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Smooth, Rich, Chocolaty Physics

Happy day-before Valentine’s Day! Do you have your heart-shaped box of chocolates, yet? Or perhaps you’re like me and are waiting for the day after to snag the tasty treats for half the price.

The saying goes: You get what you pay for. So, is the quality of those chocolates worth what your love-sick friends are paying today or what I’m paying a few days from now? That question is steeped in marketing ploys and economics, so I can’t give you a straight answer, or even a heart-shaped one.

What I can give you is a rundown for what quality chocolate means to a chocolatier and what kind of sweet physics melts into action the second it hits your tongue, teasing your taste buds for more.

What makes chocolate melt-in-your-mouth delicious is its smooth, creamy texture and the smoother, the better! Chocolatier’s create chocolate through a multi-step process. Perhaps the most important step for texture is called conching, which I’ll explain later. Suffice it say that low-quality chocolate undergoes conching for as little as four hours whereas it takes up to 78 hours of conching to produce the smoothest, highest-quality chocolate.

On a microscopic level, the texture of chocolate depends on the fat crystals’ size, distribution and overall structure. If the structure and type of fat crystals in the chocolate are not quite right, then you will either have crumbly chocolate that melts in your hands instead of your mouth or rock-hard chocolate that does not melt fast enough.

Cocoa butter is the fat inherent to the cocoa nib. After cocoa beans are freed from their pod, fermented, cleaned and roasted, then manufacturers extract the nib that is inside the bean and ground and liquefy them, which produce chocolate liquor. The liquor offers two key ingredients that you can find in your final piece of chocolate candy: cocoa solids and cocoa butter.

Chocolatiers tease out the fat crystals within the cocoa butter through a calculated process of heating and cooling. A tasty, finished product will contain fat crystals that are evenly distributed throughout the chocolate, which is where conching comes in.

A conch is a type of mixing machine that distributes the fat crystals evenly throughout the chocolate. Chocolatiers want both an even distribution and tiny crystals. The highest-quality chocolate contains particle structures that are no more than 25 microns in diameter, according to professor of food process engineering Erich Windhab at ETH Zuerich in Switzerland. Chocolate this refined can take days of conching.

Particle structures includes fat crystals as well as particles from milk powder, sugar and other additives that manufacturers infuse into the cocoa butter to create different types of chocolate like dark chocolate, milk chocolate or couverture chocolate, which is used mostly for dipping or coating.

Food scientists, like Windhab, with a sweet tooth for studying chocolate are interested in understanding how particles of chocolate interact with one another and how this affects the flow of chocolate. This area of science that involves the flow of material is called rheology, and can describe the way that chocolate melts and feels in your mouth and the properties of chocolate that lead to the smoothest melt possible.

Chocolate is a non-Newtonian fluid like honey, peanut butter and toothpaste, which tend to flow less readily than water. Windhab explains in a quick study, that the viscosity, how readily material flows, necessary for smooth, creamy chocolate is between 1.5 and 3.5 pascal-seconds, which is slightly lower than the viscosity of honey.

Credit: Dwight Burdette
When chocolate hits your tongue, it begins to melt because your tongue’s temperature dislodges the fat crystals from their rigid structure and they begin to act more like a fluid than a solid. The friction, called shear viscosity, between the fat crystals and other particles within the chocolate is related to how readily the chocolate begins to melt and how smoothly the resulting liquid coats your mouth. Customers, although they might not know it, prefer low shear viscosity chocolate.

So, when tomorrow comes and you present your valentine with a box of chocolates, I hope that you can tell your special someone that your gift is more than just tasty sweets – it’s a box of long-conched, non-Newtonian, low-shear-viscosity, heart-shaped bites containing cocoa butter and solids. Mmm!

If you're interested in learning about the chocolaty studies of today's scientists, the International Symposium on Food Rheology and Structure is a good source. Here's a compilation of abstracts from 2012.


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