Preventing mould in hot-packed maple syrup

New production practices that will prevent spoilage of maple syrup by mould are emerging as industry researchers identify the species of fungi and the conditions that enable mould to grow in hot-packed syrup.

The hot-pack temperature of 82°C has been determined to be an ideal temperature for syrup; however, the time this temperature is maintained following filling and capping may not be sufficient to kill mould spores that can be present in containers.

Pre-heating containers prior to filling

Although glass, plastic, tin containers and caps that are utilized for packing maple syrup are made of new food grade materials and arrive clean to the producer, the surfaces are not sterile. Inner and outer surfaces can be coated with spores of spoilage organisms to varying degrees, regardless of how they are stored.

Also, small to medium volume glass containers can present a particular challenge for mould growth. New designer containers are often made with glass that is thicker than standard containers. If not preheated, these containers can cool the temperature of hot syrup within seconds, down to temperatures that are no longer suitable to kill spores that may be present inside bottles and cap liners.

We know that the hot-pack temperature of 82°C will kill spoilage spores, however, ensuring complete spore destruction is time-dependent. Food processing scientists define thermal death time (TDT) as a combination of heating time and temperature that kills various spore concentrations. Two examples of TDT treatments used in food processing are pasteurization and sterilization.

Pasteurization involves application of specific combination of time and temperature to kill pathogenic bacteria in a given food; however, it does not kill all spores that may be present in the food. Sterilization requires a higher temperature and heating time to kill all spores present in food; however, it could be considered too harsh a treatment for packing maple syrup.

New designs in glass bottles have thick walls to support their unique shapes

Figure 1. New designs in glass bottles have thick walls to support their unique shapes

Recommended methods

Thermal equilibrium of thick glass containers and hot maple syrup can be maintained at high spore-killing temperatures for three to five minutes, where containers are preheated to 90 - 100°C and syrup is filled at 82°C.

A research team at the University of Maine found that preheating glass containers prior to hot-packing syrup maintained the syrup temperature of sealed containers above 70°C for 3 to 5 minutes, effectively killing mould spores. They achieved this by either boiling containers in water for five minutes; or heating containers in a dry oven at 150°C for 30 minutes. Both preheating methods also sterilized the glass surfaces. Where glass containers were not preheated prior to hot-packing syrup, the syrup temperature fell below 70°C within a few seconds as the cool glass quickly absorbed heat from the syrup.

Invert hot packed containers to kill mould spores inside caps

Figure 2. Invert hot packed containers to kill mould spores inside caps

Heating sealed containers following hot-packing

Another method to extend the duration of the heat treatment is to immerse sealed hot containers in a hot water bath. This method adopts some of the principles behind home canning or thermal processing of pickles, vegetables or sauces after containers are filled and sealed.

One maple syrup producer reported placing sealed glass containers into a hot water bath for ten minutes where the water temperature was maintained at the same recommended hot-pack temperature of 82°C. The temperature of the hot water bath should not exceed 85°C, as warmer temperatures can cause sugar sand to precipitate out and cloud the bottled syrup.

Plastic caps are not intended to be immersed in boiling water and could deform or crack, breaking the tight seal. The use of a hot water bath to extend the duration of the thermal treatment can ensure that any spores that were present inside sealed containers are killed.

Following best practices

Proper cleaning and sanitation practices of food handling areas is necessary to reduce mould spores on equipment surfaces and in the air that will cause contamination of syrup. Even the most thorough sanitizing efforts do not guarantee that spores of spoilage organisms are eliminated from surfaces or the production environment.
Achieving a minimum density of 66 °Brix in finished maple syrup will reduce the amount of water that remains available in syrup for microbial activity. Researchers at Cornell University recommend a higher minimum density of 67°Brix to give better quality syrup and less chance for spoilage organisms to survive in hot-packed syrup.

Research to fine-tune the required duration of a heat treatment tailored specifically to maple syrup and to different container sizes is showing positive results to prevent mould in syrup. Further refinements of thermal treatment of maple syrup for a variety of container shapes and sizes will be of benefit to the industry for ensuring high-quality product.


Identification of Fungal Contamination of Bottled Maple Syrup, 2014. Rafael Garcia, Beth Calder, Kathryn Hopkins and Seanna Annis. University of Maine, Research and Cooperative Extension.

Understanding food science and technology, 2003. Peter S Murano. Texas A & M University.

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