of plastics processing that I have been engaged in."

A great mystery of the technique is that despite the high pressures, no temperature change accompanies the processing. Dr Mackley says: "It is quite strange. We think it may be due to work going into changing a component of the chocolate from a crystalline structure to liquid but we can't say for sure at the moment."

Important spin offs from the work include its use in teaching rheology, the study of deformation and flow: "It is a technically demanding subject and the fact that we have chocolate exhibiting this kind of behaviour means I can use it in lectures. It's something I am sure students can readily associate with."