Chocs away: X-raying chocolate bloom

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  • Published: Jun 1, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Chocs away: X-raying chocolate bloom

Blooming delicious

An X-ray study could help explain with new clarity why the crystalline structure of chocolate changes leading to the formation of a white bloom on the surface and how that fatty bloom might be avoided.

An X-ray study could help explain with new clarity why the crystalline structure of chocolate changes leading to the formation of a white bloom on the surface and how that fatty bloom might be avoided.

Although the white fat bloom that forms on chocolate is perfectly harmless, at least in the sense that it's not toxic and ignoring the consumption of chocolate and the putative health risks associated with a high-fat diet, it nevertheless causes problems from the marketing and sales perspective in that customers may reject or return a product, uninspired by the presence of the bloom.

Svenja Reinke, from the Hamburg University of Technology (TUHH), Germany explains further. “Despite this well known quality issue, comparatively little has been known until now about its root causes," she says. Now, a team from TUHH, DESY and the food company Nestlé, well known for its many and various chocolate brands, presents details in the journal Applied Materials and Interfaces regarding fat bloom.

The bane of chocolatiers

Apparently, fat bloom forms when low-melting fats, such as the cocoa butter present in chocolate, migrate through the chocolate to the surface and form a crystalline deposit 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”, Reinke explains. Moreover, liquid fillings or ingredients such as nougat can accelerate the formation of fat bloom.

Chocolatiers know only too well that the longer their chocolate is stored and the higher the storage temperature, the more likely fat blooming is to occur. This is perhaps why consumers perhaps not entirely mistakenly assume that the presence of a white discoloration on a piece of chocolate is 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,” Stefan Palzer from Nestlé adds. “This is why fat bloom continues to be one of the most important quality defects in the confectionary industry.”

With the help of DESY’s X-ray source PETRA III, the team has investigated the underlying processes in real time of fat bloom formation. To do so, they investigated the behaviour of different mixtures of the main components of chocolate - cocoa, sugar, milk powder and cocoa butter. These laboratory chocolate samples were ground to a fine powder in order to accelerate the fat bloom process and then subjected to X-ray examination. The study reveals the structure of the fat crystals as well as the pores within the chocolate down to a scale of a few nanometres.

Melt in the mouth

In order to simulate the migration of fats from within chocolate, the team also dropped a little sunflower oil on each of their samples and observed what happened. “First of all, wetting takes place within seconds," Reinke explains. "The oil very quickly penetrates even the smallest pores, probably through capillary action." 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.”

Until now, little was known about fat bloom dynamics. “For the first time, we have been able to track in detail the dynamic mechanisms that lead to the creation of fat bloom,” explains DESY's Stephan Roth, who runs the P03 beamline on PETRA III. The team used small-angle X-ray scattering on this beamline and precisely adapted the technique to real-time investigations to allow them to observe the structural changes caused by the migrating lipids. It is a useful example of the application of this analytical technique to investigating structural changes in an everyday multi-component system.

The research might now allow chocolate manufacturers to work out how to stop fat bloom formation. One approach might simply be to reduce the porosity of the chocolate so that liquid fats cannot leech to the surface so readily. The only other alternative would require suppliers and shops to store the products at 18 Celsius, which is not likely to be adhered to, particularly in balmier climes around the world. If there were a way to make chocolate less sensitive to fluctuations in temperature that might have a detrimental effect on the palatability and texture of chocolate; for many people much of the pleasure in eating it being the obvious "melt in the mouth" feeling. However, there may well be a role for controlling polymorphism. “Cocoa butter crystallises in six different crystal forms,” explains Reinke. “The amount of fluid also depends on the form of the crystals.” Manufacturers could limit fat bloom by controlling crystallisation.

Related Links

ACS Appl Mater 2015, 7, 9929-9936: "Tracking Structural Changes in Lipid-based Multicomponent Food Materials due to Oil Migration by Microfocus Small-Angle X‑ray Scattering"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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