A Sticky Situation: The Science of Making Maple Syrup

In the fall of 2014, I was one of 1,800 international students who spent a semester studying at a university in Sweden.  During my time abroad, I made friends with a group of Canadians, and maple syrup (predictably) became a point of conversation.  While I scoured the limited American section of the Swedish grocery stores for Log Cabin, they rationed out the maple syrup they had brought from Canada at every brunch.  The following year, I visited one of my Canadian friends in Quebec.  Hoping to instill in me a sense of appreciation for “real” maple syrup, he took me to a sugar shack to learn how it’s made.    

Maple syrup comes from sugar maple trees, which grow in the northeastern region of North America.  In the summer, the leaves of the sugar maple convert sunlight to sugar using photosynthesis.  Later, to prepare for winter and the loss of leaves from the tree, the sugar is transported to the trunk and stored in the form of sap.  Sugar makers then collect this sap to turn into maple syrup.  They can’t retrieve the sap if it’s frozen solid, though, so they have to wait until late winter or early spring when the days are warm and the nights are cool.  This fluctuation in temperature creates a cycle in which the sap repeatedly freezes and thaws.  This freeze-thaw cycle leads to bursts of pressure inside the tree that cause the sap to flow.  To collect the free-flowing sap from the tree, sugar makers can insert a spout that deposits the sap into a bucket.  Alternately, they can drill a hole into the trunk and attach tubing that directs the sap to a holding vessel.  Sometimes, they will apply vacuum suction to the tubing to help gravity out and increase the amount of sap collected.  The sap is the sugar maple’s source of energy for the winter, however, so the tree can starve if the sugar maker gets greedy.  Luckily, there are strict regulations in place that dictate if and how many times a tree can be tapped for sap depending on its diameter and growth rate, ensuring that its health is not impaired.       

Sap can be collected from sugar maple trees using a spout and bucket or plastic tubing.

Once the sap has been gathered, it is ready to be converted to maple syrup.  Sap is essentially a mixture of maple sugar and water, and the first step is to remove a significant amount of the water.  To do this, sugar makers use a technique called reverse osmosis.  Osmosis is the movement of water molecules from an area where there are more water molecules to an area where there are fewer molecules.  It’s like if you placed a lot of people in the corner of a large room; naturally, they would spread out.  Reverse osmosis, then, is the opposite of osmosis—it describes water molecules traveling from an area where there are fewer water molecules to an area where there are more water molecules.  This is like if a few individuals in the center of the large room wandered over to join a huddled mass of people in the corner.  This is not an intuitive behavior in humans or in molecules, and external forces are required to make it happen.  To enable reverse osmosis in sap, sugar makers use a container that is divided in two by a membrane that allows water molecules to pass through.  First, the sap is placed in one half of the container, and some of the water molecules move via osmosis to the other side of the membrane, yielding pure water in the other half of the container.  Then, to overcome the water molecules’ natural tendency to move from the crowded pure water back to the less crowded sap, a pump is used to apply pressure force to the sap side of the container.  This force pushes the water molecules in the sap through the membrane and into the pure water side, leaving behind a concentrated maple sugar solution on the sap side.

Sugar makers use reverse osmosis to remove water from the sap and concentrate the sugar.

The concentrated sugar solution is then boiled to allow additional water to evaporate.  Once the temperature of the solution reaches about 220 degrees Fahrenheit, it achieves the viscous quality of maple syrup.  Some sugar makers use a hydrometer, which is an instrument that measures water content, to check that the syrup has the proper consistency before turning off the heat.  The boiling step also causes some residual minerals that are dissolved in tree sap to form solid particles.  The syrup must therefore be passed through a filter to separate out these particles before it is bottled.  Once packaged, the syrup is assigned a grade depending on its color and intensity of flavor.  The grade depends on when in the sugaring season the sap was collected, with earlier sap yielding lightly-colored, delicate syrup and later sap producing dark, richer-tasting syrup.

Maple syrup is graded according to its color and flavor profile.

Because tree sap contains so much water, the process of turning it into maple syrup is fairly low-yielding—in fact, 10.5 gallons of sap produces only a quarter of a gallon of syrup.  Learning how much effort is required to make such a small amount of syrup definitely gave me more respect for real maple syrup.  That’s right, I’ve removed the quotation marks around real, as it turns out that most classic American syrup brands are so artificial that that they cannot include the word maple on their label.  Today, I am a proud but cautious syrup convert—while I’ll choose authentic maple syrup over Log Cabin, I hope I never find myself using up precious luggage space on a breakfast condiment. 

Author(s)

  • Sarah Anderson is a PhD candidate in the chemistry department at Northwestern University and a science communication enthusiast. Check out her Twitter page @seanderson63 to read more of her science writing.

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