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I have learned that some makers (myself included) are lightly baking (in the oven at ~170F) their sugar on a baking sheet before it goes into the melanger. The idea being that the higher temperature of the sugar will maintain the liquor at a higher temperature allowing you to feed the sugar into the melanger more quickly. I have however heard that by baking the sugar one may be altering the flavor and/or structure of the sugar crystals, possibly affecting the flavor or rheological properties of the chocolate. While I have not performed any comparative tests to verify this I would be curious to know the science behind it.
I have always advocated in heating the sugar for the exact reason you mention. I do the same thing with my nibs and I melt any cocoa butter I am using. It all adds heat to the melanger and helps it get going faster. As you have discovered if you don’t pre-heat your ingredients the cocoa butter in your nibs may still be solid and refining goes much slower until the melanger can generate enough heat from friction to allow everything to flow.
But that is all stuff you know already. You asked about science. In the context of food, there are only a few reactions that affect flavor in regards to sugars. The main two are Maillard and caramelization reactions. And neither is applicable to heating sugar at 170 F.
Maillard reactions require amino acids and non-reducing sugars. Sucrose though isn’t a reducing sugar. And we are talking about heating only sucrose by itself so that is out on both counts.
Caramelization requires heat. And quite a lot of it. Here is a good definition from the Science of Cooking:
Caramelization or caramelisation (see spelling differences) is the oxidation of sugar, a process used extensively in cooking for the resulting nutty flavor and brown color. Caramelization is a type of non-enzymatic browning reaction. As the process occurs, volatile chemicals are released producing the characteristic caramel flavor. The reaction involves the removal of water (as steam) and the break down of the sugar. The caramelization reaction depends on the type of sugar. Sucrose and glucose caramelize around 160C (320F) and fructose caramelizes at 110C (230F).
320 F. That pretty much puts the nail in that coffin too.
“Could it be caramelizing just a little?” I hear you asking. The answer is still no. When sugar caramelizes it changes color (note it says brown color above) and the degree of color change is pretty indicative of the level of caramelization. Here is a sheet of lightly caramelized sugar I made by melting and boiling sugar for 10-15 minutes.
The main thing to note is that even with that really nice auburn hue it had a very surprisingly mild flavor. If your sugar that you are pulling from the oven is solid and unchanged visually from when you put it in, then it is unchanged. It's that simple.
You also asked about rheological differences.
Rheology is a way of describing how something flows or how thick it is. Since there are no chemical changes happening when you heat sugar at a low temperature, if there were going to be viscosity changes in chocolate it would have to come from something else. What? The only answer I can come up with is water. I would not really expect any change as dry granulated sugar has less than 0.1% water. There is significantly more than that left in roasted cocoa beans so any change from the water’s contribution should be negligible.
From anecdotal evidence I have not noticed any real difference between heating sugar or not, but that is anecdotal, and I do tend to always heat my sugar. So, a comparative test was in order. It was pretty straight forward. I needed to test to see if there was any water driven off from heating the sugar and/or if the heating the sugar had any difference in the chocolate’s final viscosity.
In one melanger I made a batch of 65% chocolate my standard way. I heated the nibs and sugar to 150 F and combined them in the melanger. The bowl and rollers I heated to 110 F. The final running temperature of the chocolate was about 120 F.
The second batch was of course also 65%, but I didn’t heat the sugar. Because I wanted to try and compare apples to apples as it were, I heated the nibs to 150 F. I also heat the bowl and rollers to 150 F to add more heat to the system since I would be adding cold sugar. In everything went and the running temperature of the chocolate was 122 F. Close enough.
The result was exactly as I expected. I could not tell the two chocolates apart. Both viscosity and flavor were indistinguishable. Maybe if you heated the sugar to the lower 200s and the running temperature was hotter for the chocolate, more moisture would be driven off from the cocoa resulting in more fluid chocolate. But that wasn’t the scope of this test.
The long story short is that heating of basic granulated sugar isn’t going to change your chocolate but will probably make it a more pleasurable experience to start.