Carey Institute of Ecosystem Studies
Excerpts from: Poughkeepsie Journal, March 24, 2013
Learn about the science of maple syrup
It takes biology, chemistry, and physics to explain the science of maple sugaring.
First, the biology. All trees photosynthesize during the growing season and produce sugars to use for growth. In the fall, when trees stop growing, excess sugars are stored for the following spring. Some of those sugars are stored in the tree's trunk in what are called rays.
If you look at the stump of a tree, you'll see rings of wood. Perpendicular to the rings, rays of cells run from the center of the tree to the bark. These rays consist of living cells that hold sugars that trees use for building new cells and repairing damage from a nail or woodpecker hole. These rays are the source of the sugar for the maple syrup.
And the cell configuration in sugar maples is just right to allow sap to flow in the spring. You can tap other species and boil down their sap, but you will not get very much syrup. As it is, the ratio of sap to syrup for a sugar maple is 40:1, so it takes 10 gallons of sap to make a gallon of syrup. For the birch, the ratio is 80:1. So tapping sugar maples is the most productive.
Physics is what makes the sap run. Nights with temperatures below freezing followed by days with temperatures in the 40's to 50's make sap flow. The freeze-thaw cycle causes a change in pressure, which forces the sap to move. At night, when the temperature cools, gases dissolve into the sap fluid, causing a decrease in pressure in those cells. The fluid in the wood freezes, compressing the gases in ice. As the temperature increases the next day, the ice melts, the sap warms and the gases expand, forcing the sap to run out to the tree and into the collecting bucket.
The boiling is where chemistry is important. As the sap boils, pure water evaporates and the sucrose concentrates.