Can syrup go bad? (+3 types of syrups)

In this article, we will answer the question “Can syrup go bad?”, and how long does syrup last?

Can syrup go bad?

Yes, the syrup can go bad. Although manufacturers allege that unopened syrup lasts indefinitely, labels come with a best-before date. 

This implies that after the indicated period, the syrup´s quality will continuously decline over time.

Transformations in quality include yellowing or darkening of the product, caused by chemical reactions involving sugars; sugar crystal formation, which is perceived as sand in the mouth; and aroma deterioration [1, 2]. 

Although these changes cause no harm to health, they may impact the sensory quality and the outputs of recipes where the syrup is used. 

Before opening, the syrup is stable to microbial spoilage due to its high sugar concentration which makes the environment hostile for them by lowering the water activity of the product.

Water activity (Aw) is a technical concept that expresses the amount of free water available in foods for microorganism growth, seeing that part of the total measured water can be bounded to other food components. 

Aw in foods varies between 0 and 1, the closer to 1, the higher the amount of water available, and the more easily the food spoils. 

The Aw of maple syrup lies between 0.84 and 0.9 [3]. Other syrups like corn and molasses have Aw of about 0.72 [3].

Fungi and bacteria require different Aw to grow. While spoilage and harmful bacteria need large amounts of free water (usually Aw>0.9), fungi start developing from Aw= 0.72. 

In other words, it is unlikely that syrup will go bad due to bacteria, but some fungi may be able to grow in it. Recent research reported maple syrup contaminated with several fungi species [4].

But calm down! spoilage by fungi becomes an issue majorly after opening – when the product enters into contact with air and external contamination – and depending on the storage conditions.     

How long does syrup last?

Before opening, the syrup can last indefinitely. However, manufacturers mark best-before dates as around 6 to 48 months, depending on the package type.

According to a producer [5], maple syrup can last unopened for 4 years if packed in glass containers and 2 years in plastic jars.

Following this date, the product´s quality starts to drop, but safety is generally not affected if good storing practices are used because the low Aw of syrup inhibits the growth of microorganisms. 

Can I get sick from consuming syrup past its best-before date?

No, it is very unlikely that you get sick from eating expired syrup because dangerous bacteria cannot grow in food products with water activity lower than 0.9, with rare exceptions.

If your syrup has been properly stored, for example, by placing it in the refrigerator in airtight containers, you will rarely have any safety issues past the best-before date, only quality faults. 

But caution. If fungi are present, discard the syrup immediately. In any hypothesis remove the mold layer and consume the remaining portion, or boil it hoping to get rid of contamination. With fungi, it is not as easy as this.

Although fungi do not cause immediate harm like some pathogenic bacteria, several species can produce toxic compounds called mycotoxins, which are said as carcinogenic [6].

The point is: if mycotoxins were produced, you do not have how to know it. And more: they are cumulative (kept stored in your body) and heat-resistant, so boiling the syrup to get rid of mycotoxins is useless. 

Last but not least, some molds are especially resistant to heat and can survive even after boiling, being able to develop again when the syrup cools down.   

Although a recent study has observed the presence of mold species in maple syrup [3], the presence of mycotoxins has not yet been confirmed.

While science pursues this answer, I strongly recommend you not take the risk and eliminate moldy syrup.  

 What factors affect the shelf life of syrup?

Storage conditions after opening are the main factors influencing the shelf life of syrup.

Although syrup is considered microbiologically stable, in case after opening it is stored in high-moisture ambients unprotected from contact with air, it can absorb water. 

As the water activity of maple syrup ranges between 0.84-0.9, while that of other syrup types is about 0.7, any further water gain or contact with contamination sources promotes microorganism growth.

Fungi predominate [3]. Remember that fungi grow in water activities as low as 0.72, while no progress of bacteria is observed below 0.9. 

Mild heating can also be deleterious for syrup because it favors darkening reactions [2], and may also encourage fungi development. Thus, storage in a cool place is important for extending quality.

Storage in the refrigerator after opening is strongly recommended by producers of maple syrup due to its slightly higher water activity  (about 0.84-0.9) compared with that of other syrups (0.63-0.72), which makes maple syrup more prone to spoilage. 

For other types of syrup, like corn or molasses, the refrigerator is recommended as an alternative store. 

How to tell If your syrup has spoiled?

You can tell that syrup has gone if you see mold floating over it. Due to its low water activity, fungi are the main type of microorganisms that can develop in syrup. Bacteria will rarely grow in such a hostile environment.

This syrup should be discarded.

Off-flavors should also account for when evaluating whether the syrup is spoiled or not. In maple syrup, musty or fermented flavors may reveal fungi contamination and should also be thrown away [2].

Types of syrup 


Molasses is the liquid that remains after sugar crystallizes from sugar cane juice. The light-colored and flavored molasses is used in baking, candy-making, and rum production. 


Honey is collected from beehives. The flavor varies with the flower source. It is used in beverage making and baking.

Corn syrup 

It is the most widely used sweetener in processed products like soft drinks, ketchup, and ice cream. This syrup is sweeter and more economical than sucrose.


In this article, we answered the question “Can syrup go bad?”, and how long does syrup last?


1. Bostan A, Boyacioğlu D. Kinetics of non-enzymatic colour development in glucose syrups during storage. Food Chemistry. 1997;60(4):581-5.


3. Schmidt SJ, Fontana Jr. AJ. E: Water Activity Values of Select Food Ingredients and Products.  Water Activity in Foods. 2020. p. 573-91.

4. Frasz SL, Miller JD. Fungi in Ontario maple syrup & some factors that determine the presence of mold damage. International Journal of Food Microbiology. 2015;207:66-70.


6. X. Zhang, L. Zhang, T. Zhou, Y. Zhou, Fungal flora and mycotoxin contamination in tea: Current status, detection methods and dietary risk assessment – A comprehensive review, Trends in Food Science & Technology, 127 (2022) 207-220.