I was reading a post earlier regarding maple syrup being left out overnight. They were asking if their beloved Maple syrup was still safe to consume. In the comments I believe someone brought up this question I am asking, I'm sure I could do the research and find out but hey whats the fun in that. So here's my question does the sugar in Maple syrup or any product for that matter inhibit the growth of mold.

Also if its true, how much sugar is needed percentage wise?

Does it matter what type of sugar?


4 Answers 4



Yes. Sugar does inhibit growth of mold.

How does this work?

Several reasons:

  • Sugar prohibits growth through osmosis / dehydration.

"The most notable is simple osmosis, or dehydration. Salt or sugar, whether in solid or aqueous form, attempts to reach equilibrium with the salt or sugar content of the food product with which it is in contact."

Source: http://www.scientificamerican.com/article/how-do-salt-and-sugar-pre/

  • Sugar weakens the molecular structure of pathogens' DNA

Salt and sugar's other antimicrobial mechanisms include interference with a microbe's enzyme activity and weakening the molecular structure of its DNA. Sugar may also provide an indirect form of preservation by serving to accelerate accumulation of antimicrobial compounds from the growth of certain other organisms.

Source: http://www.scientificamerican.com/article/how-do-salt-and-sugar-pre/

  • Do these articles talk about sugar without the presence of NaCl a.k.a. salt inhibiting mold growth? From my understanding osmosis only works with both present, but I could be wrong.
    – Chef_Code
    Commented Apr 13, 2015 at 21:50
  • 2
    @Chef_Code osmosis only requires a concentration gradient between two sides separated by a semipermeable membrane (such as a cell wall studded with ion exchange channels). It doesn't matter much what is dissolved, as long as it cannot pass through the membrane freely. Pure sugar, pure salt, salt with sugar, they all work.
    – rumtscho
    Commented Apr 13, 2015 at 22:12
  • @rumtscho oh nice to know, thanks. Just when I thought I had it all figured out.
    – Chef_Code
    Commented Apr 13, 2015 at 22:26

Water activity is the big issue in preventing microorganism growth in sugary solutions.

Water Activity of Foods Table     Includes limit points for various types of bugs. A few molds will grow down to 0.60Aw.

Another foods Aw table

Water Activity of Sucrose and NaCl Solutions     From which: 180 g sucrose + 100 g water will give you a 64% sucrose solution. That'll have a water activity of 0.87, which is low enough to prevent proliferation of most bacteria, but not molds or quite, yeasts.

Oh yes, the question: A water activity below 0.8 will prevent growth of most molds, so at room temp. your syrup (Aw ~.85) will likely eventually pick up some mold, but not much in the way of bacteria.


I can try to answer that question in terms of jam (I have a glass of strawberry jam in front of me right now).

The sugar content is about 50% by weight, so 50g of sugar in 100g of strawberry jam. The label claims there are no other preserving agents in the product and that you may store it for a long time (about a year or more) in the refrigerator.

The sugar is supposed to bind the available water, which helps to inhibit the growth of mold. I guess the osmotic effect will also drain water from the cells of the mold. It is important to keep other impurities out of the jar (like breadcrumbs), since mold might start to grow THERE and spread to the remainder of the jar eventually.

I know people who successfully store jam non-refrigerated for many months without any problems with mold.

For the type of sugar, I believe there are differences (fructose is said to react a lot faster with protein, for example) but I don't know how they affect the growth of mold. My glass of jam contains regular white, refined glucose.


Heavy Sugar syrups (including maple) are dessicants. They inhibit bacterial growth and many molds, there are however molds that grow on sugar syrups.

Those sugar syrup molds however take weeks or months to develop and they'll float right on top.

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