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Part of the appeal of chocolate is its colour and shiny surface, which give the consumer the impression that the product is going to taste nice and feel good in the mouth. However, if it is not stored correctly, it will develop an unappealing grey ‘bloom’ on its surface. It also forms as chocolate gets older. While the chocolate remains perfectly safe to eat, it does not look very attractive. Now, scientists in Canada and Sweden have worked out under what conditions this unattractive bloom is most likely to form.
Chocolate is extremely sensitive to temperature, and just a 2°C fluctuation will cause the cocoa butter it contains to melt and recrystallise. The new crystals it forms are needle shaped and scatter light, which is what gives the characteristic dull appearance of chocolate bloom.
Dérick Rousseau at Ryerson University in Toronto, Canada and Paul Smith of the Institute for Surface Chemistry in Stockholm, Sweden, carried out a careful study of chocolate as it ages, which has been published in the journal Soft Matter. They used a scanning electron microscope to look at the surface of the chocolate over time. This instrument fires electrons at the surface of the chocolate, and measures the electrons that bounce back from it to build up a very high resolution picture of the surface.
They found significant changes took place to the microstructure of the surface of a sample of plain chocolate stored at 26°C for 40 days. Cocoa butter is made up of three key triglycerides, and there are six crystal forms which are known as I to VI, in ascending order of stability. Properly processed chocolate appears glossy, as the cocoa butter forms V type crystals, which are very small – less than 5 microns in length. When the cocoa butter recrystallises with age or poor storage, it forms the most stable VI type crystals, which are larger and spike-shaped, and can be up to 100 microns long.
The surface of the chocolate is not completely smooth, with small layered crystals and surface imperfections in the form of pores and tiny depressions. They found that the more rough the chocolate surface was, the more likely the blooms were to form. Rousseau believes that if manufacturers were to minimise the surface imperfections on their chocolate, this would be a good way to reduce the formation of blooms.
They also tested filled chocolates, and found that these were even more susceptible to forming blooms. The liquid fat in the filling migrates through the chocolate to the surface, and this accelerates the formation of blooms. Ultimately, this also results in very soft chocolate.
‘As an industry, we haven’t got to the bottom of what tools we have to stop bloom formation from happening,’ says Nigel Sanders, senior research scientists at Cadbury in Toronto, Canada. ‘Companies as large as Cadbury do their own research, but it never gets published. It’s nice to see an academic study that helps the whole industry, and isn’t just for the big boys.’ |
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