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The Physicist as Chocolatier by Naveen Sinha

Since the 19th century, chocolatiers around the world have worked to make the perfect chocolate bar. DeGennes would likely have much to say about this endeavor, since chocolate falls well within his definition of "soft matter," as told in his Nobel Lecture:

  1. Complexity: Chocolate is by no means a simple solid. At the simplest level, it is an emulsion of cocoa and sugar particles in cocoa butter. However, this overlooks the delicate crystal structure of the lipid molecules, the hundreds of flavor particles form the cocoa plant, and the influence of various emulsifiers, among other characteristics.
  2. Flexibility: DeGennes related an anecdote about how Indians in the amazon basin would dry sap from trees to make rubber for their boots. This is a classic example of how a small chemical change leads to a large change in the physical properties of a substance. In chocolate, a small change in the temperature during the production can change the appearance of the resulting bar from smooth and glossy to white and fuzzy, a problem known as bloom.

How would a soft matter physicist approach the task of making the perfect bar? The first step would be to consider the the relevant length and energy scales involved:

  1. Length: The particle size distribution has a major effect on whether chocolate tastes like the traditionally smoother European chocolates or somewhat rougher American chocolates. The optimal particle size for dark chocolate is < 35 micron. The particle size distribution is most effectively found using laser diffraction. The dark chocolate is melted in vegetable oil and then placed under ultrasonic dispersion to break apart any aggregates. One commercial system available for making these measurements is the MasterSizer(R) Particle Size Analyzer for Chocolate, made by Malvern Instrume Ltd. [1]. Some typical results from a particle size distribution measurement are shown below:

AfoakwaPSD.jpg

  1. Energy: In order for chocolate to have the pleasant melting sensation in the mouth, it needs to have a melting point just below body temperature. Differential scanning calorimetry is the method of choice among food scientists.

Next, it's time for some experiments. Rheological measurements are a classic technique for studying the mechanical properties of a system. Food scientists have used Couette geometries, cone-and-plate systems, parallel plate viscometers, and a helical ribbon device to study the visoelastic properties.

Closely related to these standard measurements are texture studies, which involve sticking a metal probe into the solidified chocolate bar.

Insights from soft matter are most needed when it comes to tempering chocolate. This is often regarded as one of the most difficult aspects of chocolate-making, since improperly crystallized chocolates can be dull and crumbly.

I found a simple procedure on-line (at chow.com ) for tempering chocolate at home, so eagerly tried out the technique. I did two batches: one in which I followed the procedure and another in which I added some water to "seize" the chocolate on purpose and ruin the texture. I took some photos using a borrowed IntelPlay microscope and a trial version of the imaging software. At 200x magnification, the differences were quite dramatic:

20081225 OK Temper 200.png

20081225 Poor Temper 200.png

For even greater resolution, an electron microscope is needed. Ellen Hodges, a post-doc at Harvard, took some images of Trader Joe's dark chocolate.

Trader Joe's Dark Chocolate, frozen
Bloom on TJ's Dark Chocolate, frozen
Bloom on TJ's Dark Chocolate, RT

With the insights of soft matter physics, chocolate makers will be able to quantitatively understand the tempering process and perhaps develop new methods for making chocolate.

References:

E O Afoakwa et al. "Relationship between rheological, textural and melting properties of dark chocolate as influences by particle size distribution and composition." European Food Research Technology (2008) 227:121-1223.

E O Afoawka, A Paterson, and M Fowler. "Factors influencing rheological and textural qualities in chocolate - a review." Trends in Food Science & Technology. 18 (2007) 290-298.

E O Afoakwa, A Paterson, M Fowler, and J Vieira. "Effect of tempering and fat crystallization on microstructure, mechanical properties and appearance in dark chocolate systems." Journal of Food Engineering 89 (2008) 128-136.

E O Afoakwa, A Paterson, M Fowler, and J Vieira. "Characterization of melting properties in dark chocolates from varying particle size distribution and composition using differential scanning calorimetry." Food Research International 41 (2008)751-757.


About the author: Naveen Sinha is currently studying biofilms in Prof. Michael Brenner's group. This class is changing the way he sees the world. On his morning runs he thinks about the viscoelastic properties of his Saucony shoes. At a cafe, he contemplates the physical properties of the artful foam on his latte. When he cooks dinner, he wonders if this class could lead to some consulting jobs for the food industry.