Chocolate part 3:3 - making your own instant cocoa

During graduate studies, we often wished for a "journal of unsuccessful chemistry" for the benefit of all the failed experiments we do. What if there was a journal where we could look for the experiments we were planning, and see if someone else had tried it and failed? This is one of those stories.

Cocoa powder has this property that it is hardly wetted by water. The result is

• when making cocoa, thorough agitation or stirring is necessary to avoid lumps and cocoa powder floating on top
• eating plain cocoa powder is not a very pleasant experience
  

In my search for info on why chocolate seizes (chocolate part 1:3) and shaping chocolate (chocolate part 2:3), I came over three papers on the wetting properties of cocoa and cocoa granules. In short, they stated that if I could turn cocoa powder into a granulate rather than the fine powder it is, it might disperse more easily because it'll interact better with the water molecules. Also, I figured, it might taste better in it's dry state as it's easily wetted by the saliva in my mouth.

Would it be possible for me to make my own instant cocoa, or maybe some palatable cocoa powder for sprinkling on top of dishes? If so, I could make my own blend and use quality cocoa rather than the sugar-loaded mixtures that are sold in grocery shops.

Granulating cocoa powder
Omobuwajo et al. brought about an easy method of making granules that are readily wetted by water. They simply heated the cocoa powder on metal plates. No need for expensive industrial machinery and additives. They investigated the granulation related to various cocoa-to-sugar ratios and conclude that the two best instant cocoa mixes were made by rapid heating of sugar and cocoa in 3:1 and 4:1 ratios, using fine sugar (0.23 mm), giving granules of 0.18 mm size. Now, if heated metal plate = skillet on my stovetop, I might make my own instant cocoa powder, or...?

A mixture of 9 g plain cocoa powder and 27 g sugar was heated in a pan on medium high heat (a little below maximum) with continuous stirring for 5 minutes.

Result: yes, the cocoa is granulated (right). No, I was not able to make granules that disperse more easily in a reproducible manner (some times it was easily wetted, sometimes it wasn't). Also, the powder tasted somewhat different due to the heat treatment (surprise...), without being more palatable than pure cocoa powder mixed with sugar.

Conclusion: more testing is necessary, several parameters to vary (temperature, time, stirring). This might probably work, but I don't find this interesting enough to spend more time on experimenting.

Wetting properties of cocoa powder
Wetting by water is closely related to whether something is hydrophilic or hydrophobic. If it's hydrophilic, it loves water and wetting occurs easily; water spreads out on the surface. If it's hydrophobic, the opposite occurs, such as on Gore-Tex. Wetting is not limited to water, and other solvents also possess wetting properties. These properties are in turn closely related to the surface energy. A general rule is that media/material with similar surface energies will possess similar wetting properties.

Galet et al. reported that the surface energy of cocoa powder is considerably lower than surface tension of water. Result: water and cocoa powder don't mix very well (surprise...). The bad dispersion behaviour of cocoa is caused by a number of phenomena, and one is the hydrophobic nature of cocoa, the fat content being 10–12% in cocoa powder. The mean particle size of cocoa is commonly around 16 micrometres (0.016 mm), but the size distribution is wide. The smaller the grains, the more difficult is the dispersion. Granulating the powder (making grains of 0.02 mm - 2 mm) resulted in enhanced wetting properties.

Partly, the bad dispersion properties is because of the bad flowability and the cohesive forces of the powder. The cohesion results in solid agglomerates, and the shear forces are not sufficient to break the cohesive forces between the grains. In plain words: cocoa powder lumps up in water, simply floating on top (another surprise...).

Cocoa powder in alcohol
A conclusion that may be drawn by this, is that liquids with lower surface tension (surface energy) than water might wet cocoa more easily. Galet concludes that liquids with lower surface energies than 40 mN/m will wet the cocoa powder, and that less than 15 mN/m will give perfect wetting. Water has surface energy 72 mN/m, not to wonder why cocoa powder and water doesn't mix well. However, alcohol (ethanol) has surface energy 22 mN/m, and a 40% ethanol in water mixture has surface energy 41 mN/m.

Conclusion: making cocoa flavoured alcoholic drinks should be easier than water-based ones.



References
Galet et al. "Improving the Dispersion Kinetics of a Cocoa Powder by Size Enlargement". Powder Tech. 2003, 130, 400-406.

Galet et al. "The Wetting Behaviour and Dispersion Rate of Cocoa Powder in Water". Food Bioprod. Proc. 2004, 82, 298-303.

Omobuwajo et al. "Thermal Agglomeration of Chocolate Drink Powder". J. Food. Eng. 2000, 46, 73-81.

On chocolate for education purposes:
http://www.exploratorium.edu/chocolate

http://expertvoices.nsdl.org/connectingnews/2009/02/04/the-science-of-chocolate-just-in-time-for-valentines-day