Thinking about this from first principles, a banana is about:
- 75% water
- 20% carbohydrates (mostly sugar and a small amount of starch)
- Less than 5% combined fiber and protein
- Trace amount of fat (generally considered to be zero).
The anti-crystallization effect of guar gum works on the water component, so adding a small amount of guar gum actually should be of some benefit. The hard part is getting it all dispersed; you don't want to over-process the banana (you'd lose the creaminess) but you also need to hydrate the guar gum in order for it to work its magic. You also need to avoid over-thickening, which might be difficult even with the very small starch content in a typical banana.
Bananas convert more starch to sugar as they ripen, so I suspect you'd have more success trying this on very ripe bananas that are, essentially, little more than sugar and water to begin with.
Incorporating and maintaining air is something that I just don't think you're going to be able to do. Proteins and fats can both participate in foaming (egg whites being an example of the former, and heavy cream being the canonical example of the latter), but bananas have neither of those. There's simply nothing to whip, and thus nothing to stabilize, so neither xanthan gum nor any other emulsifier/stabilizer is going to do you much good here.
If you really want to incorporate air then you're going to need more than one ingredient. My instinct would be to say coconut milk, which has relatively high fat and pairs well with banana (and obviously is also vegan / non-dairy), but I can say from experience now that coconut milk just doesn't have enough fat. You could try cooking it down first to get the fat ratio up, or using coconut oil, but I'm not sure how it would taste.
Traditional ice cream is about 60% water, which means the amount of guar gum that most people seem to get the best results with (about 1 tsp/qt) is about 0.87% of the total water. For a small (100 g) banana, that's about 75 mL of water corresponding to very close to 1/8 tsp of guar gum.
So, I'd give this a try with 1/8 tsp of guar gum, forget the xanthan gum. Try adding it after you've frozen the banana for the first time, when you first pop it into the blender.
No guarantees, but, you can probably afford to waste one banana in the attempt.
Update:
I thought I'd give this a try myself since (a) it seemed pretty easy and (b) I can always use more non-dairy dessert recipes. Here's how it came out with 1/4 tsp of guar gum for 2 bananas, after being frozen for two days:
You can see that ice crystal formation is pretty minimal, and although some parts did harden a bit (much like ordinary homemade ice cream with low overrun), they returned to normal consistency within just a few minutes.
Of course it still tastes like frozen bananas, so if I were to do this again I think I'd probably try adding a few other flavours. Also note that even though the texture is decent, the colour is getting dark, and I have no doubt that this would eventually turn black after enough time in the freezer - and the lemon juice trick doesn't really work so well when the whole thing has been blended up.
I think the problem is your cream, but not your fault if your recipe didn't mention it anymore. For a Bavarian cream, you normally whip cream and put it in the mixture in the end, to make it more light and also less liquid.
In that case, I think your amount of gelatin would be enough, since you'd only have a good 250 ml of liquid (but not all gelatin sheets are as large as others etc.).
I can't say if you let it boil long enough or not. The "back of a wooden spoon test" doesn't work for me. I normally let it cook till I see it has thickened a bit.
Best Answer
Many hydrocolloids are very heat tolerant.
Hydrocolloids Primer
Some of the options that might work for you are:
Agar agar
A polysaccharide made from boiling seaweed, agar agar is heat stable up to about 90C. It gels when cooled to 40C. It is inexpensive and easy to find, especially in Asian markets. It would be relatively easy to coat the inside of a cone by heating the agar agar to 90C, cooling it almost to 40C and pouring into very cold cones and pouring out the excess ungelled liquid.
Obviously it won't work if your liquid is hotter than 90C.
LM Pectin
Like agar agar, pectin is a polysaccharide but it is extracted from fruit. Regular, off the shelf, pectin that requires a lot of sugar is HM Pectin. It will melt easily. LM Pectin on the other hand, requires calcium to gel. Once gelled it is heat stable and will not melt again.
Unless you have a well stocked specialty pastry supply store handy you may have to buy this online and it will definitely cost more than the agar agar.
Sodium Alginate
Like LM Pectin, sodium alginate gels in the presence of calcium. Often used to make "caviar" preparations of random liquids. It is easier to find for sale as part of molecular gastronomy kits.
It forms a heat stable gel but you'd have to be creative about how to coat a waffle cone with it. Perhaps you could form a sheet of it and "glue" it into the waffle cone.
All in all, if your liquid is cooler than 90C, for cost and ease of use I would recommend going with the agar agar.