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. 2) 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. In the chocolate industry, there are at least two main measurements in use: 1) Yield stress: 2)
Chocolate is an ideal system for the study of soft matter, since there is a clear correspondence between its physical properties and the experience of consuming it.
- Rheology: The satisfying snap of a high-quality bar of chocolate is the result of a careful balance of particle size distribution, lipid composition, and tempering. The high viscosity of molten chocolate can result in a pasty sensation that persists in your mouth.
- Emulsions: chocolate is an emulsion of sugar and cacao particles in a continuous lipid phase. When chocolate melts in your mouth, it undergoes a phase transition to become an emulsion with a continuous aqueous phase.
- Thin films: to produce the proper particle size distribution, molten chocolate is passed through a system of rollers and is drawn along their surface as a thin film. The shearing action of the rollers breaks up large particles and aggregates.
- Phase transitions: chocolatiers have spent decades perfecting the craft of tempering, to produce a chocolate bar with the ideal physical properties. By switching between higher and lower temperatures, the chocolate makers can produce uniform lipid crystals with the correct form. Without tempering, the chocolate is soft and difficult to remove from molds.
For more information, see: E. O. Afoakwa, A. Paterson, and M. Folwer. "Factors influencing rheological and textural qualities in chocolate - a review." Trends in Food Science & Technology. 18 (2007) 290-298.
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.