When winters are long and very cold and have no episodes of thawing, the first sap flow may sometimes be off-flavour. Maple researchers believe the off-flavour is caused by a buildup of amino acid in the maple trees, which normally would dissipate during winter thaws. Researchers refer to this off-flavour as ‘tree metabolism’. Syrup made from off-flavour sap will have an unpleasant flavour, which cannot be blended out with good syrup. The flavour will usually clear in subsequent sap runs.
For this reason, producers often discard the first sap run. Most commercial producers used the first sap flow to rinse collection tubing systems, then discard the sap.
Sap sugar concentration below 2 °Brix often occurs in the first flow of sap.