Today the sap is flowing steadily and we expect a good run. The maple tree cell structure is somewhat unique in that the cells contain both carbon dioxide and liquid. The cell structure is quite complicated but for our purposes the fact that the tree contains gas is the reason the sap runs even when the tree is still dormant in the spring. Under warm conditions the gas in the tree trunk, limbs, and twigs expends creating a pressure inside the tree which pushes the sap to the tips of the branches and also down the trunk and out the tap hole.
The sap runs when the pressure around the tap hole exceeds the atmospheric pressure and usually stops in the afternoon as the tree cools and the gas pressure decreases. As the temperature continues to fall the gas contracts further and the gas pressure inside the tree actually turns negative pulling fresh sap up from the roots and stores it in the tree ready for the next day’s sap run.
Sap is also pulled back out of the pipe line as well. This is not desirable as this sap is often infected with algae from the pipe line which hastens the healing and sealing of the tap hole, decreasing the sap flow. Usually, however, the temperature drops quickly below the freezing point and the sap is held in the pipe line with no problem.
This simple explanation of gas expansion and contraction does not fully explain all the reasons for sap flow but it is the major factor, and warm days and cold nights are eagerly awaited by the sugar maker.
The tank of crystal clear sap shown in the photograph contains 2000 gallons of sap and will make about 50 gallons or 200 litres of syrup. 1600 litres of water will be removed from this sap in the reverse osmosis (RO) machine and a further 350 gallons during the boiling process. The RO has made a major improvement to the efficiency of our process as it requires only 10% of the energy needed for the boiling process! Small RO units are now available so even the producer with a few hundred taps can consider this addition to his or her operation. A boiling step is still required to produce the distinctive flavour of maple syrup, however.