How do I concentrate the flavor in orange juice?

How about using orange zest instead of the juice? That way you'll get a lot of the aroma and flavor we think of as "orange" without really changing the sweetness or acidity.

You might try zesting the orange, reducing the juice a bit, and then adding the zest to the syrup and then cooking it down a bit more. You may or may not wish to strain it after letting the zest cook for a minute or so.

If you do this, take care. I believe cooking it too long will make the zest overly bitter.

Alternatively, you could use frozen orange juice concentrate.

You have a couple of solutions:

• Make it thicker with agar agar instead of starch
• Use something like jam or marmalade

The latter might be to close of the sugary paste you dislike. However agar agar contrary to starch has a really wide range of thickening. You can just make things from a tiny bit thicker than water to jelly. More over the boiling needed will reduce it a bit already.

https://en.wikipedia.org/wiki/Agar#Culinary

You might want to experiment a bit with it to find the right dosage. Jelly cubes between your meatballs would work rather well. As contrary to a liquid sauce they won't spread, keeping flavors separated.

The only good way I know of to concentrate fresh orange juice and maintain the flavor is by removing the water. But not by heating it, as the heat will destroy the flavor.

You need a vacuum distiller or freeze dryer for your orange juice. Pulling a vacuum will boil the juice at room temperature, removing the water without adding any heat.

If you have enough time, you could make an orange extract. Just peel pieces of zest from several oranges (avoid the white pith) and soak them in vodka for a couple of weeks, then filter it. Then you could add it to the pan like wine, boiling off the alcohol (assuming you don't want alcohol in your sauce). You could also follow a homemade orange liqueur recipe, but they might add too much sugar for your use (Here is one from Serious Eats).

It's probably easier to just use zest directly in most cases, though.

I've poached white fish in orange juice with hot chili peppers (I used dried chipotle or even habanero, but I think the recipe called for fresh).

I reduced the cooking liquid a bit, and poured some onto the rice I served the fish with.

It was pretty potent, esp. the time I made it with habanero, though! There was some nice orange flavour, and the heat of the chilis made the sugar of reduced orange juice not a problem. (It was juice from concentrate, not fresh-squeezed. I maybe could have mixed up concentrate with less water instead of reducing for as long after taking the fish out. :P)