The science behind maple syrup | NCPR News - North Country Public Radio

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Mar 18, 2021 — You can get the sugar out of a lot of trees, but there's something special about the sugar maple. Its trunk is highly efficient at storing and moving sap. That's in part because the sap is stored throughout the trunk, rather then down in the roots, as with most trees in winter. Martha Foley and Curt Stager look at that other "sweet science," the one behind our favorite breakfast condiment.

Sap drips from a maple tree into a collection bucket. Photo: Tom Gill, Creative Commons, some rights reserved

Natural Selections is a weekly conversation about the natural world heard each Thursday on The Eight O'Clock Hour.

Martha Foley: I want to talk about the science of maple syrup. Because as you know, here in this territory, we grow up with maples and maple syrup and boiling sap, but you one of your colleagues wrote this really cool book that tells me a whole lot I never would have guessed.

Curt Stager: Yeah, my former colleague and longtime friend, Mike Rechlin, we worked together Paul Smith's College for years, and he was involved with the maple syrup and sugar production operation that the college runs. He's got a book out now kind of summarizing his knowledge of maples and sap. It's called Maple Syrup: An Introduction to the Science of a Forest Treasure. And it's just full of great little science insights into something that, you know, we all kind of know about maple syrup is something to eat. But the science behind it's really fascinating.

MF: Yeah, of course, there's the kind of cultural knowledge, the traditional knowledge about how to make syrup. But I want to ask you so many questions. You said that maples are different than other trees. And that's one reason that we get so much sap and so much sugar out of them.

CS: Yeah, you can get the sugar out of a lot of trees, the sap, that's kind of one of the things it's for. But one of the reasons that you focus on sugar maples is because of how the trunk is built. More of the trunk is involved in containing and storing and moving the sugars than your average tree.

MF: I think of the inner bark as the place where everything is traveling up and down in the tree.

CS: Right. And that would, of course, be happening. Like, if you look at black cherry, let's say, and you had a log of firewood from black cherry, the wood is sort of pale, there's a ring of pale wood around the outside. And then the center is kind of this reddish, darker brown color.

MF: Cherry wood color. Beautiful red color.

CS: You call that the heartwood, let's say, and the sapwood is the outer pale part. And the inner heartwood there is pretty much dead. And it's got some resins and things in there to prevent decay and stuff. But with the maples, you don't normally see that it's pretty much pale and clear all the way through. And that's because it's full of these conducting vessels that have the sap in them.

MF: So it's going out, as well as up and down.

CS: Well, that was another neat thing that Mike had pointed out. I thought, you know, you say the sap is rising in the spring right? And I always thought, well, it's stored in the roots because the sugar was made in the leaves in the previous summer, got sent down to spend the winter in the roots, and now it's coming up. And he says no, it's not necessarily that way at all. In fact, it's all through the trunk. And you could say the tree is pressurized on the inside. And so when you do put a hole in it, it's not necessarily that it was coming up like a fountain and you diverted it. But because it's just so pressurized inside, it's just coming out of the tree itself.

MF: That's fascinating, really, that changes my whole mental image of what's happening,

CS: I had just always thought of the vertically oriented little conducting tubes in the trunk of a tree. And for maple, about a quarter of the wood is tubes, you could say, or cells that are oriented horizontally, they're called ray cells, and they store sugars and starches and they can move things around horizontally. So a lot of the sap is traveling through the ray cells, you know, in and out of the middle of the trunk as well.

MF: So why does it run? Why does that happen?

CS: Well, according to Mike, there's still active debate on that it's not a simple thing. And maples are a little different from your average plant. So one idea is that really just the whole tree is kind of pressurized on the inside. And one of the things going on is at night in the winter when it's really cold and the liquids in the tree are freezing up. If that happens inside these little conducting tubes that are going up and down later in the day when the sun comes out and things start to thaw and liquefy what used to be big frozen ice shrinks down into a liquid and that leaves spaces or bubbles or gaps inside the tubes and the tree has a way of flushing those out but it makes pressure and extra liquids in there to do that. And so it may be related to that, the tree is just trying to flush out the bubbles that are happening as the thaw begins.

MF: Well I have a whole different picture of what's going on in my maple trees now. Thanks very much Dr. Curt Stager, at Paul Smith's College. I'm Martha Foley at St. Lawrence University.

Mike Rechlin's Book Maple Syrup: An Introduction to the Science of a Forest Treasure can be found here, and with many other fine booksellers.

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