Cornell University's Director of the Uihlein Forest, Michael Farrell, explains the collection of walnut sap through vacuum tubing. Freezing temperatures in early spring cause the tree to go into negative pressure and brings water up from the ground into the tree's cambium layer. Warming temperatures during the day cause the tree to thaw again which releases the water, now in the form of sap, back down through the cambium. It's the freezing and thawing events that cause sap to flow back and forth in late March and early April. Michael Farrel explains the flow of sap which enables them to draw it out with a vacuum tubing system in order to make walnut syrup.
Michael Farrell demonstrates how to tap a walnut tree and collect its sap with a bucket. Before tapping a tree, the bark should be checked for any knots or defects. A clean surface on the bark without any defects should be tapped. It is critical to tap into the white sap wood of the tree. After tapping the tree, the shaving should be examined to determine if they are white sapwood. This will ensure the best sap flow from the tree. Walnut trees by nature have more heartwood and less sap wood than maple trees, which are composed primarily of sap wood. The thin layer of sap wood with walnut trees means that less sap flows throughout the tree. In turn, less sap can be collected for syrup.
Mike Farrell discusses conservative tapping guidelines. In order to obtain sap from healthy walnut trees for years, conservative tapping should be practiced. Every tap hole leaves a stain column within the tree which will not yield very much sap if tapped twice within the same column. Large trees can support two taps at a time, but smaller trees should only be tapped once yearly. When using a vacuum system, it's best to set up the tubing to have gravity work with it. The system demonstrated in this video is placed so that the main line is going downhill to meet the vacuum pump at the collection tank at the end. The lateral lines go from the tree to the main line, bringing sap from the entire sugar bush down to the collection point.
Walnut trees contain juglone, a component that causes an allelopathic reaction in many plants, but not all. This causes competing plants to die out, leaving little understory in walnut stands. Michael Farrell, Cornell University's Director of the Uihlean Forest, discusses the characteristics of the walnut tree and cautions anyone who is allergic to nuts from trying walnut syrup. While it is possible for any proteins within the syrup that may cause allergies to be denatured during the boiling process, there has not been enough research to verify whether this is the case. Anyone with nut allergies should err on the side of caution and avoid consuming syrups from nut-bearing trees.
Maple and walnut trees are quite different, especially when it comes to the amount of heartwood vs. sapwood within each species. Maple trees primarily have more sapwood and little heartwood, whereas walnut trees have a greater amount of heartwood and less sapwood. This translates into walnuts producing less sap, as there is less sapwood within the tree. Maple trees can be tapped at around 20 to 25 years of age while walnut trees can be tapped at 10 to 15 years of age. The reason that a walnut can be tapped at a younger age is because the sapwood that should be tapped will turn into heartwood anyway. Another difference lies within the sap itself. Walnut sap has much more pectin in it. This is the same kind of pectin that is used in jams and jellies. Filtering the syrup is therefore much more difficult in the presence of pectin.