When maple sap boils, pure water evaporates and the sucrose concentrates.
Sticker shock drove my family to start making maple syrup several years ago. In the long hours around the cooker, we figured out the science of maple sugaring. Satisfyingly, it takes biology, chemistry and physics to explain the process.
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 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 quart of syrup. For a 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 40s to 50s make sap flow. These conditions occurred in January 2012, and February of this year, but in past years didn’t happen until March or April. 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 of the tree and into the collecting bucket. If the temperatures remain below freezing during the day or well above freezing at night, those pressure differences will not occur and the sap will not flow.
The boiling is where chemistry is important. As the sap boils, pure water evaporates and the sucrose concentrates. During the boiling, foam must be skimmed from the surface every few minutes. The foam is the product of a chemical reaction that occurs as the sap heats. The cooking also causes minerals in the sap to precipitate as solids. This precipitate is commonly called niter. It is harmless but somewhat gritty and not very pleasant to eat. It consists largely of potassium, calcium, silica, magnesium, sodium and some other minerals that occur naturally dissolved in the fluid in trees. The skimming of the foam as the sap cooks reduces the amount of niter in the end product. The minerals stick to the organic compounds that make up the foam.
You know the season is over when the syrup begins to taste like an old shoe. This happens when the trees start to use sucrose for growth and other compounds become dominant in the sap. When this happens, it’s time to put away the equipment and start planning the garden.