Science Explainers | Growing Food
How Do Trees Make Sap? The Science Behind Syrup
By Julia Agostino | December 3, 2024
For many of us, maple syrup is a pantry staple reminiscent of the arrival of colder weather and the clamoring of radiators. Besides serving as the obligatory glaze for pancakes or the added zhuzh in a salad dressing, maple syrup is a mainstay of the fall flavor profile. While most of us know that maple syrup comes from trees, many of us could find ourselves asking why or even how a tree has any business creating something so distinctly nutty and caramelly. Interestingly enough, this seasonal sweetener is all courtesy of a tree’s remarkable anatomy.
A sugar maple (acer saccharum) displays its colorful foliage at the Native Flora Garden in the Brooklyn Botanic Garden. Photo by Steven Severinghaus on Flickr.com.
Maple syrup begins its journey as sap. Essentially the life blood of a tree, this sticky substance enables the tree to sustain itself. Packed with nutrients and sugars, sap keeps trees well-fed and energized.
In fact, tree sap more broadly refers to the combination of two substances: xylem and phloem. Xylem, derived from the Ancient Greek word for ‘wood’, carries water, minerals, and hormones (trees–they’re just like us!) from a tree’s roots to its canopy. As the tree grows, it builds new xylem tissues around the old ones. This process creates annual xylem rings that can be used to age trees. Phloem, derived from Ancient Greek word for ‘bark’, distributes the sugars made during photosynthesis from the leaves throughout the rest of the tree for energy. Phloem tissue is located just beneath a tree’s bark. A tree’s vascular system shuttles sap to branches as they begin to bud, not unlike our own blood vessels.
Clear sap trickling out of a tree. Photo by pfly on Flickr.com.
During the growing season, sap circulates the tree during a process called transpiration. When water evaporates through tiny pores on leaves known as stomata, this creates negative pressure in the crown of the tree. To stabilize its pressure throughout the tree, the tree draws water up from its roots. In the absence of leaves during the winter, however, trees use stem pressure to transport its food and nutrients.
“When the tree doesn’t have leaves, it can’t move sap through transpiration. There are no stomata. There are no leaves. So the tree actually relies on a mechanism called ‘stem pressure’,” relays Emily Burke, a naturalist at Grass River Natural Area in Michigan. “This is pretty unique to maples and a couple other trees here in the northern hemisphere.”
The mechanics of stem pressure involve the hollow fiber cells that surround the xylem tissue. Air stored in hollow fiber cells diffuses across a thin barrier into the xylem itself, peppering it with air bubbles. As the tree freezes and thaws in the winter, the presence of gas and water inside the xylem expands and contracts the liquid as well. The contraction of xylem creates another pressure differential between the internal tree pressure and outside atmospheric pressure.
“Water is then drawn up through the roots to replace that negative pressure, sort of like a straw. That’s why we tap maple trees during the cycle of freezing and thawing, because that’s when the sap is really running. When it freezes at night, the water is drawn up into the xylem, but then it thaws during the day. You can imagine there’s this huge buildup of pressure in the tree because now it has all this extra fluid in it,” Burke explains.
When trees are relieved of this pressure, out trickles a sapstream. The practice of collecting sap and boiling it into syrup is known simply as maple sugaring. Typically, the sugaring season lasts a couple weeks from late winter through early spring. Sap owes its sweetness to sugars stored in a tree’s stem. As temperatures cool, carbohydrates from the previous season are stored as starches. As temperatures rise again, enzymes convert the starches to sucrose, the main sugar found in sap. While other native maples can be tapped for sap, sugar maple trees (acer saccharum) are the most ideal because of their higher sugar content.
Tapping a sugar maple tree using a spile. Photo by Chiot's Run on Flickr.com.
Collecting sap requires drilling a three inch hole into the tree and inserting a spout (this doesn't hurt the tree). The sapstream will slowly drip into a bucket. Once enough sap is harvested, it can be processed into syrup. A maple tree’s sap contains only two percent sugar, and the rest is water. Because of this, the sap is boiled to evaporate the water and concentrate the sugars. When it reaches 66% sugar by volume, sap has officially transformed into syrup. Surprisingly, boiling 40 gallons of sap yields just one gallon of syrup. As the season progresses, the syrup's color darkens and its taste deepens. This results in maple syrup’s grading system, based on flavor and translucence. Compared to refined sugars, maple syrup contains a higher mineral content and antioxidant properties.
But before maple syrup lined grocery aisles in leaf shaped bottles or wound its way into Starbucks lattes, maple products were used by indigenous Northeast Americans, who are credited with inventing maple sugaring. The first written account of maple sugaring was published in 1557 by a French monk traveling across America’s northeast. The practice was soon adopted by early European settlers and has since evolved into the billion-dollar industry it is today.
In fact, the state of New York is a leading producer of maple syrup in the United States, and the sugar maple even stands as New York’s state tree. You can spot maple trees all around New York City or stop by Brooklyn Botanic Garden’s Maple Grove during your next visit. Look for our Fall Fiesta sugar maple (Acer saccharum 'Bailsta') in our Discovery Garden or pay homage to our good ol’ sugar maple in the Native Flora Garden. From vitalizing a tree to vitalizing our palates, tree sap’s transition from natural byproduct to maple syrup is nothing short of fascinating.