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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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4 Answers 4

up vote 6 down vote accepted

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 '13 at 15:45
    
@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. –  sourd'oh Oct 16 '13 at 16:21
    
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 '13 at 0:37
    
@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. –  sourd'oh Oct 23 '13 at 15:37

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 at 9:15

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 '13 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. –  Blessed Geek Oct 17 '13 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". –  Blessed Geek Oct 17 '13 at 7:23
    
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 '13 at 15:14
    
I suppose if your syrup is that thick, then sure. I'm operating on the assumption that that the syrups used in cooking would be roughly as thin as those used in mixology. Sounds like that's not the case, which explains a lot! –  wastubbs Oct 17 '13 at 15:37

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? –  sourd'oh Oct 19 '13 at 18:46

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