Beet sugar is extracted by soaking sliced beets in hot water and pressing or straining out the juice, which starts at only about 4-9% sugar, so making syrup means boiling off 86-94% of the liquid. For beets with 15% sugar and 90% of it extracted, evaporating the water takes about 177 MJ per 100 lb of beets, which is roughly 0.8 to 1.8 tons of wood per ton of beets depending on how well the pan uses the heat.
Beet extraction is the transfer of sucrose from sliced roots into hot water, followed by straining or pressing and by evaporation to concentrate the juice.
Hot Water Pulls The Sugar Out Of Sliced Beets
The sugar guide’s extraction steps are: wash the beets and remove tops (feed them to livestock); slice thinly or shred, because more surface gives better extraction; soak in hot water at 150-170 F for 1-2 hours, stirring, using roughly equal weight of water and beets, which diffuses sugar from the cells into the water; strain through cloth to remove the pulp (feed it to pigs); clarify; and boil down, reducing the liquid by 80-90% until a thick syrup forms.
Pressing the pulp after the soak recovers liquid that stays in it. The mass balance below treats pressing as a way of raising the share of sugar recovered.
Worked Example: Mass Balance For 100 lb Of Beets
Assumptions: beets with 15% sugar (the middle of the guide’s 12-20%), 5% fibre (an assumed value), and the remaining 80 lb as water; 100 lb of added water (the guide’s equal weight); a fraction e of the sugar ends up in the liquid you collect; and a finished syrup of 66% sugar by mass (the concentration at which maple syrup is made; the guide gives no target for beet syrup).
- Liquid water: 80 + 100 = 180 lb. Sugar in the liquid: 15 × e.
- At e = 0.9: sugar 13.5 lb; liquid 180 + 13.5 = 193.5 lb; concentration 13.5 ÷ 193.5 = 7.0%.
- Syrup mass: 13.5 ÷ 0.66 = 20.5 lb. Water to boil off: 193.5 − 20.5 = 173.0 lb, which is 89.4% of the liquid.
- Ratio of liquid to syrup by mass: 193.5 ÷ 20.5 = 9.5 (or 0.66 ÷ 0.070).
| Beet sugar | Extracted share e | Juice sugar | Liquid boiled off | Liquid-to-syrup mass ratio |
|---|---|---|---|---|
| 15% | 50% | 4.0% | 93.9% | 16.5 |
| 15% | 70% | 5.5% | 91.6% | 12.0 |
| 15% | 90% | 7.0% | 89.4% | 9.5 |
| 12% | 90% | 5.6% | 91.6% | 11.8 |
| 20% | 90% | 9.3% | 85.9% | 7.1 |
Across these cases the juice runs from about 4% to 9% sugar and 86% to 94% of the liquid must be boiled off. So the guide’s 80-90% reduction holds only for rich beets and good extraction; poorer beets or a weaker extraction need 91-94%. This ratio is for beet juice with this starting sugar; a different crop with a different starting sugar has a different ratio and must be derived separately.
Worked Example: The Fuel
Inputs recalled from physics and handbooks: latent heat of vaporization of water 2,257 kJ per kg; 1 lb = 0.4536 kg; air-dry wood about 14-15 MJ per kg (14.5 used); pan efficiencies of 15%, 25% and 35% are assumed scenarios, not measured values.
- Water boiled off per 100 lb of beets (e = 0.9): 173.0 lb × 0.4536 = 78.5 kg.
- Latent heat: 78.5 × 2,257 = 177,000 kJ, about 177 MJ. This ignores the heat to warm the liquid to its boiling point, which adds more.
- Per short ton (2,000 lb) of beets, 20 times that: 3,543 MJ.
- Wood needed = energy ÷ (14.5 MJ per kg × pan efficiency): at 15%, 3,543 ÷ 2.175 = 1,629 kg; at 25%, 977 kg; at 35%, 698 kg. A short ton of beets weighs 907 kg, so wood-to-beet weight is 1.8, 1.1 and 0.77, which is the lead’s range of about 0.8 to 1.8.
- For an acre of 15-25 tons: at best, 15 tons at 35% = 10.5 tonnes of wood; at worst, 25 tons at 15% = 40.7 tonnes. The kitchen guide’s whole annual cooking wood is 12-18 tons (Community Kitchen Design & Operation).
The kitchen’s 12-18 short tons is 10.9-16.3 tonnes, so boiling an acre’s whole crop to syrup would burn roughly 0.6 to 3.7 times the wood the community cooks with in a year (10.5 ÷ 16.3 = 0.64; 40.7 ÷ 10.9 = 3.7). For the sugar quantities a community of 100 actually wants, see Yield per Acre.
What You Need
- Slicing or shredding tools and a washing tub
- Hot water and large soak vessels held at 150-170 F
- A cloth strainer and a press (even a weighted board) for the pulp
- A wide, shallow evaporating pan and a large supply of dry wood
- A thermometer, because boiling point rises as the syrup thickens
Method: Extract And Concentrate
Step 1: Slice thin and weigh the beets. Weigh an equal amount of water. Step 2: Soak at 150-170 F for 1-2 hours, stirring. Step 3: Strain and press the pulp to recover liquid. Step 4: Clarify the juice (Clarification and Crystallization). Step 5: Boil in a wide shallow pan, skimming, until thick.
Common Mistakes
- Not weighing. Without weights you cannot tell extraction or fuel use.
- Boiling in a deep pot. Surface area sets how fast water leaves.
- Overheating the soak. The guide’s range is 150-170 F.
Safety
Boiling sugar solutions, steam, and a very large fire
Concentrated sugar is hotter than boiling water and sticks to skin; juice foams over without warning. Keep children away, use long-handled tools, and never leave a pan unattended. A large evaporator fire needs the clearances and extinguishing water in Fire Safety & Suppression. Steam volume is enormous, so boil outdoors or under a roof with open sides. Cool burns at once under running water (Thermal Injury Care).
What’s Next
- Learn next: Clarification and Crystallization — cleaning and finishing the juice
- Combine with: Boiling and Grading — the same evaporation arithmetic for maple