I've actually been under the impression that it does not need to be refrigerated, and I have successfully left cakes frosted with cream cheese out for a day. What I read recently, and I can't recall where, is that the sugar in frosting actually prevents it from going bad outside of the fridge. Apparently bacteria does not like sugar. I hope someone corrects me if I am wrong. In either event, if you choose to refrigerate, make sure you take it out with enough time for the frosting to come to room temp and soften.
You may have read that right here as I posted it recently, but I must have been a little sloppy with my comments if I left you thinking bacteria don't like sugar -- quite the opposite. But bacteria can't survive in an environment with a high enough concentration of sugar (think "jam").
Cream cheese frostings made with at least 4 cups of sugar (roughly a pound) per 8 ounces of commercial cream cheese are considered safe to leave unrefrigerated.
I never refrigerate mine... But I was unaware of the whole "bacteria" thing as well.
Cynthia is a trusted source on Bread/Baking.
When the sugar in cream cheese icing binds up free water, it is more difficult for bacteria to grow, since one of their requirements is moisture. I've not had a problem leaving things iced with cream cheese icing at room temp for up to 8 hours. If left out longer than that, the dairy portion will begin to sour.
Dairy products like milk and cream are pasteurized as opposed to sterilized. Pasteurization is designed to kill harmful bacteria but others, like lactobacillus, survive. At warm temperatures, lactobacillus readily converts lactose into lactic acid, souring the product.
While it's true bacteria need water to replicate, there's plenty of water in maple syrup yet it is well preserved from bacteria. When the concentration of sugar is high enough, water is drawn through bacteria's cellular walls by osmosis, desiccating it. The key being a sufficient quantity of sugar.
It's also recommended on the label that maple syrup be refrigerated after opening ;)
True, but that's to prevent mold -- an entirely different animal. So to speak.
I must have read it here then. Thanks for clarifying! I'm not sure I totally understand your point to boulangere about souring. Is this or is this not a concern with frosting left outside?
Yes, souring is a concern. Enough so that boulangere refrigerates her cream cheese icing. Maybe she's being extra cautious or maybe her recipe(s) don't include sufficient sugar to preclude bacterial growth, not all do. I am certainly not challenging her expertise nor her experience.
We know for a fact that sugar, in sufficiently high concentrations, will kill bacteria. The foodborne illnesses we're discussing and dairy souring are both caused by bacteria.
Remember that pathogenic bacteria aren't detectable by taste or smell. While food that smells fine isn't necessarily safe, food that smells bad is always suspect -- where there's smoke there very well could be fire.
Hypothesis: If a cream cheese frosting formula is prone to souring, it probably doesn't contain enough sugar to kill the nasty bacteria either and therefore should be refrigerated.
It's possible that lactobacillus has a different kill point than pathogenic bacteria. But the ratios I learned came from food safety experts who oversee county fairs where food sits out in warm buildings for countless hours and is then sampled by judges. Neither type of bacteria would be acceptable; souring would cause the cake to lose the contest, campy would cause the judges to lose it.
Experiment: Take two samples of commercial cream cheese, adding powdered sugar to one until frosting consistency and leaving the other plain. Place in glass prep bowls and cover. Allow to stand at RT for one week, checking daily for off odors.
Observations: After 24 hours at 65-70F, plain cream cheese soured markedly. Sample was discarded before it stunk up the entire kitchen. Sugar mixture remained sweet both on the nose and on the tongue, despite temperatures > 80F at times for 7 days whereupon the test was terminated.
Conclusion: Sufficiently high concentrations of sugar will preserve cream cheese. Please note I did not attempt to quantify the amount of sugar necessary to prevent souring.
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