All the sugar in maple syrup comes from maple sap, but sap and syrup are not the same thing. Sap is thin, clear and only about 2 percent sugar. Syrup is sap boiled down until it's about two thirds sugar, and on average that takes 40 gallons of sap for every gallon of syrup.
What sap is
Sap is the liquid that drips from a tapped maple between late winter and early spring. Vermont's maple rules define it plainly: the unprocessed liquid that comes from a maple tree. Taste it straight from the tree and it's only slightly sweet, because it's mostly water, with about 2 percent sugar. A USDA Forest Service summary puts the average at 2 to 3 percent, and says it varies a lot from tree to tree.
The sugar is sucrose, the only sugar in sap when it leaves the tree. It starts out as starch, stored in the wood over the growing season, and turns to sugar once the wood warms to about 40°F. Freezing nights and thawing days then build the pressure that pushes sap out through the taphole.
Sap also spoils fast when it's warm. Sugar and water make a perfect place for bacteria to grow, so sugar makers keep sap cool and boil it as soon as they can. Sap that warms up before it's boiled can make darker syrup with off flavors.
What boiling does
Boiling takes the water out. Sugar makers boil sap in an evaporator. The water leaves as steam, and what stays behind gets thicker and sweeter. Many run their sap through a reverse osmosis machine first. It pulls out part of the water before the boil, so boiling takes less time and energy. On a modern evaporator, sap takes about 45 minutes to go from the inlet to finished syrup.
Heat does more than thicken it. The color and the maple flavor both develop as the sap cooks. Cornell's maple program points to one of the reactions behind that, the Maillard browning reaction, which can darken syrup and strengthen its flavor.
Boiling concentrates everything else in the sap, too. A faint off flavor in the sap becomes a strong one in the syrup. And if it boils too far, syrup comes out darker than it should be and can taste scorched. Syrup made later in the season also tends to be darker and more robust. Our guide to the grades covers each color.
How thick syrup has to be
With syrup, thick really means sugar. Sugar makers measure density in degrees Brix, the percent of solids by weight. Most of those solids are sugar, so Brix is often read as percent sugar.
Federal rules set the floor: at least 66 percent soluble solids by weight, all of them from maple sap. Vermont asks for syrup a little thicker than that, and our guide to what Vermont means on a maple syrup label has the exact numbers.
Getting the thickness right matters. Syrup that's too thin can mold or turn sour. Syrup that's too thick tends to grow sugar crystals in the bottom of the container. In between, it stays shelf-stable.
So how do sugar makers know when to stop? Sap starts out boiling at about the same temperature as water. As the water leaves, the sugar gets more concentrated and the boiling temperature climbs. Syrup is done when it boils about 7°F hotter than water does. At sea level, that's about 219°F.
The catch is that water's boiling point moves with elevation and with the weather, so sugar makers check it every day. They also check density directly, with a hydrometer or a refractometer.
Why it takes so much sap
To get from about 2 percent sugar to about 67 percent, nearly all the water has to boil away. Averaged out, that takes about 40 gallons of sap to make a gallon of syrup, though it can be anywhere from 20 to 100. Sweeter sap needs less. Sap sugar shifts from tree to tree, from one run to the next and from year to year. Our guide to how much sap one maple tree makes has the rule sugar makers use to work out the ratio, with the tree-by-tree numbers.
At that average, an 8 fl oz tin of ours takes about 2 1/2 gallons of sap. A 16 fl oz tin takes about 5. That's a lot of steam.
So, is it just sap?
In one sense, yes. Federal rules describe maple syrup as the liquid you get by concentrating and heating maple sap, and every bit of its sugar has to come from that sap. That rule is also what separates it from pancake syrup. The same rules do allow a few extras, such as salt or a preservative, as long as the label lists them.
In another sense, no. By the time sap becomes syrup, nearly all the water is gone and the sugar is more than 30 times as concentrated. Heat has turned a clear, faintly sweet liquid golden to very dark and developed its maple flavor. Sap is the raw material. Syrup is what a long boil makes of it.
Ours is one ingredient. The front of the tin says "100% pure maple syrup," and it comes from a single Vermont forest, boiled over a wood-burning fire, with nothing added. Pour Amber when you want something warm and smooth with caramel notes, the one that goes on everything. Reach for Dark, deep and robust, when you're cooking or baking. To follow the whole trip from tree to tin, see our maple guide.
