The Science Behind Maple Syrup
Let's get sap happy
On April 1st, over 60,000 people will gather at the world’s largest maple syrup festival in Elmira, Ontario. There are plenty of tributes to this liquid maple gold at the event, from pancake breakfasts to high-fiving the pancake-shaped mascot (aptly named Flapjack) and visits to a sugar bush full of maple trees to see how the syrup is tapped. It’s here in the sugar bush that visitors to the festival may find themselves most perplexed. In its simplest terms, maple syrup is a sugary sap that runs through the maple tree and gets tapped, boiled down and bottled for human indulgence. But once you learn the science behind the process, you’ll discover that tapping maple sap is nothing short of botanical magic.
Maple syrup’s infancy is during the summer prior to the harvest, when the tree’s leaves are photosynthesizing sunlight into starch. While much of that energy-rich starch is used by the tree for its growth and maintenance, some of it will be stored for sustenance when sunlight is less readily available in the winter, a bit like how humans store fat.
As the weather cools, the maple tree loses its leaves and that starchy energy reserve is converted to sucrose, which is dissolved in the tree’s sap at a concentration of 2.5 to 3 percent. When the springtime thaw arrives and temperatures rise above freezing, positive pressure develops within the tree. When you ‘wound’ it by hammering a metal spout called a spile into the trunk, the pressure build-up forces sugary sap to flow out of the wound.
But the sap flow doesn’t just continue indefinitely. Temperatures need to fall below freezing at night, triggering the tree to draw water up from the soil through its roots and replenish or recharge the sap. This freeze-thaw cycle is essential for sap flow. Too warm at night or too cold in the day? No sap. Ideally, maple sap harvesters or “sugarmakers” will experience that freeze-thaw cycle five to ten times before the tree naturally heals over the tap wound. Even better is a more drastic fluctuation between daytime warmth and night-time freeze, which produces a higher volume of sap flow the next day.
What makes maples unique in the tree world is a serendipitous coming-together of botanical oddities. To commercially produce syrup from a tree, it’d have to exude enough sap with a potent, palatable sugar concentration. Most trees either don’t produce sap in a way that can be easily tapped, or don’t produce sap with a high enough sugar concentration to make production worthwhile, or leak a bitter resin that mixes into the sap when wounded.
Lastly, the trees need to grow in a region that experiences a springtime freeze and thaw cycle, focusing sap-producing maples to North America’s Northeast. Out of all the tree species, maples are the only variety that satisfies all these needs. A few producers in Alaska, Russia and Scandinavia make syrup from birch trees, but the result is more savory than sweet (and a lot more expensive).
Does this harm the tree’s future growth? Not according to the Cornell University Maple Program. As long as producers tap only healthy trees over 40 years old with a trunk wider than 12 inches in diameter, there won’t be any damage to the tree. Tapping only removes about 10 percent or less of the tree’s sap so it’s too small an amount to damage a healthy tree. So enjoy your pancakes in peace.