Scientists have used X-rays to study the formation of an unsightly white layer that occasionally forms on chocolate.
The white fat bloom can form when liquid fats, such a cocoa butter, migrate through the chocolate to the surface and crystallise there.
"This can happen when liquid chocolate cools down in an uncontrolled manner and unstable crystals form, for example. But even at room temperature, a quarter of the lipids contained in chocolate are already in a liquid state," said the main author of the study, Svenja Reinke, from the Hamburg University of Technology (TUHH).
"Although fat blooming is perfectly harmless, it causes millions in damage to the food industry as a result of rejects and customer complaints," Reinke said.
The longer chocolate is stored and the higher the storage temperature, the more likely fat blooming is observed. This is why the white colouration of the product surface is often taken to be a sign of old or spoiled chocolate.
"Even though fat bloom does not actually constitute any deterioration in the quality of the product, the visual alteration associated with it can lead to a large number of consumer complaints," said Professor Dr Stefan Palzer, from the food company Nestle.
"This is why fat bloom continues to be one of the most important quality defects in the confectionary industry," Palzer said.
With the help of Deutsches Elektronen-Synchrotron (DESY)'s x-ray source PETRA III, the scientists have now been able to observe the underlying processes in real time.
To do so, they investigated the behaviour of different mixes of the main components of chocolate, which are cocoa, sugar, milk powder and cocoa butter.
These technical chocolate samples were ground to a fine powder so as to speed up the processes and then X-rays were shone through them.
In order to study the migration of fats, the researchers also placed a few drops of sunflower oil on each of their samples and observed what happened.
"First of all, wetting takes place within seconds. The oil penetrates very quickly into even the smallest pores, probably through capillary action," Reinke said.
Secondly, the liquid fat alters the internal structure of the chocolate.
"Over a period of hours, the liquid fat dissolves additional crystalline lipid structures, which makes the entire structure of the chocolate softer. This in turn increases the migration of lipids," Reinke said.
The observations will allow the food industry to develop concrete approaches for reducing fat bloom.
"One consequence might, for example, be to reduce the porosity of the chocolate during manufacture, so that the fat migrates more slowly," said Reinke.
"Another approach is to limit the amount of fat that is present in a liquid form by storing the product in cool, but not too cold, conditions. 18 degrees Celsius is ideal," Reinke said.
