It's best to boil everyday and finish all the sap. That said, many can't do that. If you must hold sap, keep it as cold as possible. I used to use 90% shade cloth over my collection tanks and I also banked snow as long as I had it to bank against the tank. It is best not to concentrate (RO) until you are ready to boil, just hold it as sap. Concentrating also concentrates the micro organisms. If by concentrate you mean what is in the evaporator and has been boiled, it will keep well for a few days. When you do get to boil the older sap, you will get less syrup because the micro organisms have been feeding on the sugars and the syrup will be darker. It can still be good syrup, just less of it than if you had boiled it when it was fresh.
This said, I have never boiled sap that smelled real bad, but before my RO I did boil a few times with sap that started to have a slight sour smell and had turned cloudy.
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.