The last time we spoke, I discussed Kemp Station’s small sugar bush and our newly installed tubing system for gravity-flow sap collection. Mark your calendars for our open house on Saturday, April 11 from 9am to 3pm. We plan to demonstrate our maple sap collection system as well as our boiling, filtration, and bottling methods. As of last week Tuesday, sap was flowing into our collection tank, so maple season is here. Did you ever wonder how sap flows from maple trees? I thought we could explore that today.
Maple sap flow for maple syrup production occurs during a narrow window in late winter and early spring. We need cold nights with temperatures below freezing and warm days with temperatures above freezing. This freeze-thaw cycle creates pressures within the tree moving sap through the wood xylem to the tap holes.
During the freeze cycle, temperatures drop below freezing, gas (mainly CO2) in the tree's fiber cells (adjacent to the water-conducting vessels) contracts and some of it dissolves into the sap. This creates a vacuum or negative pressure, which draws water up from the roots and soil to replenish the tree. You might be thinking that the ground is frozen. How can liquid water be available. Well with adequate snow cover, frozen ground only goes down a few inches. As of last Tuesday, the ground temperature at Kemp Station was 32 degrees at 2 inches down and 32.2 degrees at 4 inches down. Snow is a good insulator.
During the thaw cycle, temperatures rise above freezing. The ice that formed in the fiber cells melts and the gases expands. This gas expansion generates positive internal pressure. This pressure can significantly exceed atmospheric pressure, forcing the sap out of any "wound" in the tree, such as a broken branch or a drilled taphole. We have a pressure gage connected to a tap hole on one of our trees at Kemp Station. At night, you see a negative pressure form in the tree. As is warms up during the late morning, the pressure will move from negative to positive approaching 25 psi. That is close to the pressure in your car tires!
To make use of this tree pressure, a small hole is drilled into the sapwood, and a spout (spile) is inserted. Sap will drip from the spout during the thaw cycle and stop when the pressure falls to zero. Commercial producers that use vacuum pumps on their lines can continue to vacuum sap from the tree even after the natural internal pressure falls to zero.
Sap is not just water. It contains sugars, which are stored in the ray parenchyma cells. During the dormant season, these living cells convert stored starch into sucrose, which acts as a natural antifreeze for the tree. In the spring, these sugars are concentrated in the xylem sap to support the upcoming bud burst. These sugars are the fuel to help build the new canopy leaves in the spring. Raw sap is typically 98% water and 2% sugar. To create syrup, producers must remove most of the water through boiling. It takes approximately 40 gallons of sap to produce just one gallon of finished maple syrup.
The sap season is short, typically lasting 4 to 6 weeks. The flow naturally concludes when the "freeze-thaw" cycle stops or when the tree begins to "bud out". Once buds begin to develop, the chemical composition of the sap changes, resulting in a "buddy" flavor that is unsuitable for high-quality syrup production. By leveraging the science of the freeze-thaw cycle, maple syrup producers tap into the first sweet harvest of the year.
Watch for our April 11th program announcements in the coming days. This demonstration project will offer hands on examples of sap collection and maple syrup processing.
