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Energy & Environment
On the second Tuesday of every month at 7:45 a.m. and 5:45 p.m., we hear from our contributors in the field. Susan Knight and Gretchen Gerrish both work for the University of Wisconsin-Madison’s Center for Limnology at Trout Lake Station. Scott Bowe is the Director of Kemp Natural Resources Station.

Field Notes: Maple Sap And Sweet Science

Scott Bowe

In this month's installment of Field Notes, Scott Bowe of Kemp Station discusses how maple trees produce sap for real maple syrup.

Every spring at Kemp Station, we tap a few sugar maple trees to make our own brand of real maple syrup called Kemp Gold. Not only is this a fun family activity, but we use the bottles of Kemp Gold as speaker gifts for our summer outreach programs. Nothing says thank you like real maple syrup on homemade pancakes! Most people have a general understanding of how real maple syrup is made – you tap the trees, collect the sap, then boil the sap to condense it to a syrup consistency. Today, I would like to talk about the sap and how the tree gives us this valuable resource.

First, any maple species can give us maple sap, but some species have a higher sugar content than others. Sugar maple, the species we tap at Kemp Station, has the highest sugar content of the maples at about 2%. As a rule to thumb, it takes 40 gallons of sap to produce 1 gallon of syrup. Red maple and silver maple are two other common maples in Wisconsin. It will take more sap from these maples to make the same amount of syrup because the concentration of sugar is lower. Did you know that syrup can be made from other tree species like birch? We made a small batch of birch syrup this year. The ratio of sap to syrup is more than 100 to 1 since the sugar content is less than 1%. Birch syrup is very dark and has a molasses-like taste. I think we will stick to maple syrup in the future.

The sugar in maple sap is the product of photosynthesis that occurred during the previous growing season. In past broadcasts, I’ve talked about how trees make sugar and use it for energy and a building material. The tree will use the sugar to build new leaves and wood in the coming weeks, we just borrow a small amount to make syrup. In fact, we follow established rules on tree size and how many taps to place in a tree. We don’t want to stress the tree by taking too much sap.

We can expect decent sap flow when warmer days are followed by below freezing nights. Sap flow is especially good on warm sunny days where the sun warms the dark colored tree bark. On a night when it does not freeze, sap will not flow the next day. Fluctuations in wood temperature below and above freezing create periods of alternating negative and positive stem pressures. Positive pressure in the sugar maple stem will force sap out of the tap.

Maple trees have many hollow air-filled fiber cells, which surround the sap conducting vessels. When freezing conditions are present, small ice crystals begin to form inside each of these air-filled fiber cells. As the ice crystals form, the humidity within the fiber cells falls rapidly, causing moist air to be drawn in from adjacent vessels. As this moisture is pulled into the fibers, the layer of ice crystals within each fiber thickens and the air bubble becomes increasingly compressed. The movement of sap into fiber cells pulls water from the vessels, and this negative pressure or suction is transmitted throughout the branches, trunk, and roots. This process continues until all the sap in the tree is frozen.

When the temperature rises above freezing the next day, the wood of the tree begins to thaw. The pressure at the taphole quickly changes from negative (suction) to positive. The thawing of the ice in the fiber cells allows the compressed air to expand and push the sap back into the vessels. Two other forces including gravity and osmosis work with the change in pressure. First there is a downward force of the standing column of sap above the taphole. Second, as the water moved into the fiber cells the previous night to form ice crystals, it could not carry the sugar molecules through the cell wall. As the tree warms, osmosis forces the water back through the cell wall to the areas of higher sugar concentration in the vessels. These combined forces result in a positive pressure pushing sap out of a taphole.

Not only does real maple syrup make homemade pancakes great, but it also makes for great science!

Photo: A sap bag collects sugar maple sap at Kemp Station.