Blooming chocolates under the microscope

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  • Published: Jun 15, 2008
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Blooming chocolates under the microscope

To maintain the seductive and lustrous brown gloss of chocolate, so enticing to chocoholics the world over, food technologists must find a way to prevent fat bloom from forming on the surface and turning the surface an unappealing grey. Now, scientists from Canada and Sweden have found new clues to understanding the microstructure of chocolate and what happens when it turns grey with age.

Dérick Rousseau and of the School of Nutrition, at Ryerson University, in Toronto, Ontario, and Paul Smith of the YKI, Institute for Surface Chemistry, in Stockholm, Sweden have used temperature-controlled environmental electron scanning electron microscopy (SEM) to look at the needle-like spikes of cocoa butter that scatter light and make chocolate turn grey either if it has been stored too long or has been exposed to even small fluctuations in temperature as small as 2 degrees Celsius. Fat crystals on the surface can melt and then recrystallise when the temperature drops forming a lacklustre grey surface.

Writing in a forthcoming issue of the Royal Society of Chemistry journal, Soft Matter, Rousseau and Smith explain how chocolate is composed of a particulate mixture consisting of cocoa particles, sugar crystals and milk powder (if milk chocolate) dispersed within a continuous cocoa butter fat phase.

Cocoa butter itself is made up primarily of three triglycerides [1,3-dipalmitoyl-2-oleoyl-glycerol (POP), 1,3-distearoyl-2-oleoyl-glycerol (SOS) and rac-palmitoyl-stearoyl-2-oleoyl-glycerol (POS)]. It is polymorphic and has six known forms with transitions between each polymorphic form occurring in the solid or else via a melt transition. Chocolate manufacturers endeavour to make the form "V" polymorph bas it confers on chocolate the desirable melting and sensory characteristics with which chocolate lovers are so familiar. Crystals in the V form are less than 5 micrometres long.

The team explains that, since the 1970s, various microscopy techniques, including polarised light microscopy, fluorescence microscopy, scanning electron microscopy, magnetic resonance imaging, atomic force microscopy, laser scanning microscopy, confocal microscopy, and X-ray tomography have been used to investigate the microstructure of chocolate. "Yet, there still remains a tenuous understanding of the microstructure of chocolate," the team says, "and consequently microstructure-based means of controlling fat bloom."

The researchers stored samples at 26 Celsius for 40 days, which led to significant changes in the chocolate's microstructure. The SEM images revealed that chocolate with a microscopically rough surface was more likely to form grey fat bloom, presumably as the higher surface area provides a great number of nucleation sites - pores, pits, and other imperfections - for recrystallization to take place. Rousseau says that if manufacturers could find a way to make the surface of their products smoother they could be stored longer and would be less temperature sensitive. In contrast, control samples underwent much slower changes and displayed little fat bloom.

The team demonstrated the same effect in chocolates with soft centres. Filled chocolates were even more susceptible to fat bloom as the liquid-state fat found in the fillings migrates quickly through the chocolate to the surface, accelerating bloom formation and ultimately making the chocolate very soft. Needle-like crystals predominated on the "solid" and filled chocolate samples, the researchers say, but the filled chocolates also produced spherulites with large crystals upwards of 100 micrometres in length.

Nigel Sanders, senior research scientist at Cadbury in Toronto, Canada, says that "as an industry, we haven't got to the bottom of what tools we have to stop bloom formation from happening." Large companies do their own research, but this is usually never published in academic journals. Sanders adds that, "It's nice to see an academic study that helps the whole industry."

Incidentally, chocoholics worried about their muse turning grey need not worry about ill effects. Fat bloom is essentially nothing more than the growth of existing fat crystals and usually has little effect on taste or texture. As with products designed to hide grey hair any efforts to mask the presence of a chocolate fat bloom would be nothing more than cosmetic.

Rousseau, getting a microscopic view of chocolate

Chocolate spiked (Credit: Rousseau and Smith/RSC)
Chocolate spiked

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