That's correct, reheating any syrup makes it darker, the longer you take to reach the desired temperature the darker it gets.
That's why I try to store syrup in barrels that are at the lighter end of any grade, so that as it's reheated to pack it I'll still have the grade I want. I made a mistake once regarding that, (I put a batch of 68% light transmission [LT]syrup in my water jacketed bottler and bottled some, it came out as 57% LT but I still had about 6 gal left in the bottler, and I had to leave. I failed to shgut off the WJ bottler,. I returned the next day, checked density and added a little distilled water to get it to the density I wanted (66.9%) and I checked the grade, it was in the low 40's LT, I hads made about 6 gal of Amber into Dark by that mistake.)
Since that time , when I open a barrel to repack, I do re filter it, where as I used to heat it to 190-200 in my finisher, I now only heat it to 170, then I filter it into the WJ bottler. Once there I heat it to 185+/- 2 degrees F retest density and grade and bottle it, if I ever need to leave, with syrup still in the bottler I shut it off. When I return to finish bottling I still need to verify density and grade and it always needs some distilled water and it's a little darker, but doing it this way i only get a slight decrease it LT and I'm still in the grade I wanted.
If you want to learn more about this or anything maple related, buy a copy of The North American Maple Producer's Manual or to get it free, download it. It's available at no charge, only the print version costs you.
Dave Klish, I recently bought a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.