How Thermally Stable Is Your Chocolate?
How thermally stable is my chocolate?
This question has come up in our product development repeatedly as we worked to formulate our vegan, sugar-free, soy-free all-natural chocolate. I have searched the web and beyond and all I could find in the open literature were data for the less relevant oils like sunflower, soybean and cottonseed. The cocoa butter information has been very hazy and imho left intentionally vague.
So, I ran my own analysis which I hope might be of some interest to this group.
Let me get the provisos and disclaimers out of the way:
- Thermogravimetric analysis is a tool that has been universally used to evaluate solid state reactions such as oxidation and thermal decomposition, two processes that are very relevant to chocolate. You can consult Wikipedia write up on the technique here: http://en.wikipedia.org/wiki/Thermogravimetric_analysis. Its pretty good, and what it lacks in scientific rigor, it makes up for in ease of understanding.
- There are many ways to define decomposition temperature of a substance. Historically, in polymer literature it has been linked to 10% weight loss of a material. In other materials, lower weight percent thresholds have been used. I settled on 1% weight loss because waxes such as triglycerides (cocoa butter is a mixture of 3 principal glycerol fatty acid esters) are relatively low molecular weight as compared to polymers and they are more sensitive to weight loss before the properties start to go out the window. 1% is a pretty good indicator, but not an absolute one.
- If you want to talk about triglycerides and their ratios and other species present as we span the global sources of cocoa butters/cocoa liquors from around the globe (Indonesia, West Africa, Peru, Central America, etc.), Id love to have that discussion. I merely used what we use and what we have readily on hand and what tastes better. I am sure there will be subtle differences among other sources. Within reason, I will be happy to run your cocoa butter/liquor on TGA if you want to send me a sample and build up a database of sorts.
- The most rigorous method (imho) results from isothermal holds at fixed temperatures resulting in whats known both as Kissinger Equation or Arrhenius Plots plotting log of weight loss vs inverse of temperature. The linear fit of that line provides several key pieces of information, one of which is accuracy of ones assumptions regarding the physic-chemical nature of decomposition. Another is the activation energy of decomposition. I am not posting any results from this study because it is more time intensive and time is a luxury we do not have at the moment.
So, here is the plot (in air, heating rate = 10C/min):
The green curve is for the pure cocoa butter. The red curve is the cocoa liquor. The black curve is for our chocolate sweetened with our Sweet LUV sweetener. 1% weight loss points are labeled for each curve, but you can pick out your own from the plot.
Some observations: cocoa butter in air is good to 230C for brief excursions to that temperature before you start breaking apart its chemistry. 230C is over 450F, so under normal circumstances, there should be no need for anyone to take it up to that temperature to do anything.
That out of the way, however, addition of cocoa powder (and whatever other stuff is left behind when the butter is hydraulically squeezed out in the press), really reduces that 1% weight loss temperature all the way down to below 140C (red curve). If you dont believe this, try gently heating cocoa powder over a low flame and see how far you get before your loved one kicks you out of the kitchen. So, the lesson here is, compounding with cocoa liquor is much less forgiving that with cocoa butter. I had to find this out the HARD way, of course.
Finally, our sweetener does not worsen/lower the thermal threshold for cocoa liquor. In fact, it can be argued that it imparts some additional stability to it.