When to filter, and how hot
Finished syrup carries sugar sand (niter), the mineral deposit that falls out of sap as it concentrates. Filtering removes it, and the one rule that matters most is temperature: filter hot, right off the heat. Hot syrup is thinner and runs through quickly; it slows sharply as it cools. The Producers Manual gives 180 to 200°F (82 to 93°C) for pressure filters, and UVM's filter-press brochure says to heat a batch to 200°F so it is still at least 180°F when it leaves the press.
The catch is the top of that range. Reheating syrup above about 195 to 200°F forms new sugar sand (the 2022 manual and Cornell say 200°F; the 2006 manual, UMaine and UNH say 195°F). So filter when the syrup is freshly finished and hot, rather than letting it cool and reheating it later. If you must reheat, do it gently and filter again. The full story is in why syrup goes cloudy.
Filter types, from coffee filter to press
| Filter | What it is | Capacity and notes |
|---|---|---|
| Coffee filter | Household paper filter. | Cornell allows it for a very small producer. No capacity figure in the sources. |
| Felt cone ("felt hat") | Wool or, more often now, synthetic felt (polyester or acrylic, sold as Orlon), hung over a clean lead-free pot. | About 2 to 3 gallons of syrup before a change with good practice; sometimes much less when niter is fine or pasty. Synthetics are more durable and shrink-resistant. |
| Flat filter | The same felt laid flat over a frame or screen. | Same material and capacity as a cone. Cornell: give each layer its own screen so prefilter and felt don't touch. |
| Prefilters | Paper or nylon liners laid on top of the felt. | Catch the coarse niter; peel off as they clog. Stack three to five (2022 manual) or two to three (2006). |
| Small plate press (5-inch) | A small plate-and-frame press used with food-grade diatomaceous earth (DE). | Made to filter 2 to 10 gallons per charge. Needs food-grade DE and a pump. |
| 7- or 10-inch press | The standard commercial press, with a pump. | Typically 50 to 60 gallons per charge; as little as 10 to 15 of dirty syrup, up to 100 of clean. |
Cheesecloth belongs on the sap side. Ohio State lists layered cheesecloth, along with a coffee filter or canister filter, for straining bark, insects and debris out of sap before it goes into the pan. None of the sources we rely on offers it for finished syrup, where the particles are far finer. Filter all sap before boiling, and change sap filters often.
Filtering through a felt cone or flat filter
- 1
Dampen the filter and prefilters slightly
A dry filter soaks up syrup and stalls. The manual says prefilters should not be dry when syrup is introduced, but only slightly damp, so you don't thin the syrup. New filters should be boiled in clean water before first use to remove manufacturing residue. - 2
Stack the prefilters inside the cone
Three to five paper or nylon prefilters inside the felt (2022 manual). On a flat setup, Cornell says prefilter and felt work best when they are not in direct contact. - 3
Pour the syrup hot
Right off the heat, in the 180 to 200°F range. Keep the waiting syrup covered so it stays hot; cold syrup crawls. - 4
Peel prefilters as they clog
Lift off the top prefilter when flow slows, instead of swapping the felt. Expect about 2 to 3 gallons per felt before it needs changing. - 5
Keep the filtered syrup hot for bottling
Pack at 180°F or a little above. If it has cooled below that, reheat gently and stay under 195 to 200°F so you don't make new niter.
Filter presses and diatomaceous earth
A plate press forces hot syrup through paper held between plates, with food-grade diatomaceous earth (DE) as a filter aid that builds a porous cake. The 2022 manual notes small 5-inch presses made for hobby producers that filter 2 to 10 gallons. The working numbers:
- How much DE: the common rule for a standard 10-inch press is 1 cup per 5 gallons of average syrup (50 gallons, 10 cups). UVM's table gives a 7-inch press about 5 cups per 40 gallons. More for dark or late-season syrup.
- Charging: stir half the DE into 3 to 5 gallons of hot syrup, recirculate until it runs clear, then add the rest to the batch.
- Pressure: presses start below 20 psi. Stop and clean as pressure approaches 50 psi (Cornell: generally don't exceed 50 to 60 psi). Tighten the plate stack first; a loose stack squirts hot syrup.
- When it plugs, add more DE, not less. Pressure builds because niter plugs the pores; more filter aid makes more pore space.
Food-grade DE only
Settling instead of filtering
Still a valid method for small operations, in the 2022 manual's words. Let the hot syrup stand undisturbed, then pour or siphon the clear syrup off the sediment. The manuals say it takes several days to a week or more; UMaine's beginner bulletin says 12 hours or more. UNH recommends it for small volumes precisely because felt filters soak up so much syrup.
The drawbacks: a fine haze may remain, long contact with the sediment can add off-flavor, and it raises the lead risk if any lead-bearing equipment was used, because sugar sand concentrates metals. Early in the season, niter can be too fine to filter at all; UVM's brochure suggests letting the barrel stand a month and filtering again.
Washing and storing filters
- Hot water only. No soap, detergent or bleach, ever. The fibers hold the smell and pass it to the syrup; the manual lists detergent-cleaned filters as a cause of off-flavor.
- Turn the filter inside out, scrape off the niter, flush from the clean side toward the dirty side, rinse, and air-dry completely.
- Wringing: the editions differ. The 2022 manual says never wring or twist a filter (it tears fibers). The 2006 manual says never twist, but running filters through an old-style wringer removes water effectively.
- Store clean, fully dry and open to the air, away from mothballs, scented soap or any strong smell. Next season, check for tears, mold and odors before use. Damp-stored filters give syrup a musty, “dirty sock” flavor.
Common mistakes
- Filtering cool syrup. It crawls, and a cooled batch means a reheat, which risks new niter.
- Reheating too hot. Above 195 to 200°F you make the sugar sand you just removed. Double boiler, or stir constantly.
- Skipping the second filter. Near-syrup filtered on the way to a finishing pan must be filtered again when finished, and Cornell suggests re-filtering any syrup reheated from cold, especially for glass.
- Dry filters, or soaked ones. Slightly damp is the target.
- Soap, bleach or mothballs anywhere near the felt.
- Less DE when the press slows. It's the opposite.
- Fighting a late-season batch that plugs every filter instantly. The manual reads that as the season ending; so should you. The other signs →
