can anyone explain chocolate?

[eub]

It seems to exist in at least two different solid phases, "solid tempered" (dark and shiny) and "solid untempered" (tan and crumbly). Descriptions of the tempering procedure suggest that liquid tempered chocolate is stable between 83 and 95 °F, while untempered chocolate is solid up to 115. When it melts (into untempered liquid chocolate?) it can be converted to liquid tempered chocolate by adding chunks of solid tempered chocolate. (By rapid cooling? Or one recipe says it's seeded crystallization; I could buy catalysis anyway.) Or by cooling with stirring, apparently. But I've certainly seen that cooling by letting it sit out leads to untempered crud. On the other hand, we know liquid tempered chocolate can cool to solid tempered chocolate -- maybe when it's cooled faster?

What are the thermodynamic phases reached in cooking with chocolate? I wonder if the tempered solid phase is only metastable. I guess I want a phase diagram that also sketches the metastable regions.

Comments

[jpbl.livejournal.com]

If you want a reasonably good explanation of tempering, borrow Ralph's Cookwise book sometime and read about it. (This book is wonderful.) There are apparently 4 phases of chocolate, and the phase is determined by the type of crystal into which the cocoa butter crystallizes. Of course, acording to this, once chocolate is melted, it looses all tempering and must be re-tempered on solidification. Dropping solid tempered chocolate into liquid chocolate probably does cause "seeded crystallization," since the solid chocolate contains cocoa butter crystals that will act as a template for future crystals to form. Catalysis involves speeding up a chemical reaction, so I doubt that it has anything to so with solidifying the chocolate.

[ralphmelton.livejournal.com]

Ya know, I was going to comment to the effect of 'borrow my Cookwise book and read about chocolate', but apparently I'm not the first one to say that.

[eub.livejournal.com]

I will definitely have to borrow that. I don't know anything about this sort of probably-not-really-liquid-phase chemistry.

There is that mantra that catalysis speeds up a reaction rather than shifting its equilibrium, but you can still use catalysis to favor an unstable state. The reason is that a catalyst doesn't necessarily get you to the global minimum faster -- it lowers the barrier for a particular transition. So if you have a situation where the energy surface is like [er, diagram pushed to end -- <pre> is hosing formatting after it?] you can catalyze A<->B to kinetically favor B over C, and then freeze the system into the excess-of-B state. (Thermodynamically unstable, but now it can't get over the hump to C.) For chocolate you might (just speculatively) have A = liquid, B = good solid, C = bad solid.


I have no point here, I would just like to say "the hazard that all chocolate on earth will be converted to chocolate-9."

      /\ /\
   \ /  A  \
    B       \ /
             C

[ralphmelton.livejournal.com]

I just read the discussion in Cookwise last night. It is indeed very nifty. It doesn't provide a full phase diagram, but it does point out that the 'tempered chocolate' form is the beta crystal structure, as opposed to gamma, delta, or beta prime. (I wonder what happened to alpha.)