How Maple Is Made

A gallon of maple syrup begins as roughly forty gallons of something that tastes like cold, faintly sweet water. Sap from a sugar maple typically runs at about 2 percent sugar; finished syrup must be at least 66 percent. Everything between those two numbers is weather, plumbing and fire, and it happens in a few weeks at the end of winter. This piece follows the sap from the tree to the grading bench, with the arithmetic of how much sap each jar takes. The maple syrup guide covers the rest of the subject, from grades to cooking.

The tree and the weather

Sugar maple and black maple are the two species most used for syrup. The sap runs only when the weather swings: overnight freezes followed by daytime thaws create positive pressure inside the tree, and that pressure pushes sap out of any opening. Penn State Extension puts the working numbers at days above 40°F and nights below 32°F. When the nights stop freezing, the flow often stops, and it starts again when the nights drop back below freezing. In the Northeast, Penn State says, sap begins to flow in late January or early February, and the most productive runs come from mid-February through March, occasionally into early April. Once the buds begin to swell, the sap makes off-flavored syrup that sugarmakers call "buddy," and collecting stops.

A sugarmaker cannot choose when to harvest. The weather chooses, and a run lasts only as long as the freeze-thaw swing does.

Tapping and collecting

A tap is a small spout set into a hole drilled 1½ to 2 inches deep into the white wood of the trunk, at a slight upward angle so the sap drains out. Penn State's guidance is to leave a tree alone until it is 10 to 12 inches in diameter, to put no more than one tap in a tree up to 18 inches across and up to two in a larger one, and to move each year's hole at least two inches to the side and twelve inches above or below the last one. Tap holes usually close within three years, and Penn State notes reports of trees tapped for 100 years.

The sap goes either into buckets hung on each spout or into plastic tubing that runs downhill from tree to tree to a collection tank. Most larger producers attach vacuum pumps to the tubing, which creates negative pressure in the lines and, in Cornell's words, dramatically increases the amount of sap a tree gives.

The arithmetic of sap

Sap strength varies; a Cornell Small Farms Program article gives a range of 1 to 4 percent and a typical 2 to 2.5. Sugarmakers turn that number into a volume with one division: 86 divided by the sap's sugar percentage gives the gallons of sap needed for one gallon of syrup. The Kentucky extension gives two worked examples: 2 percent sap needs 43 gallons, 1 percent sap needs 86. The table below runs the same division from 1 to 3 percent, plus sap that has been pre-concentrated before boiling, and shows the water that has to leave for every gallon of syrup made.

The middle column is a rule of thumb, not a measurement, but the right-hand column is where most of the work goes: at typical sap strength, about 42 gallons of water have to leave for each gallon of syrup. Every step that follows is a way of getting that water out.

Taking water out before the fire

Most commercial producers now remove part of the water first, with reverse osmosis. Sap is pumped under pressure against a membrane that lets water molecules through and holds sugar back, so what reaches the evaporator is sweeter and there is less of it. Cornell's maple program reports that reverse osmosis cuts processing time and energy by 75 percent or more. One sugarmaker's years of small-scale trials, published through the Cornell Small Farms Program, took 2 percent sap up to 4 to 5 percent with a small unit and to 10 to 12 percent with a larger system. Boiling time per run and the wood burned both fell by about half, with no identifiable change in the taste or quality of the syrup.

The evaporator

An evaporator is a long, shallow, divided metal pan set over a firebox. Sap enters at the back, a float box holds its level steady, and the sap works forward through a series of channels. The back section, the flue pan, is where Ohio State's maple program says "all the heavy lifting" of the boil is done; on a drop-flue evaporator the program keeps sap 1.5 to 2 inches deep from back to front. The exhaust can run between 600 and 1,000°F. On a modern evaporator, sap takes about 45 minutes to travel from the inlet to the draw-off valve at the front, arriving as syrup.

