Based on my searching here and elsewhere, brewing with birch sap is quite uncommon with brewers (although I did find one brewer that mentioned brewing with maple sap). So I'd like to get a list of things to think about as I develop a beer recipe based on birch sap. Brewing with sap is different than using syrup because knowing the flavor of the sap once concentrated is unknown in the case of using sap.
Specific Gravity / Flavor Concentration
From what I have been able to ascertain so far, the sap, right out of the tree is in the 1.004 to 1.007 range. One approach would be to heat the sap and use it as my mash and sparge water. In that case, there may be very little birch flavor in the final product. On the other end of the spectrum would be to gently evaporate1 the sap until the specific gravity gets to a place that would make a beer on it's own, without grain. That would yield the strongest birch flavor, but might be "too much".
I have tasted birch sap out of the tree2, and it's almost like pure water, but I don't know how the flavor would change if the sap was concentrated by evaporation. Based on what that taste would be, then hops could be selected that would mesh appropriately.
If the out-of-the-tree specific gravity is, say, 1.005, is there any way to tell how much of that will be fermentable sugars, and what will be left as residual sweetness? Or even how much of that gravity is caused by dissolved sugars versus something else (minerals?).
Are there any rules of thumb for birch sap based beer recipes? Or rules of thumb from maple sap based beer recipes? What techniques could be employed to select a reasonable starting point for the brewing of a birch sap beer?
1. Boiling apparently causes a poor shift in flavor towards "burnt"
2. I cut a branch off of a river birch tree in my yard last spring and the sap dripped for weeks. I had to taste it.