using ethanol to stop mold growth

[Shalmanese]

After accidentally leaving a bottle of elderflower syrup out and having mold grow on the top of it, I was wondering if there's any reference for what proportions of sugar, water & ethanol prevent mold growth. I tried doing a quick google search but didn't find anything that seemed relevant.

[vice]

I can't recall having seen any extensive testing, but I've found 2:1 syrups to be quite stable at room temperature.

[Cim Ryan]

According to Chocolates and Confections by Greweling, at less than 60% free water, no microbial growth occurs.

[bostonapothecary]

the minimum for keeping a mixture of sugar, water and alcohol preserved i'm pretty sure are the ratios found in vermouths. 15.5% alcohol can hinder acetic acid forming bacteria and 18% alcohol can inhibit the growth of lactobacteria. apparently increasing sugar to around 170g/l, like in a sweet vermouth, allows you do reduce the amount of alcohol to 16% or so.

i had thought i've seen mold or bacteria at higher percentages but what i've really observed has been precipitated pectin that has floated to the top.

without being frighteningly sweet, a syrup would definitely have to turn into a full on liqueur to be preserved.

[slkinsey]

Dave Wondrich has reported that as little as a half-ounce or so of 100 proof vodka in a bottle of simple syrup stored at room temperature has been enough to prevent the mold growth he had otherwise been getting. Honestly, I don't quite understand how or why this works, but apparently it does.

In my own experience, a highly saturated syrup has stored just fine in an unopened liter bottle (with occasional pours out into the service bottle). This was probably a 3:1 syrup with gum arabic to prevent recrystallization, and did include around an ounce of 100 prof vodka on top.

[Shalmanese]

the minimum for keeping a mixture of sugar, water and alcohol preserved i'm pretty sure are the ratios found in vermouths. 15.5% alcohol can hinder acetic acid forming bacteria and 18% alcohol can inhibit the growth of lactobacteria. apparently increasing sugar to around 170g/l, like in a sweet vermouth, allows you do reduce the amount of alcohol to 16% or so.

I've never known wine to grow mold or, for that matter beer. Then again, I haven't ever left beer unopened for a few months. Can anyone report as to whether beer does indeed get moldy?

[mkayahara]

the minimum for keeping a mixture of sugar, water and alcohol preserved i'm pretty sure are the ratios found in vermouths. 15.5% alcohol can hinder acetic acid forming bacteria and 18% alcohol can inhibit the growth of lactobacteria. apparently increasing sugar to around 170g/l, like in a sweet vermouth, allows you do reduce the amount of alcohol to 16% or so.

I've never known wine to grow mold or, for that matter beer. Then again, I haven't ever left beer unopened for a few months. Can anyone report as to whether beer does indeed get moldy?

Oh yes, beer will get moldy. As anyone has learned who's ever found some empties that weren't rinsed out.