Moldy maple syrup?

Why does syrup get moldy?

Maple syrup is typically pretty resistant to molds, which need water, oxygen and nutrients to grow. While syrup appears packed with water, the water is bound up with sugar and therefore hard for microbes to access. Not many molds can grow in syrup, but those that can are deemed xerophiles because they’ve adapted to grow in harsh conditions. In fact, several molds that have been documented in maple syrup are also common in another low-water environment: house dust, according to David Miller, a research professor at Carleton University who has studied maple syrup mold.

Often, when mold grows in syrup it’s due to a production error. The syrup’s water content may be too high, opening the door to mold growth. Spores could have landed in the syrup or jug during processing and bottling.

Can you just strain out the mold and boil the syrup?

Straining and boiling moldy syrup used to be common advice, but it’s not enough to undo the damage wrought by the mold, especially if your syrup looks anything like the examples in this post. 

First, some reasons why you don’t want to eat unidentified mold in any scenario:

• Airborne mold spores can cause allergic reactions if sensitive folks inhale them. 
• Molds “digest” the substances they grow on and release an array of compounds in the process; they can make food taste bad.
• Some molds produce toxins we don’t want to consume.

If you were to simply strain out the mold, some of its enzymes, spores and other components could easily slip through your strainer and stay in the syrup. And boiling in a home kitchen is typically insufficient because it takes intense heat and pressure to be confident you’ve killed spores, Miller explained. Plus, by the time you see the mold, it’s already diminished the sugar content in your syrup.

Boiling at home, Miller said, “won’t undo the damage to the product that the mold has done.”

What does maple syrup mold look like?

Catherine Belisle, a research support specialist at the Cornell Maple Program, told me that maple syrup producers have reported a “rainbow of colors,” but she most often sees white to tan molds, like in the photos here. The mold, which could be any of a number of species, can form in little circles or squidgy masses. It can spread into a dusty-look sheet across the surface of your syrup. It typically floats on the surface because it needs access to oxygen in the air to grow. 

In 2015, Miller and a team of researchers identified 23 species of fungi when they examined syrup from 68 producers in Ontario. The molds they found were colors including greenish yellow and brown shades of gray. 

The molds detected were known for inhabiting dry or very sweet surfaces. Many were quite common, including some known for growing on sweet foods like cakes and jams. But one was a particularly exciting find for the scientists as it had only ever previously been documented on sugarcane in Hawaii, Miller said.

I shared the images in this post with Miller. While he noted that it’s impossible to identify a mold from a photo alone, he said, “one of them appears to be the maple syrup mold first recognized more than 100 years ago, Wallemia. This fungus grows on high sugar foods all over the world.”

If you find something floating atop your syrup is there a chance it’s not mold?

It’s probably mold. There aren’t any other common syrup imperfections that would result in a mass floating in your syrup.

Can mold form at the bottom of a bottle of syrup?

You’re unlikely to find fungi settled at the bottom of the bottle, Belisle said, given their need for oxygen to grow. Syrup can have other imperfections, like “sugar sand,” which is naturally occurring minerals and organic acids that typically sink to the bottom. Syrup can also be contaminated by a sandy substance used to filter it, but that will also settle at the base of the jug.  

How does syrup get contaminated with mold?

Maple sap is basically sterile while it’s inside the tree. After properly boiling the sap into syrup it should be pretty clean.  

“Like with bottling anything from jam to beer, the filling and bottling process is where there can be contamination” Miller said. 

Miller explained that syrup bottles and caps are typically shipped separately. If bottles aren’t carefully stored, especially if they’re saved from one season to the next, airborne mold spores could settle in.

And, if the syrup’s sugar isn’t concentrated enough, the spores can more easily grow into a mat of mold (the FDA requires a minimum sugar content for maple syrup). Miller’s research found that some syrup makers were poorly calibrating their syrup’s sweetness because they got inaccurate readings from sugar-measuring tools. 

“We found that some of the instruments that farmers use to determine sugar content were inaccurate sometimes because they were old and had been bumped around,” he said. “Too low sugar content means more mold species can grow more quickly.”

Belisle noted that packing the syrup when it isn’t hot enough and not properly sealing the bottles can make mold growth more likely, as can poor cleaning and maintenance of the “sugar shack” where syrup is made. Producers can also guard against microbial growth by holding their bottles upside down soon after filling them with hot syrup. This ensures that the hot syrup touches–and helps sanitize–all the inside surfaces of the bottle. The hot syrup eventually cools and contracts, creating an anaerobic vacuum, where mold cannot grow, according to Ontario’s Ministry of Agriculture, Food and Rural Affairs.

That’s why unopened syrup can be stored at room temperature, but opened syrup should be stashed in the fridge, or even the freezer. 

Since his 2015 research, Miller said major maple syrup producers in the U.S. and Canada have adopted a number of best practices, like more accurate sugar measurement and hot packing syrup, which make mold growth less likely. If he were to repeat the study today, he predicted he’d find many fewer instances of mold. 

“My long experience is that farmers really care about their product, including the many maple syrup producers I have met,” he said.

But, he added, smaller producers might not be up on best practices. Because mold typically shows up until weeks or months after bottling, a syrup maker may not know they have a problem. If you do happen to discover mold on syrup, tell the producer! You could help them solve a problem they didn’t know they had. 

Can maple syrup be contaminated with mold in your kitchen?

Miller said mold contamination is most likely during production. But you should still take care to prevent it at home. 

Ann Charles Vegdahl, an extension associate and microbiologist at the Food Research Lab at Cornell University’s AgriTech program, noted that, “Every time the maple container is opened for use, it is exposed to microbes in the air, on the consumer’s hands. This can potentially contaminate the maple syrup and cause mold to grow.”

A properly produced and packaged bottle of maple syrup should theoretically be microbiologically fine indefinitely, Vegdahl said. However, chemical changes that lower the syrup’s quality could eventually occur. 

For more on shelf life check out our Behind The Date Label articles. 

What’s the best way to store maple syrup? Should you refrigerate maple syrup?

To keep your syrup in tiptop shape, store it in the fridge or freezer after you open it. Refrigeration temperatures slow microbial growth; but the freezer can buy you even more time to use your syrup; and it won’t even freeze solid. No matter where you store it, keep the lid tightly sealed. That way you’ll reduce the odds of airborne spores landing inside.

Miller also suggests buying reasonably sized bottles so it doesn’t sit around for a while. 

“Don’t buy five gallons of maple syrup at once,” he said.

How to avoid moldy maple syrup:

• Buy from producers who follow best practices. If you aren’t sure – ask!
• Cap your syrup when you’re not using it.
• Once the seal is broken, store syrup in the fridge or freezer.
• Buy small amounts of maple syrup so you don’t have to store it for extended periods.

Before we go, check out this video of some extra squidgy maple syrup mold from Caroline, the EatOrToss reader who supplied these photos. Thanks Caroline!