Cool Chocolate

"When it comes to the crunch, some chocolates simply taste better than others. But researchers are just beginning to understand that the taste depends on which crystalline forms predominate as the chocolate cools in the factory. Some of these crystalline forms are not very palatable--others are sensational. The British chocolate manufacturer Cadbury is helping to finance the research.
Although Cadbury has been making chocolate since 1831, only now are researchers at Heriot-Watt University in Edinburgh and the Central Laboratory of the Research Council at Daresbury teasing out the secrets of the perfect chocolate.
"Cadbury knows about the process of making chocolate," says Alexandra Rossi of Heriot-Watt. "But there was not a lot of information about how it crystallises."

Cocoa butter, the main solid fat in chocolate, can crystallise into six different arrangements of molecules known as polymorphs. The form that predominates in the best-tasting chocolate is form V. This polymorph also makes the chocolate look glossy and melt in the mouth. The best bars do not taste too sticky or thick. They are also more resistant to "fat bloom" --a cloudy discolouring seen on some chocolate bars. The Heriot-Watt researchers have been working out which conditions will allow form V to dominate, and how to stop the other forms from displacing it.
In conjunction with researchers at Daresbury, Rossi and her colleagues constructed a test cell to mimic what happens in the factory. They pumped liquid cocoa butter heated to 50 °C into this cell. The liquid was "crash cooled" at a rate of 7 °C to 8 °C a minute until it reached 22° C, and then the mix was held at that temperature. All the time the cocoa butter was being monitored to see when and why the different polymorphs emerged.
The researchers irradiated the cooling cocoa butter with X-rays and measured the diffraction patterns produced. The different crystalline forms scattered the X-rays in distinctive patterns, enabling the team to work out the precise temperature at which the different forms appeared and disappeared.
The temperature in the test cell can be raised or lowered and the cocoa butter mixed more or less vigorously. By varying the conditions, Rossi and her colleagues discovered the importance of mixing. Without mixing, form V does not appear, and less palatable crystalline forms predominate (see Diagram).
They also found that small differences in temperature can create big differences in the taste of the chocolate. The unpalatable II and III forms appear between temperatures of 22·3 °C and 23·55 °C but form V makes its appearance at only slightly higher temperatures of 23·86 °C. Rossi says that now the researchers understand how cocoa butter reacts to cooling and mixing they will start to make the process more representative of chocolate manufacture by combining the solid fat with more of the ingredients in chocolate bars."

Copyright New Scientist May 9th 1998.