How to prevent pureed blueberries from gelling?

Blueberries, and especially underripe blueberries, have a lot of pectin. Blueberries have about .4g per 100g compared to apples which have .5g. As you suspected this is almost definitely causing the problem. Many blueberry jam recipes consist of just heating pureed blueberries with sugar and acid- no added pectin needed.

When you heated your pureed blueberries the pectins dissolved out of the cell walls and developed a negative charge that would keep them from gelling. The acid in your recipe, as well as the sugar, created perfect conditions for those pectins to re-tangle and gel.

Solutions to this problem would be:

1. Use riper blueberries- ripe blueberries are lower in pectin.
2. Use less sugar
3. Add less acid

Obviously the last two suggestions are easier but will change the flavor of the recipe somewhat.

You could also add less fruit but, as you noticed, a little pectin goes a long way. I fear you would have to drastically reduce the amount of fruit which would be sad.

An interesting paper about pectin says in reference to LM pectins: "The presence of acetyl groups prevents gel formation with calcium ions but gives the pectin emulsion stabilising properties."

Of HM pectins it says: "Acetyl groups prevent gelation and the DM within the group of high methoxyl pectins determines the setting temperature of a gel."

Other papers point out that the more acetic acid that can be extracted from a fruit the poorer the gelling properties of its pectin.

Wikipedia concurs that acetylation prevents gelling of pectin. Acetylation of salicylic acid to make aspirin is done with Acetic anhydride. If you could get your hands on this compound it seems likely it would drastically decrease the gelling ability of your pectins.

I included this last suggestion for the sake of the anonymous hordes on the internets because, as Acetic anhydride will react with water in the air to form acetic acid, it reeks of vinegar. And everyone knows that you hate that smell.

A couple simple, practical things to go with Sobachatina's suggestions:

First, you can break up the gel with a serious blender, not just a whisk. If it gets liquid really flowing, it'll disintegrate pretty well.

Even easier, though: just don't chill it, at least not that much. What exactly you can get away with depends on your ice cream maker, the temperature of your freezer, and the ambient temperature, but in general, it sounds like your situation can handle this. Make sure everything's cool before blending it (keep your fruit in the fridge), then blend it and put it straight into the ice cream maker. I've done this with several things, probably with an ice cream maker like yours and a much warmer apartment.

Note that you should definitely not try this with a hot custard (i.e. French-style ice cream); it's unlikely your ice cream maker can handle that. I'd also suggest trying it with previously-chilled ingredients (blended fruit from the fridge, like above) before trying it with something that's actually room temperature. If you have the frozen canister style ice cream maker, and you push it too far, it's not fun. At that point you can try to salvage it by alternating a couple minutes in the freezer and a couple minutes churning - but you'll probably still get some frozen on the sides. The safe solution at that point is just to scoop it out, refreeze the canister, and start over a few hours later.

In my experiments with pureed blueberries as a photosensitizer in optical detectors, I've experienced that the pectin tends to leave the top fluid phase when centrifuged a lot. 4000G, for 60 minutes, has done the trick - at least I don't experience gelling of my samples.

Admittedly, it's of somewhat limited usefulness, but you never know.

The enzyme, Pectinase AKA Polygalacturonase, might prove useful to you. Apparently, winemakers use it to reduce cloudiness. You can buy it online.