Use for the rind after making Citrus Sugar

There are noted resources for flavor pairing, many of which you can find on this site. Check out flavor. You'll find:

1. Why some flavors work better together than others
2. A list of books and websites providing information on flavor combinations, including suggestions for
   • FoodPairing
   • Khymos
   • The Flavor Thesaurus
   • cuuks (here's a start for your combo)
   • The Flavor Bible
3. How to determine if two ingredients will go well together

For your specific question of what goes well with citrus rind, you already know chocolate is a good match and that you are putting the sauce on brownies. Nuts in your brownies will be a good match too, especially pecans. Caraway, thyme, cardamom, or black pepper might add an unexpected note with orange. Rosemary is a very nice match for lemon. I recently made cookies using orange zest and black pepper that came out quite well.

If you want the citrus flavor to dominate I would leave out the berries. Otherwise I'd definitely use the raspberries, which go well with citrus. The bluberries would be fine, but I wouldn't consider them your best choice.

There are three (to the best of my knowledge) main factors at work here:

1. Pectin. This is a gelling agent, a bit like gelatin. Fruits such as apples and plums are very high in pectin; citrus fruits are not that high in pectin but citrus peels are incredibly high, so if you're using any of the peel, you're getting tons of great pectin.

   Obviously, the more pectin, the faster and firmer it's going to gel. If the fruit doesn't contain much pectin then you're going to have to cook it down a lot or just add actual pectin (you can buy it in stores).

2. Acidity, as you suspect, makes a big difference. It's not just acidity, it's the relationship of acidity to pectin as above. For more details read The Role of Acid in Jelly Formation. Basically, pectin gels best at a pH of 2.5 (almost exactly the pH of lemon juice) to 4.5 (about the pH of sweet grapes).

   It's best to hit the sweet spot in the middle rather than the extreme ends, so you won't generally be using pure lemon juice, you'll be adding some sugar and other ingredients. Which leads me to the final factor:

3. Sugar. As described in The Role of Sugar in Jelly Formation, sugar precipitates the pectin, basically making the pectin molecules less soluble which mimics the effect of a higher concentration. Therefore, more sugar makes the jelly set faster.

So: Pectin-rich fruits + high acidity + high sugar content = fast setting.

There are actually various other factors that affect the pectin (which is pretty much the only gelling agent at work when working with natural fruit jellies) like salts and even alcohol, but in the vast majority of recipes, it's acidity and sugar that are your primary concerns as well as the actual pectin content of the fruit itself.