The Paradox of Sugaring

By Laura Yayac

saptap (1)It flavors creemees, cotton candy, and liqueurs. It’s poured over pancakes and snow, and is used in countless recipes. And right now, the raw sap is running from trees into buckets and webs of tubing then onto sugarhouses, where it’s boiled into maple syrup in all its amber glory.

Sap runs when the nights are cold and the days warm, but something about this does not make sense.

Before I get to that, though, a bit of history. European settlers learned about maple sugaring from native tribes, who in turn have a variety of legends as to how they discovered that maple sap could be boiled into a liquid sugar. Written accounts of maple tapping date from the 1550s, and it isn’t just people who love maple syrup. One of the explanations for human discovery of syrup is watching red squirrels (Tamiasciurus hudsonicus). These critters have been documented using their teeth to cut into sugar maples, then returning over the next few days, after much of the water has evaporated, to lick the sweet blobs that are left behind.

Most Vermonters know the story and process of sugaring, but I’ll offer a refresher for those who, like me, did not grow up next to a sugarbush. Sugar maple (Acer saccharum) trees are the type most commonly tapped. Sap flows through the sapwood of the tree, the layer between the bark and the center heartwood. In the fall, trees turn their sugars into starches to store them in their roots and trunk for the winter. In early spring when the days are above freezing the starches are returned to sugars and the sap begins to flow. In a tapped tree, it flows out of the tap, and it is then drawn back up into the tree at night when the freezing temperatures return. Recent innovations such as a one-way valve for tapping, patented by Tim Perkins of the Proctor Research Center, help to keep microorganisms from being pulled back into the tree when this occurs.

But wait. If raw sap is 98% water, shouldn’t it expand when it freezes, and therefore be pushed out of the tree rather than pulled back in at night?

Maples are unusual among tree species in that the cells surrounding their vessels (the sap-carrying veins) are filled with gases instead of liquids. During freezing temperatures, the carbon dioxide (CO2) in these spaces contracts, creating room for the sap to move out of the veins and into the surrounding cells. As the sap does this, more sap is drawn up to replace it. (Other tree species have liquid-filled cells, so during freezing temperatures at night, the liquid-filled cells and the sap freeze. Because the sap has no open cell to move into, and it expands as it freezes, any sap that remains in liquid form is expelled).

When temperatures rise above freezing during the day, a combination of forces propel the sap out of the tree. Expanding CO2 gas bubbles push the frosty sap out of the cells and gravity pulls liquid sap down into the sugarmaker’s tubes. It’s from this harvest that 40 gallons of sap are boiled down to one gallon of maple syrup, and that Vermonters can once again revel in all things maple.

With help from: and

Photo by Bryan Pfeiffer.

Laura Yayac is an Ecological Planning student who uses maple syrup in as many recipes as she can.

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