This is a really good question and one I have received over the years quite a bit. And I have to say it stumped me for a while. Really, it makes perfect sense…in theory, but it fails in practice…or does it?
CAUTION: SCIENCE CONTENT AHEAD (but only a little)
So why does it not work? Let’s look at what you are trying to do, and what you are and are not doing if you don’t reduce the temperature to 28C/80F.
Solid chocolate is by definition 100% not liquid. I don’t say 100% crystallized because technically it can be amorphous or without crystal structure, or a mixture of amorphous and crystal structures. In totally untempered solid chocolate we can estimate that 20% is Type V and the rest are types I-IV (this probably actually isn’t true due a whole host of reasons (rates of crystallization in contaminated polymorphic system would sum it up) but close enough for the example). With that number in hand, we can see that we can have up to 20% Type V crystals when we bring the temperature down to ‘seed’ temperature, i.e. that point where the chocolate just starts to thicken a little. The cooling step of 28C/80F. In actuality, it is probably much less than 20%. Maybe 10-20% of that, the rest being Types III-IV (types I-II form much lower) and un-crystallized liquid chocolate. So, with a little rough math, that means there is probably 1-2% type V crystals.
Now, what I find terribly interesting about my ‘back of the envelope’ calculation is that that number is very close to the recommended amount of seed chocolate (i.e. 100% type V) you should add if you are tempering by seed. Why? I’m glad you asked. It comes down to a concept called nucleation. In layman’s terms you have to have a nucleus or SEED for the rest of the chocolate to form around and experience shows that for chocolate you need at least 1-2%. Below that and either the crystal formation is too slow, or there are too many places in the solidifying chocolate that are ‘un-seeded’ so other crystals start to form instead.
So, why does it not work to bring your chocolate to 32C/88F and then let it cool? Well, if you followed what I was saying above, it can only be because you don’t have enough Type V crystals. You are below the 1-2% threshold limit for proper Type V propagation. And that is because crystallization is heavily temperature and concentration dependent, and polymorphic crystallization is even MORE dependent on those two conditions. What that boils down to is that you are not giving it enough time at 32C/88F. (DISCLAIMER: I am totally making this next part up, but keep in mind I’ve many years as an organic chemist, so have a reasonably good gestalt feel for this kind of thing. And I am backing it up with the fact that we know that holding at 32C/88F doesn’t work.) In order to reach 1-2% type V at 32C/88F you may well have to keep it at that temperature for 24 hours. Or 48 hours. Or longer. Even if it was only 4 hours, that is quite a while if you want to temper NOW. And to make matters worse, generally speaking, crystallization is not a first order rate of reaction in regards to temperature. This means that if at one temperature (say 28C/80F) you are forming 0.1% Type V per minute, a couple of degrees warmer won’t reduce it just a little to 0.6% or 0.8% per minute, but will probably reduce 1-2 orders of magnitude, meaning just a couple degrees higher and you are down to 0.01 or 0.001% per minute…and you need to get to 1-2% and you want to raise it 5-10 degrees? You can see why it does not work.
Basically what you should get out of this is that the melting point of a crystal (94 F for Type V) is not really related to its formation speed at all, except the formation rate is zero at the melting point, i.e nothing can form if it is melting. All other bets are off.
To explicitly answer your question, the purpose of reducing your chocolate to 28C/80F is to create 1-2% Type V crystals ‘in a reasonable amount of time’.
One final science tidbit that I won’t (and can’t, I just don’t understand it well enough myself. Basically a variant of a variant of a Feynman quote "If you can't explain it to a six year old, you don't really understand it.") explain. It is in regard to your comments about stable and unstable forms. There is a general truism of polymorphic systems (i.e. compounds that form multiple crystal structures). It is that the LEAST stable forms are the forms that form first. So, Type V is the least stable. And really, this makes sense. If it were the most stable, we would not have to work so hard to form it and keep it. It would just happen…but it doesn’t, does it?
Class adjourned.
Go rest your brain and have some chocolate.
And keep those questions coming.
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