Mold, crystals and fizzy syrup: what went wrong in the jar

Sediment is its own story: why syrup goes cloudy. The rest is below.

For decades the standard advice was to skim the mold, bring the syrup back to a boil, and repack it. That advice is now being withdrawn, and because readers will still find both versions, here is exactly what each source says, newest first.

• Producers Manual, 2022: “Do not ‘skim and reheat’ moldy syrup.” Moldy syrup should be discarded, because the fungus is more than the visible mat on top. Its food-safety chapter adds that skimming, reboiling and filtering, once widely accepted, is no longer an acceptable practice; mycotoxins cannot be filtered out or destroyed by heating. The same page says mycotoxin research in maple syrup is preliminary and the risk is still being assessed.
• Cornell Maple Program mold fact sheet, 2024: moldy syrup must be discarded. Skimming, boiling and repackaging it is adulteration under the US Food, Drug, and Cosmetic Act and it cannot be sold; customers should be told to throw moldy syrup away and refrigerate after opening. The same sheet notes that no recalls or illnesses from mold in maple syrup have been reported, while some of the mold genera found can produce mycotoxins.
• UMaine bulletin 7036 (rev. 2025): discard, because of potential contamination by a microorganism that can cause foodborne illness.
• UMaine research, 2011 and 2014: fungi were found in 8 of 14 submitted moldy samples (Penicillium, Wallemia, Aspergillus, Trichoderma and others); the authors called skim-reboil-eat the conventional practice and possibly risky. The 2014 paper asked producers to remove jug-label wording saying mold can be scraped off and the syrup reboiled, and reported preliminary evidence that one isolated species could produce a mycotoxin in syrup under optimal conditions.
• Ohio State consumer sheet, 2010: do not remove the mold and reheat; discard, because some microorganisms make toxins.
• Ontario (OMAFRA), 2022/2026: prevention only. It explains that mold roots can run deep into food unseen and that spores are too small to filter out, but gives no instruction for syrup that has already molded.

The older advice, still in print:

• Cornell beginner notebook, 2020: heat to 190°F, skim, repack.
• Producers Manual, 2006: for growth found in bulk syrup, remove all visible growth, evaluate for off-flavor, heat to at least 180°F, filter and package. The 2022 edition says the opposite: if growth is present, discard that syrup.
• Massachusetts Maple Producers Association FAQ: an archived 2013 version called the mold harmless and said to bring the syrup to a slight boil, skim, and pour into a clean container. That sentence is gone from the association's live FAQ.

Finished syrup is a low-risk food: at 66 °Brix its water activity is about 0.85, and most disease-causing bacteria cannot grow below 0.91. Molds are the exception. Some dry-loving molds grow down to 0.70, and UMaine's moldy samples averaged 65.6 °Brix with most above 66, so correct density alone does not make syrup mold-proof. The 2022 manual lists mold as the result of bottling at too low a temperature or improper storage after opening; the 2006 edition adds not inverting capped containers.

Prevention, in the sources' words:

• Density of at least 66 °Brix; below that, Cornell says syrup will ferment or mold irrespective of pasteurization. Many Ontario producers aim for 66.5 or a bit more.
• New, clean, sealed containers.
• Pack at 180°F or hotter, in all the syrup, when the cap goes on. Preheat small glass.
• Invert or lay on the side so hot syrup touches the cap and headspace.
• Minimal headspace.
• Cool, steady, dark storage, and refrigeration after opening.

The full temperature table and the inverting debate are in how to bottle maple syrup.

Rock-candy crystals in the bottom of a jar mean the syrup was finished past the legal maximum and then sat. The editions differ on where that starts: the 2022 manual says 68.9 to 72 °Brix stored a long time at room temperature or cooler; the 2006 edition said 67.5 to 70; UNH's backyard sheet says over 67. Cornell's confections notebook explains the mechanism: syrup cooked to a density that became supersaturated once it cooled.

The crystals are pure sucrose with little maple flavor, so the syrup left behind has lost a little sweetness and a little volume. It is a quality defect, not spoilage. None of the sources we read gives a consumer fix; the producer fix is to bring the density back into range with the manual's water-addition table, and supersaturated syrup should not go into retail containers. Hit the target next time with the thermometer method or a hydrometer.

Fermented syrup tastes sweet, fruity or alcoholic, can be foamy or fizzy, and may bulge its container. Henry Marckres, retired from the Vermont Agency of Agriculture, says it may “work like apple cider”. The 2022 manual traces it to low-density syrup, packing too cool or in unclean containers, and pails or barrels without an oxygen barrier or not filled full. Yeasts are converting the sugar to alcohol.

None of the sources we read gives a way to salvage fermented syrup, and it cannot be Grade A. Prevention is density (at least 66 °Brix) and hot packing. Note the distinction from the fizz in the table above: a slightly fizzy, baking-soda feel on the tongue with no foam or bulge is the burnt-niter off-flavor, which is a pan problem, not a spoilage problem.

Syrup darkens in the jar mainly through oxidation: oxygen passing through the container wall and sitting in the headspace. Heat adds to it (reheating for packing itself lowers light transmittance, and so does stack burn from boxing hot containers), and so does time. UVM's trials measured it: syrup in ordinary uncoated plastic jugs lost on average 2.6 points of light transmittance per month, against 0.8 in oxygen-barrier jugs, and crossed from Amber to Dark within about two months. The jug maker funded that study. Flavor loss is a much smaller effect than the color change.

Darkened syrup is not spoiled. It has moved down a color grade, which for a producer means relabeling; the darkening is oxidation, and the manual treats any flavor change as a smaller effect. The manual also notes a rare “layering”, where the top of a container is darker, slightly higher in invert sugar and slightly less dense than the bottom; the cause is unknown.

• Sealed, hot-packed, no growth: fine. Cornell says a properly packed container is stable at room temperature without a time limit. Quality slips first in metal (6 to 12 months), then plastic (up to 2 years), then glass (about 4 years).
• Darker than you remember: fine. See above.
• Crystals on the bottom: fine to eat; a density defect.
• Cloudy, or sediment on the bottom: fine for home use. Cornell calls sugar sand harmless, though the 2006 manual says it can upset digestion in quantity.
• Opened and kept in the fridge or freezer: fine. Syrup never freezes solid.
• Mold, foam, a bulging lid, or a fermented smell: discard. No current source gives a salvage method for either.