Chocolate

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Written by Kevin Tian, AP 225, Fall 2011 --Ktian 22:00, 5 December 2011 (UTC)

Chocolate as a substance is quite interesting for a wide variety of reasons, though one must certainly acknowledge it's more delectable qualities. It's purposes as a food aside, chocolate is also quite interesting from the perspective of science as well, as well as how the tried and true methods of chocolatiers are in fact well grounded in science; for the most part they are simply unaware of the fascinating properties of the things they're working with. Since this has changed in the recent decades, with massive efforts towards exciting the cooking world with the Science of Cooking, much more has been learned about chocolate than ever before.

Basic Components

The essence of chocolate can vary depending on who you ask however, in physical terms, chocolate is simply an emulsion. More specifically it is an emulsion of cocoa mass particles suspended in the cocoa butter, with an emulsifier to help things along (Lecithin). Thus is the general consensus among the scientific and culinary community that three ingredients are essential to a chocolate[1]:

  • Cocoa Butter
    • This is the fat that acts as the continuous phase (or the medium) of the chocolate emulsion
  • Cocoa Mass
    • This is the mass that acts as the dispersed phase of the chocolate emulsion
  • Lecithin
    • This serves as the emulsifier to help disperse the cocoa mass in the cocoa butter.
    • There is actually a small amount that is naturally occurring, however since we like having an easier time, more was added so one could use less cocoa butter.

The basic scientific principle at work is that the lecithin stabilizes the dispersion of cocoa mass in the cocoa butter by lowering the surface energy at the interface between the two phases. If this did not occur then the two phases will phase separate, in which case we don't get much of a chocolate.

However all the components of chocolate itself comes from a cocoa bean. However from the cocoa bean to the final chocolate product involves a long process of harvesting, fermentation, drying, roasting, deshelling and ground into a paste that can be separated into cocoa mass and cocoa butter via hydraulic press or the Broma Process [2].

Phases of Chocolate

Obtained from [3]. As one can see there are 6 known phases of chocolate that melt at different temperatures. Phase V, as indicated in the image, is what is most desired for chocolate making.

In chocolate there is a well known process referred to as "tempering". This process involves heating up and cooling down chocolate to specific temperatures several times. The result is a chocolate that is glossy, firm, snaps well and has a melting temperature most conveniently near body temperature.

This may seem like magic but the reason that this works is has its origins in the phases of chocolate. There are 6 known phases of chocolate, which for simplicity are numbered I-VI, in order of their melting temperature. The process of tempering essentially involves melting all phases of the emulsion and forming seed crystal of phase V to act as nucleation sites for further phase V crystal growth.

The process thus is as follows[4]:

1. Bring chocolate up to 45°C to melt all six phases of chocolate.

2. Cool chocolate to 27°C to allow crystals of phase IV and V to form. This step is accompanied by agitation to create seed crystals of these phases.

3. Heat up chocolate to 31°C to melt all phase IV crystals.

4. Do not heat the chocolate any further or risk ruining the tempering. Allow the chocolate to cool.

One notes that the crystallization is the crystallization of the cocoa butter. Tempering takes advantage of two things.

  • Different melting temperatures (i.e. Phase transition temperatures) for each crystal phase
  • Nucleation

The first is straightforward, as one notes that each phase melts at different temperatures, so if an undesired phase exists, one can simply melt it by heating the chocolate. Seeding the chocolate with phase V crystals is a nice trick that encourages a specific kind of crystal structure to form. Once a nucleation site has been formed, the further increasing of the site beyond its current size actually becomes favorable (releasing energy) instead of consuming it. Thus forming phase V crystals becomes favored over other phases, and allowing for the glossy chocolate to be made in large quantities.

However one notes there is still a 6th phase of chocolate. Interestingly enough Phase V is not the most stable phase at room temperature, and the phenomenon known as "chocolate bloom" may also be the result of phase transitions away from phase V to phase VI. However not enough research has been performed on this to verify whether the phase transition is the underlying cause of bloom, or whether other theories are better suited to explain it (another one attempts to explain it by phase separation).

References

Chocolate in Surfactants from Lectures for AP225.

[1] A.C. Rowat et al. "The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation". Journal of Chemical Education, 2011, 88 (1), pp 29–33.

[2] http://en.wikipedia.org/wiki/Cocoa_bean

[3] http://www.eng.buffalo.edu/Courses/ce457_527/ce457_pro/g4_doc.htm

[4] http://en.wikipedia.org/wiki/Chocolate#Tempering

Keyword in references:

The Science of Chocolate: Interactive Activities on Phase Transitions, Emulsification, and Nucleation