Pipelines and Pressure: The Story of Maple Syrup
by Casey Twanow
Picture a tall stack of pancakes—so big their edges hang off of your plate. Now, spread on some soft butter, and for the final taste treat, pour on golden-brown, deliciously sweet, pure maple syrup. Mmmmm. Not much beats the rich flavor of maple syrup fresh from the tree.
What? Fresh from the tree?
That's right—maple syrup is made from the sap of sugar maple trees, which grow in the midwestern and northeastern United States and eastern Canada. Maple syrup was first made by Native Americans hundreds of years ago, and when European settlers arrived, Native Americans taught them to make syrup from maple sap.
The Sap System
Sap is the watery substance that flows through every tree's roots, trunk, and branches. Sap provides food—energy for growth and maintenance.
In spring, the food-energy in sap is sugar made by the tree's leaves during the previous summer. Sugar maples have the sweetest sap. Farmers who own a sugarbush, a woods full of sugar maples, drill small holes in the trees and collect the sap as it drips or “runs” out of the holes. (Don't worry; they don't collect enough to hurt the trees.)
Farmers can only collect maple sap for a few weeks each year—usually during February or March, when the weather is just right to start the sap running. During most of the year, hardly any sap would drip from a hole drilled in a maple tree. On a warm spring day followed by a freezing cold night, however, two to three gallons might drip from a hole in a single maple tree. Scientists don't completely understand maple sap flow, but they do know that daily thawing and nightly freezing cause pressure changes in the wood of a maple tree and make the sap run.
If you looked at a tree's wood under a microscope, you would see long cells stacked on top of each other to make tiny pipelines. These move water, food, and sap up and down the tree. Spring thawing and freezing create positive pressure in the sap flowing through a maple tree's pipeline system. This means that sap presses against the walls of the tiny pipes, so if there is a hole in the wood (like the small ones drilled by sugar maple farmers), the sap drips out of the “leaky pipes” much like water drips from a leaky pipe under your sink.
The Spring Run
All trees have a pipeline system that carries food, water, and sap to their parts. But spring weather does not raise the pressure in all trees—just in sugar maple trees and their close tree relatives. This group of trees is unique because in the layer of tiny pipelines just under their bark, a lot of gas bubbles float around. These bubbles are like little balloons full of carbon dioxide. The bubbles help create positive pressure inside maple trees in spring.
If you have ever carried a balloon from a warm house into cold air outside, you may remember that the balloon shrank (if you haven't seen this, you can blow up a balloon and put it in your freezer for a few minutes to see the change). This is because gases compress, or take up less space, at colder temperatures. So during a cold spring night, the gas bubbles inside a maple tree compress. Water is pulled up from the roots and through the tree's pipeline system to fill extra space around the shrinking bubbles.
If the next day's sun heats the maple tree above freezing, the gas bubbles expand (you can watch your balloon grow to its original size when you take it out of the freezer). Gases expand, or take up more space, at warmer temperatures. The growing bubbles crowd their pipelines because of the extra water sucked up during the night. Water moves from the crowded pipelines into any available space in the tree's pipeline system. It ends up crowding sap-carrying pipelines deeper in the trunk and the sap starts pushing against the walls of its pipes—it has positive pressure! This high pressure causes the sap to run into any open space (where pressure is lower)—even outside the tree—through a broken twig or a hole drilled by a farmer.
Sap dripping from a maple tree looks, smells, and tastes almost exactly like water, and about 97 percent of it is water. The rest is sugar made by the leaves, and dissolved minerals. Surprisingly, the two or three percent sugar in sugar maple sap is still a higher concentration than in other trees. Researchers are trying to discover if this extra sugar creates osmotic pressure, another force that could help maple sap run.
Osmotic pressure is created as water moves between cells in the maple tree, because for some reason water tries to even out the sugar mixture in different cells. If you made two glasses of lemonade, and put too much sugar in one, you might even out the drinks by diluting the sugary one with more water. Well, since cells in a maple's pipelines hold so much sugar in the spring, some scientists think that water moves from the roots (where there is less sugar) into the pipelines to dilute the sugar in those cells. This water from the roots could crowd the sap-carrying pipelines even more, and help produce high-pressure sap that will drip out of a drilled hole.
Collecting and Cooking
As long as warm days alternate with freezing nights, sap will run and can be collected for syrup. On traditional sugar maple farms, a farmer fits a metal or plastic spout, or spile, into the hole drilled in each maple tree and hangs a tin bucket from each spile. Sap drips through the spiles into the buckets, which are covered with tin roofs to keep out rain and insects.
In most commercial sugarbushes, however, vacuums actually suck sap from the trees. Technicians fit a spout into the hole drilled in each maple tree, and attach clear, plastic hoses to the spouts. The hoses from each tree join into a larger hose connected to the vacuum. This doesn't hurt the trees, and often collects about twice the sap of traditional methods.
Nothing is added to sap to make maple syrup. It is poured into a large, deep, metal pan called an evaporator, and heated to a boil. As it boils, water evaporates from the sap and it becomes sweeter and thicker and turns a gold color. When the boiling temperature reaches 219.1 degrees F (a farmer may test it with a thermometer, but some modern evaporators monitor temperature automatically), the liquid is 66 percent sugar—pure maple syrup. It is quickly drained from the evaporator, and is ready to be bottled and sold. Since sap is so watery, it can take more than 40 gallons of sap to make one gallon of syrup!
So the next time you enjoy maple syrup–drenched pancakes, take a moment to think about the special weather conditions and pressure changes that made that maple sap run, and the hard work someone put into collecting it and making your syrup. Your breakfast will taste even better!
- Sap contains food materials for the tree. Through which kind of vascular tissue does the sap flow?
- During certain times of the year, what effect does heating and cooling have on the pressure inside of a maple tree's pipeline system? Write a short paragraph explaining the effect of heating and cooling. Then draw two diagrams that show the sap in a pipeline inside of a maple tree. The first diagram should show what the sap looks like on a cold night. The second diagram should show what the sap looks like on a warm day following a cold night. Label the parts of your diagram.