Anyone concocting soda flavors at home will sooner or later run into the problem of storing the syrups they make and buy. One solution is to refrigerate, but ever-multiplying syrup bottles fighting for valuable fridge space will eventually lose the battle and get booted.
Cheaper syrup brands typically contain preservatives like sodium benzoate to maintain shelf stability (Torani, Davinci), so there's no reason to worry about these. Higher quality syrup brands contain only sugar and sometimes acidulents for preservation (Monin, Giffard, craft brands), but oddly they are not always labelled with 'refrigerate after opening'.
I store most of my syrups at room temperature, so inevitably some show mold growth a few months after opening. It's just surface bloom but it's not the most appetizing look.
To dig further into how to preserve syrup longer at room temperature I wanted to understand how pH and Brix play a role. What these two factors do in combination is decrease the water activity of the solution, and lower water activity is a direct indication of how well spoilage organisms will be inhibited in all foods.
This paper gives a formula for Aw (water activity) as tested against common sugar+acid containing foods:
Aw(25C)=[0.95+0.03(pH)+1.02x1E-3(Brix)+5.21E-4(pH*Brix)-3.95E-3(pH^2)-1.07E-4(Brix^2)]^1/2
They recommend Aw<0.85 to be in the safe range, but for syrups Aw<0.90 can be shelf stable. At an Aw of 0.90 there is no bacterial activity, and the yeast Saccharomyces cerevisiae is effectively halted.
To put this into practice, say you are starting with a sour fruit juice with pH of 3.0. If you add sugar in the range of 56-65% by weight you will be in the Aw=0.90-0.85 range. For a neutral simple syrup with a pH of 7, you need to be above 66deg Brix for Aw=0.90. That's simply a 2:1 simple syrup.
Both pH meters and Brix refractometers are under $15 online, and they are great tools for any home brewer/chef/bartender.
Cheaper syrup brands typically contain preservatives like sodium benzoate to maintain shelf stability (Torani, Davinci), so there's no reason to worry about these. Higher quality syrup brands contain only sugar and sometimes acidulents for preservation (Monin, Giffard, craft brands), but oddly they are not always labelled with 'refrigerate after opening'.
I store most of my syrups at room temperature, so inevitably some show mold growth a few months after opening. It's just surface bloom but it's not the most appetizing look.
To dig further into how to preserve syrup longer at room temperature I wanted to understand how pH and Brix play a role. What these two factors do in combination is decrease the water activity of the solution, and lower water activity is a direct indication of how well spoilage organisms will be inhibited in all foods.
This paper gives a formula for Aw (water activity) as tested against common sugar+acid containing foods:
Aw(25C)=[0.95+0.03(pH)+1.02x1E-3(Brix)+5.21E-4(pH*Brix)-3.95E-3(pH^2)-1.07E-4(Brix^2)]^1/2
They recommend Aw<0.85 to be in the safe range, but for syrups Aw<0.90 can be shelf stable. At an Aw of 0.90 there is no bacterial activity, and the yeast Saccharomyces cerevisiae is effectively halted.
To put this into practice, say you are starting with a sour fruit juice with pH of 3.0. If you add sugar in the range of 56-65% by weight you will be in the Aw=0.90-0.85 range. For a neutral simple syrup with a pH of 7, you need to be above 66deg Brix for Aw=0.90. That's simply a 2:1 simple syrup.
Both pH meters and Brix refractometers are under $15 online, and they are great tools for any home brewer/chef/bartender.
