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Pressing and Boiling

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Sorghum Syrup
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Pressing and boiling

Sweet sorghum stalks are pressed through a roller mill to extract juice, and the juice is boiled in a long shallow pan, skimmed, and cooked to about 230 F. If the juice starts at 15% sugar and the syrup finishes at 78%, each gallon of syrup takes about 6.8 gallons of juice and about 59 MJ, which is roughly 13-27 kg of dry wood at 15-30% pan efficiency, far less than maple needs per gallon.

Part of Sugar Production & Sweetener Sourcing

Sorghum pressing and boiling is the roller-milling of sweet sorghum stalks to release juice and the evaporation of that juice to syrup in a shallow pan.

A Roller Mill, A Shallow Pan And A Finishing Temperature

The sugar guide says to press the stalks through a roller mill, animal-powered or hand-cranked, with a three-roller press traditional; to boil the juice in a long shallow evaporator pan, skimming foam continuously; to cook to 230 F for proper syrup consistency; and to jar hot. It gives no juice sugar content, no extraction rate and no finishing concentration, so the starting figures below are labelled assumptions.

Worked Example: The Boiling Ratio For Sorghum

Assumptions: sweet sorghum juice sugar content of 12%, 15% and 18% (a commonly cited range, recalled; measure yours); a finished syrup of 78% sugar by mass (an assumption: the guide gives only the temperature of 230 F, which is 18 F above the boiling point of water at sea level, a far higher figure than the 7 F used for maple, so sorghum syrup is thicker); densities from sucrose-solution tables (recalled): juice 1.0485, 1.061 and 1.0745 kg per litre at 12%, 15% and 18%, and syrup 1.38.

  1. Mass ratio of juice to syrup = 78 ÷ juice sugar %: 6.5 at 12%, 5.2 at 15%, 4.3 at 18%.
  2. Volume ratio = (0.78 × 1.38) ÷ (juice sugar fraction × juice density). At 15%: 1.0764 ÷ (0.15 × 1.061) = 1.0764 ÷ 0.1592 = 6.76 gallons of juice per gallon of syrup. At 12%: 8.56. At 18%: 5.57.
  3. For the guide’s 100-200 gallons of syrup per acre at 15% juice, that is 676-1,353 gallons of juice to evaporate.
  4. Energy per gallon of syrup at 15%: syrup 3.785 × 1.38 = 5.22 kg; juice 6.76 × 3.785 × 1.061 = 27.2 kg; water to evaporate 21.9 kg. Latent heat 21.9 × 2,257 = 49.5 MJ. Warming the juice from 20 to 100 C: 27.2 × 4.18 × 80 = 9.1 MJ. Total about 58.6 MJ (latent heat 2,257 kJ per kg and 4.18 kJ per kg per K are standard physical values, recalled).
  5. Wood at 14.5 MJ per kg of dry wood and pan efficiencies of 30% and 15%: 58.6 ÷ (14.5 × 0.30) = 13.5 kg and 58.6 ÷ (14.5 × 0.15) = 26.9 kg per gallon. At 12% juice the same calculation gives 76.2 MJ and 17.5-35.0 kg; at 18%, 46.9 MJ and 10.8-21.5 kg.
  6. In round numbers for 15% juice: about 6.8 gallons of juice and 59 MJ per gallon of syrup, and 13-27 kg of wood (13.5 and 26.9 rounded).
  7. Per acre at 15% juice: 100 gallons × 13.5 kg = 1.35 tonnes up to 200 gallons × 26.9 kg = 5.4 tonnes of wood.

This is the sorghum ratio, derived from sorghum’s own starting sugar. It is not the maple ratio (about 43 gallons of sap per gallon of syrup, at a different starting sugar and a different finishing concentration) and must not be swapped for it (Boiling and Grading). The wood per gallon is roughly a seventh of maple’s, since the juice is several times sweeter than sap.

The pressed stalks, dried, can also serve as fuel (a qualitative point; no heating value is given here), or as animal feed.

Reading The Thermometer

The 230 F figure is an absolute temperature at sea level. Test your thermometer in boiling water on the day and finish at 18 F above that reading, since boiling temperature drops with altitude (about 1.8 F per 1,000 ft, recalled) and weather. The guide gives no tolerance, and I cannot source how a degree of error changes the syrup, so check the finished syrup’s thickness against a retained reference jar.

What You Need

  • A roller mill (hand-cranked or animal-powered), with three rollers if possible
  • A long shallow evaporator, firebox and chimney
  • A skimmer, filters and jars
  • A thermometer and a pot of water to calibrate it
  • A large supply of dry wood

Method: Press, Boil, Finish

Step 1: Feed the stalks through the rollers, repeating if the juice is not fully expressed. Step 2: Strain the juice into the pan. Step 3: Boil, skimming foam continuously. Step 4: Test for the finishing temperature. Step 5: Filter and jar hot.

Common Mistakes

  • Using the maple ratio. The starting sugar and the finishing point differ.
  • Assuming one juice sugar value. Measure.
  • Narrow, deep pans. The shallow pan is for surface area.

Safety

Roller mills, boiling syrup and foam-over

A roller mill pulls fingers and clothing in and crushes them: feed stalks from a guarded position, never clear a jam while the rollers can turn, and keep children away from the animal or crank. Syrup near the finish is far hotter than boiling water, sticks to skin and foams over; use long-handled tools, keep the pan under watch at all times and cool burns under running water (Thermal Injury Care).

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