As long as it stays cold the sap will be good. Being in milk jugs you can't throw the ice out, but when it is half ice and half liquid, you could boil the now concentrated sap. But without another jug to replace it, you would further dilute new sap that ran into the jug, as it melted the very low sugar ice.
Dave Klish, I recently ordered 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.