This is also where the flavor is made. Cornell notes that the heat of boiling caramelizes a small amount of the sugar, creating a robust flavor, and the finished syrup ranges from light to dark brown. Maple is a cooked product, in the same family as the reactions in the browning and caramelization guide.

Finishing: temperature and density

Syrup is done when it reaches a set density, and the first sign is temperature. Penn State's rule is to add about 7.5°F to the boiling point of water on that day, because barometric pressure moves the boiling point. The rule matters in Colorado: elevation lowers the boiling point of water, and so it lowers the finishing temperature of syrup by the same amount.

Colorado State Extension gives the second figure and an approximate drop of 2°F for every 1,000 feet of elevation. For a home cook reducing syrup for candy or maple cream above 5,000 feet, a sea-level recipe temperature will overcook it.

Temperature is then confirmed with a hydrometer or refractometer. The standard is 66 Brix, meaning 66 percent sugar, measured at 60°F. Below that, syrup is thin and can spoil; much above 67 or 68, it can form sugar crystals in the jar. The margin a sugarmaker works in is one or two points of sugar.

Filtering, grading and bottling

As sap concentrates, minerals fall out of solution as fine grit called sugar sand, or niter. Hot syrup is pushed through wool or Orlon, a synthetic fabric, usually behind paper pre-filters, to remove it; left in, sugar sand makes syrup cloudy.

The filtered syrup is then graded by color. Under the USDA standard, Grade A syrup falls into four color classes measured by how much light passes through it: Golden Delicate, Amber Rich, Dark Robust and Very Dark Strong. Grade A syrup must be pure, clear, free of off-flavors, and taste of maple in a way that fits its color. Syrup that falls short of those requirements is Processing Grade, which goes to food manufacturing rather than the table. Penn State's guidance is to bottle at 180 to 190°F.

Why a late-season syrup tastes darker

Color and flavor follow the calendar. The first runs of the season give the lightest syrup, and as the season warms the sap changes and the syrup comes out darker and stronger.

Runamok, the Vermont maple house founded by Laura and Eric Sorkin, makes the case that the fire changes the syrup too. The Sorkins moved to Vermont in 2000, farmed organic vegetables for a decade and tapped their own woods in 2009. Their Sugarmaker's Dark (shop Sugarmaker's Dark) comes from smaller Vermont sugaring partners who boil over wood-fired evaporators, a slower boil that the maker says brings out different characteristics than a modern oil-fired rig. The WhistlePig Rye Whiskey Barrel-Aged syrup (shop WhistlePig Barrel-Aged) goes past the evaporator entirely: it is organic syrup rested in the distillery's rye barrels, for a length of time that depends on each barrel's age and how saturated its staves are. The Runamok producer guide covers the full range, and the maple Old Fashioned recipe is one place the barrel-aged style goes.

Past the syrup

Cooked further, the same syrup becomes maple cream, maple sugar and candy. A maple caramel is the next step along that road, and the caramel guide picks up from there. The shop is at 263 Josephine Street in Cherry Creek, Denver, where visitors can ask staff questions, and products ship across the continental U.S. from theambrosianpantry.com.

Sources & Further Reading

• Penn State Extension: Maple Syrup Production
• Cornell Maple Program: About Maple Syrup
• Cornell Small Farms Program: Working with Little Reverse Osmosis Units for Syrup Production
• Ohio State University Maple Program: All Things Evaporators, Part I
• Ohio State CFAES: Consumer's Guide to Pure Maple Syrup
• University of Kentucky: About Maple Syrup
• USDA Agricultural Marketing Service: United States Standards for Grades of Maple Syrup
• Colorado State University Extension: High Elevation Food Preparation
• Runamok Maple: Our Story
• Runamok Maple: Sugarmaker's Dark, Wood-Fired Vermont Maple Syrup
• Runamok Maple: WhistlePig Rye Whiskey Barrel-Aged Maple Syrup