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I'm curious about plain simple syrup and simple syrup with other ingredients. For instance, cardamon simply syrup. Do other ingredients make a difference?

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  • I'll point out that the solubility of sugar depends on the temperature of the solution. So sugar could be more concentrated at room temperature than in fridge.
    – MaxW
    Oct 27, 2015 at 4:16

6 Answers 6

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The key factor in syrup's shelf life is the water activity in the syrup, rather than the ingredients used to make it. Generally, the water is all 'bound up' with dissolved sugar so microorganisms can't use it to grow, but the lighter the syrup, the more available water it will have.

In my experience, simple syrup is usually kept refrigerated except for small portions that will be used within a day or two.

For a chart of water activity (aW) of related foods, check here: Water Activity Table

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    Sorry, Sour, your outcome is correct but the reason is not. Sugar is dissolved in water, but it doesn't bind up the water in any way. The preservative quality of high concentration syrups is due to the desiccation of pathogens via osmosis.
    – SAJ14SAJ
    Oct 16, 2013 at 15:45
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    @SAJ14SAJ I didn't figure an in depth discussion of water activity was warranted in this answer, so I added quotes to clearly indicate my use of metaphor. The water is still unavailable to microorganisms, whether by osmotic pressure or whatever other function.
    – SourDoh
    Oct 16, 2013 at 16:21
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    FWIW, I've had a simple syrup that got moldy after a few weeks in fridge; I guess it may have been on the lighter side of syrups. In regards to original question, do other ingredients make difference?
    – Dolan Antenucci
    Oct 23, 2013 at 0:37
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    @dolan The other ingredients shouldn't make much of a difference, unless one of them has a really strong antimicrobial effect, but I can't think of anything you'd put in a syrup that would be strong enough to do that.
    – SourDoh
    Oct 23, 2013 at 15:37
  • I have to admit that the Water Activity Table link was completely useless to me, I'm not up to speed on my Osmophilic yeasts. But now I am curious. + 1
    – John C
    Dec 17, 2016 at 8:50
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I've done a lot of reading on this subject - as well as quite a bit of my own experimenting - and this is what I've concluded:

A highly concentrated simple syrup produced in a sterile environment and stored in sterile containers (with sterile caps) has a shelf life of at least a month as long as the containers remain unopened. I recommend glass bottles with phenolic or otherwise lined caps.

I use a 2 to 1 ratio (2 sugar, 1 water) and simmer my solution for at least 15 mins to reduce it and to allow my other ingredients to absorb. I primarily use whole vanilla beans and various spices.

I do recommend refrigeration after the bottles have been opened to prevent any microbes from sneaking in.

Use best kitchen practices and keep everything clean and your syrups will likely be fine.

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Adding a tablespoon of vodka/cup will extend the life of the syrup significantly.

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    I will give you the benefit of doubt and not downvote, but the answer does not pass my gut test. There is too few alcohol in this case to preserve anything, all microorganisms will continue to multiply merrily. You usually have to reach about 5% alcohol to start seriously killing bugs (that's why most fermented alcohols including beer are at this concentration, it happens when the alcohol in the drink kills the organisms which have been fermenting it).
    – rumtscho ♦
    Jan 26, 2014 at 9:15
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There's a few things to consider, but let's start with a general term to encapsulate the safety issue, call it "bacterial potential". That is, how numerous would bacteria be in the solution before it's stored, and how much sugar is available to fuel whatever bacteria are present.

I start off a basic simple syrups with a rolling boil to kill the bacteria and make the sugar dissolve quicker. This method generally results in a syrup that's shelf stable for at least a month, so long as the bottle in which it's stored is sanitized ahead of time. Very low "bacterial potential".

I haven't experimented much with ingredients beyond sugar & water, but depending on the additional ingredients, I would strongly consider forgoing the boil. You can create a simple syrup at room temperature. Here's a good article from SeriousDrinks that outlines the process. The "bacterial potential" when you haven't boiled the solution is going to be higher, and unless you had a very good reason not to, I would refrigerate the syrup and discard it after no more than a month.

It's called 'simple' for a reason - there's not much stopping you from whipping some up on-demand.

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  • Bacterial potential sounds like made up pseudo-science.
    – SAJ14SAJ
    Oct 16, 2013 at 16:54
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    I'm am sorry to say that I don't think you are aware of what you are criticizing when you say "pseudoscience". This person exhibits an engineering approach to the issue. Engineers start off by measuring/estimating a potential and then mitigating that potential. 30 years ago, when engineering and analytical methods took a foothold at economics, we were accused of "pseudoscience". Today you probably can't get an economics nobel without engaging is such analytical attitudes.
    – Cynthia
    Oct 17, 2013 at 7:17
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    Similarly, when engineering methodologies and design-of-experiments began creeping into pharmaceutical research. We were accused of "pseudoscience". Today, you will never get your drug approved without what you purport as "pseudoscience".
    – Cynthia
    Oct 17, 2013 at 7:23
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    While your answer generally isn't wrong, in this case it isn't right either. The main thing preserving Syrup is the high sugar and low water content. For that very reason honey for instance has a long shelf life despite never getting sanitized (if it's good honey) in any way. I believe the threshold is somewhere around 18% water content. The same applies to most syrups with a water content in that area. Any bacteria will simply die because the water gets "sucked out of them" by the sugar.
    – Anpan
    Oct 17, 2013 at 15:14
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    The reason why this isn't good science is that basing it solely on "bacterial potential" and available sugar is highly inaccurate. If I dumped sugar on the floor and then measured the bacteria on it over time, it would most likely go down over time as bacteria requires both nutrients and water to thrive. I actually have to do shelf life studies on products, and I can tell you that bacterial counts on products with low water availability will usually drop over time.
    – SourDoh
    Oct 17, 2013 at 17:00
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Wow! A simple question about simple syrup and the nerds go nuts. Put it in the fridge and throw it away when it looks funny or smells funny. Simple.

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Simple syrup should be used immediately. That which is left over should be poured down the drain. Storing it, even by refrigeration, is asking for trouble. If you don't use that much of it, or don't use it very often; may I suggest that you purchase a bottle of agave syrup and use it instead, storing it in accordance with the instructions on the bottle?

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    If agave nectar is of similar concentration to your syrup, why would it be any less hazardous than the simple syrup?
    – SourDoh
    Oct 19, 2013 at 18:46

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