Without claiming to know a thing about the science behind it, I made a few test batches of syrup this past week before brewing a Dubbel. Nothing to report about the beer itself yet since it's still in the fermenter, but for what it's worth here's what I discovered about the syrup.
I started with Syrup #4, the 290F version, and made three batches with different sugars: raw organic cane sugar, refined white cane sugar, and dextrose. Cane sugar was the only one worth talking about, the other two turned out terribly. I then made a fourth batch without DAP, to see how pure caramelized sugar stood up to the rest. It was flat and one-dimensional, not something I'd put in my beer.
The 290F cane sugar batch was incredible. Smelled great when cooking, like caramelized sugar and toast. The final result is a smooth, richly dark syrup that days later remains crystal-free. It tastes incredibly complex. Rich, dark fruits like plum and cherry. A bit toasty (though the lighter stages tasted more so), with hints of chocolate and rum.
But that said, there's a slight hint of burnt bitterness that I'm not as fond of at 290F. I suspect it's just a shade too hot. A very quick change occurs between 285F and 290F, the syrup goes noticeably darker in a hurry and bubbles far more furiously. For me anyway, I think the magic number is 285F, after that point the caramelization seems to take off and butt up against the burnt end of the register.
What's clear to me after all this is that the nitrogen provided by the DAP is absolutely critical to facilitating the maillard reaction and creating such a rich complex syrup. Next time I'll keep the temperature a bit lower, but I had no problems brewing with the 290F syrup as it was, the complexity I was looking for was there in spades.