This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
The birch tree contains a colorless acid and sweet sap, which may be obtained by boring holes about one or two inches deep into the trunk during spring, and putting tubes into the holes, with cups at the end. It is said that fifty white birch-trees, of about eighteen inches diameter, yield in four days about 350 pounds of sap. This sap contains sugar, extractive matter, acetate of calcium, etc. A very excellent sparkling beer or wine can be made from this sap by adding to it from eight to ten per cent. of its weight of sugar, and 0.2 to 0.3 per cent. of tartaric acid. According to an authority, the best product is made by adding to 100 pounds of the sap about six ounces of tartaric acid and eight to ten (or if a stronger product is wanted, sixteen to twenty-four) pounds of sugar, and three ounces of a strong almond milk. The mixture is fermented in the usual manner, put in bottles with a little more sugar, and securely sealed.
Another method of preparing birch beer is: take birch bark, one-half pound; hops, one-half pound; allspice, one-quarter pound. Boil these ingredients in a few gallons of water for a few minutes, add the liquid to ten gallons of water, mix, and when below 100° F. add about one pint of yeast and let ferment until suitable.