Studying microstructural changes in chocolate could help confectioners stop that seductive shiny surface from turning an unappetising dull grey in poorly stored bars and boxes.

Chemists in Sweden and Canada have used environmental scanning electron microscopy to examine how filled and plain chocolates develop fat bloom - the unappealing dull grey coating that can develop on the surface of hoarded Easter eggs, boxes of pralines and other chocolate treats. Dérick Rousseau at Ryerson University, Toronto, Canada, says that understanding chocolate microstructure could reveal ways to control fat bloom.

 

The surface of chocolate close up is akin to a mountain range

 

Fat bloom forms on chocolate when tiny temperature fluctuations as small as +/- 2 °C cause cocoa butter crystals to melt and then recrystallise, forming large needle-like structures that scatter light giving a dull appearance.  'Temperature fluctuations are the death knell of chocolate - and once it leaves the factory floor, there's no way for companies to control it,' says Rousseau.

To get an insight into the way the bloom formed, Rousseau, along with Paul Smith at the Institute for Surface Chemistry in Stockholm, Sweden, studied chocolate structure as it aged. 'We found that the chocolate surface was very heterogenous - and that bloom crystals only grew from specific points on the surface,' says Rousseau. So controlling chocolate making to minimise surface imperfections could be a good way to control bloom, he adds.

The team also looked at filled chocolates, and found that liquid-state fat from the filling migrates through the chocolate, accelerating bloom formation and ultimately making the chocolate very soft.

'This is certainly a problem that haunts the whole chocolate industry,' says Nigel Sanders, senior research scientist at Cadbury in Toronto, Canada, who adds that Rousseau's study confirms 'quite a few of the mishmash of ideas others have had regarding bloom formation'.

'As an industry, we haven't got to the bottom of what tools we have to stop bloom formation from happening,' adds Sanders. 'Companies as large as Cadbury do their own research - but that never gets published, so it's nice to see an academic study that helps the whole industry, and isn't just for the big boys.'

James Mitchell Crow