Okay, I screwed up. I was supposed to let the jello ‘thicken’ before adding fruit and pouring into the mold. I got distracted, and when I remembered the jello it was already completely hard.
I was wondering: I know Jello melts when it gets warm, but is it reset-able? Could I microwave this solid lump, stir in the fruit, and then have it turn back into a solid in the fridge?
Does anyone know? Or, if I try, will I be forging new grounds for the world of science?
Hello, 2003. Here’s the answer for your question so future googlers will know: yes, you can reheat jello and re-set it but make sure you do it on a low heat since there’s sugar in it.
Why would it have to be low heat? You can boil it and no harm will come to the sugar, since that’s how you make it the first time around.
OK, before anyone comes in with the zombie jokes, that was the good kind of bump, one that adds new relevant information to a thread. Welcome to the SDMB, findokf, please stick around and contribute more.
EDIT: And of course someone named CookingWithGas would advocate for boiling something 
Really, I think that how you heat it, and how much, will depend mostly on what sort of container it’s in.
Gelatin is an animal protein, the same thing you see that looks like gelatin when you put cooked meat in the fridge and it gels. I use it for glue and reheat it many many times and it always resets, however I don’t have sugar in mine so you may not want to get it too hot.
Actually, I’d be more concerned about boiling the gelatin. 212F is at the upper range of gelatin’s tolerability. Higher heat (which is going to be possible since your mixture has sugar in it) or holding the liquid at 212F for an extended period is going to weaken the gelatin’s ability to firm up.
Perhaps it crystallizes with reheating?
Sugar will carmelize if heated too hot, so I would be careful heating.
Did you need answer fast?
Sugar carmelizes at 340 F. If you’re heating a solution of gelatin in water, it can’t possibly get that hot unless all the water boils away.
If you’re heating sugar dissolved in water, the water will basically never exceed the boiling point. But the starting material is solid gelatin, so it might be possible to heat the bottom of the pan past the boiling point before the rest of it melts. Certainly this could happen if you heat the pan first and dump in jello chunks later.
It has nothing to do with the sugar…at all. It needs to be low heat because high heat (boiling) will destroy the proteins that allow Jell-o to set in the first place. 104 is the minimum temp to dissolve gelatin. 212 (boiling) is way too high. I think 120-130 seems reasonable…120 is the average temperature of hot tap water.
Boiling doesn’t harm the sugar. It harms the gelatin.
The lower temperature of harm is due to the protein.
Harming the protein may well harm some sugar at the same time… Mallard reaction ?
protein + sugar = caramel ? flavour.
But yeah the jelly being a solid is trapping the stuff down there near the heat source, so convection is not effective… the heat is trapped down there , so the stuff at the bottom gets a too high density of heat… it gets hot down there instead of it all just warming up as one.
Mallard reaction:
http://images.shoutwiki.com/nothingiscanon/thumb/2/2d/Duck.jpg/250px-Duck.jpg
Maillard reaction:
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Where were you guys when I actually needed you? 
I see I never gave an update: I decided to take the easy way out. I cubed up the jello, mixed it with the fruit chunks and a whole lot a cool whip and called it a dessert.
And nearly 20 years later, you still remember this? I’m impressed.
My memory is fantastic for useless stuff. You want to know what my locker combination was back in seventh grade? OTOH, what the hell did I come into the kitchen just now to get?
Yes, gelatin is the protein. Sugar can withstand such high heats…may not degrade at 120 -degrees which is enough to dissolve gelatin.
Maillard reaction…missed the “i”…we got ducks below.