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Boiling and Grading

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Maple Syrup Production
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Boiling and grading

Maple sap is boiled until it reaches about 66% sugar, which is 7 F above the boiling point of water on the same day (219 F at sea level), and the volume ratio follows the rule of 86: gallons of sap per gallon of syrup equal 86 divided by the sap’s sugar percentage, so 43 gallons at 2%. Evaporating that water takes about 430 MJ per gallon of syrup, equivalent to roughly 99-198 kg of dry wood depending on how much of the heat the pan captures.

Part of Sugar Production & Sweetener Sourcing

Boiling and grading is the evaporation of maple sap to syrup of a defined sugar concentration and the classification of the finished syrup by colour and flavour.

Sap At 2% Is Boiled To 7 F Above The Boiling Point Of Water

The sugar guide says maple sap is approximately 2% sugar and takes 40 gallons of sap to make 1 gallon of syrup. It boils in a wide shallow pan outdoors, skimming foam; syrup is done at 7 F above the boiling point of water (219 F at sea level); it is filtered through wool or felt while hot and jarred immediately at 185 F or above for shelf stability. Yield is 1-2 quarts per tree.

Worked Example: Where The Ratio Comes From

The target is syrup near 66% sugar by mass (the concentration implied by the 7 F rule; recalled as the usual finished syrup). Densities are recalled from sucrose-solution tables: syrup about 1.32 kg per litre, sap about 1.008.

  1. Mass ratio of sap to syrup = syrup sugar % ÷ sap sugar % = 66 ÷ 2 = 33. That is by weight.
  2. Convert to volume. Gallons of sap per gallon of syrup = (0.66 × 1.32) ÷ (0.02 × 1.008) = 0.871 ÷ 0.0202 = 43.2. This reproduces the maple industry’s rule of 86, written 86.4 ÷ sap sugar %.
  3. The parent’s 40 gallons then corresponds to a sap of 86.4 ÷ 40 = 2.16% sugar; its “approximately 2%” gives 43.
  4. For other sap strengths, the same formula gives: 1.5% sugar, 57.6 gallons; 2.5%, 34.6; 3%, 28.8. A sap that tests weak needs a lot more fuel for the same syrup. Test your own sap.

This ratio belongs to maple sap at the sugar content shown. Sorghum juice, beet juice and birch sap each start at a different sugar content and need their own ratio, worked out the same way (Pressing and Boiling, Extraction by Boiling and Pressing).

Worked Example: The Fuel

Inputs recalled from physics and handbooks: latent heat of vaporization 2,257 kJ per kg; specific heat of water 4.18 kJ per kg per K; air-dry wood 14-15 MJ per kg (14.5 used); sap starting near 0 C; pan efficiencies of 15% and 30% as scenarios, not measurements.

  1. One gallon of syrup is 3.785 L × 1.32 = 5.0 kg. The 43.2 gallons of sap are 43.2 × 3.785 × 1.008 = 164.9 kg.
  2. Water to boil off: 164.9 − 5.0 = 159.9 kg.
  3. Latent heat: 159.9 × 2,257 = 360,900 kJ, about 361 MJ.
  4. Warming the sap from 0 to 100 C: 164.9 × 4.18 × 100 = 68,900 kJ, about 69 MJ. Total about 430 MJ per gallon of syrup.
  5. Wood: 430 ÷ (14.5 × 0.30) = 99 kg at 30% efficiency; 430 ÷ (14.5 × 0.15) = 198 kg at 15%. That is about 218-436 lb of wood per gallon.
  6. For 50-100 gallons: 50 × 99 = 4.9 tonnes up to 100 × 198 = 19.8 tonnes, against the kitchen’s 10.9-16.3 tonnes a year for cooking (12-18 short tons). So syrup boiling is from about a third to nearly double the community’s annual cooking wood.
  7. Per flowing day: 100-400 gallons of sap each need about 430 ÷ 43.2 = 10 MJ per gallon, so 1,000-4,000 MJ a day, or about 230-1,840 kg of wood (1,000 ÷ (14.5 × 0.30) = 230; 4,000 ÷ (14.5 × 0.15) = 1,840).

Reading The Thermometer

The finishing rule is relative to the boiling point of water on the day, not a fixed 219 F. Water boils at lower temperature at altitude (about 1.8 F lower for each 1,000 ft of elevation, recalled) and as the weather pressure changes, so test your thermometer in boiling water at the pan each day and finish at 7 F above that reading. Two failures bracket the target:

The margin is narrow, which is why the thermometer must be calibrated every day. The parent’s temperature rule is the only finishing test it gives, and I cannot source how sensitive concentration is to a degree of error, so keep a hydrometer or a measured sample if you can.

Grading

Maple syrup is graded by colour and flavour: golden with a delicate taste, amber with a rich taste, dark with a robust taste and very dark with a strong taste (recalled from modern grading practice; the parent gives no grades). The standard grades are defined by how much light a sample transmits, which needs an instrument, so without one, make a reference set: jar the first, light syrup of the season, keep it, and compare later batches by eye against it. The grade names describe flavour, not safety.

What You Need

  • A wide shallow pan on a firebox, outdoors, and a large dry wood supply
  • A calibrated thermometer and a pot of boiling water to test it
  • A skimmer, and wool or felt filters
  • Clean jars and a way to heat them, to fill at 185 F or above

Method: Boil, Finish, Jar

Step 1: Fire the pan and keep it going; add cold sap a little at a time so the boil is never interrupted. Step 2: Skim the foam as it forms. Step 3: When the temperature reaches 7 F above boiling water, draw the syrup off. Step 4: Filter hot through wool or felt. Step 5: Jar immediately at 185 F or above, and cool.

Common Mistakes

  • Using 219 F at altitude. Calibrate to the day’s boiling point.
  • Letting the pan run dry. The sugar burns and the pan can be ruined.
  • Boiling indoors. The steam load is enormous.

Safety

Foam-overs, scalding syrup and steam

Syrup near the finish is far hotter than boiling water, is viscous, sticks to skin instead of running off, and foams over without warning. Keep children and bystanders well back from the pan, wear sleeves and gloves, use long-handled tools, and never move a pan of hot syrup full. Treat a scald by cooling it under running water at once (Thermal Injury Care).

A large wood fire and a dry pan

A pan that boils dry can overheat, burn the sugar, and ignite the fire around it. Never leave an evaporator alone, keep a bucket of water or sand beside it and follow the clearances in Fire Safety & Suppression.

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