Everywhere people are concerned over whether or not certain things will spoil if left out since cakes are popular for weddings, etc. . .
But why can some things that spoil at room temperature be used in certain recipes and not in others? (i.e. why can't you use cream in a mousse filling but it's okay for ganache?)
What are the ratios to "spoiling ingredient" that are used to prevent it from spoiling?
I remember my graduation cake for 6th grade has a Belgian Chocolate Mousse, why was it fine even when it was 70 degree weather for three hours?
I'm curious too. I don't add milk to buttercream because I worry about spoiling, but then again I don't worry about leaving plain cake out even though it has egg in it (albeit, cooked egg) I really worry about this, but know that in many places the things we refrigerate are actually left out and never refrigerated at all. I bet t here is SOMETHING that has general rules in it out there!
Milk in icing is okay at cool room temp for about three days. All of the sugar in the icing acts as a preservative.
In France, eggs are never refrigerated. Here in my house, if an egg sits out for 6 hours or more, I throw it away. Kind of an ingrained habit, I s'pose.
Lots of people are wiggy about salted butter being left out at room temp, but that's the only way I use butter (one stick at a time, haha, not boxes of butter sitting on my countertop). Butter bells are making quite a comeback!
I'd like to know the magic ratios, too.
Have you ever noticed that boxes of eggs in supermarkets are just displayed on shelves, not refrigerated. They must be fine like that until the 'use by' date, otherwise they wouldn't be able to sell them like that. BUT, once you refrigerate them, you have to keep them in the fridge (aside from taking them out, so they can come to room temp shortly before you bake with them!) otherwise the eggs can develop salmanella (apparently something to do with the increase in temperature, so my DH tells me - he used to manage a supermarket a few years back!)
Kelly
Eggs are actually fine to sit out at room temp. Salmonella (sp?) is actually only on the egg shell, not in the egg. About 1 in 500,000 eggs will have salmonella on it. However the US gov. has made a guide line that all eggs should be refrigerated. This is because of that 1 egg in the 500,000. Refrigerating it will help slow down the process of the germ doubling itself every 20 minutes or so. That's why if you watch cooking shows they crack an egg on a plate instead of on the side of the bowl. It helps to keep small egg shell fragments from getting pushed up into the egg.
As far as ganache it has such a high content of milk that it needs to be in the frigerator, because the sugar content does not balance it out.
In buttercream there is little milk as compaired to sugar, as someone stated above, it can sit out at room temp (unless the temp. is high or humidity) for 3 days after that in to the frige it goes. I use water instead of milk just because I'd rather not deal with it.
As far as jam's from a jar or sleeve a lot of people don't refrigerate them unless out for 2 days. However, if the back of the jar says "once opened--refrigerate" Than it needs to be in the frigerator the whole time.
IT can get confusing I know--that's why when in doubt just put it in the frigerator.
when you ad heat to certain ingredients it changes the chemical structure of them. That's why it's important to take a "FOOD SCIENCE" or BAKE TECH class,
". Heat also kills pathogenic microorganisms like, salmonella.
4. COAGULATION OF PROTEINS:
When egg and gluten proteins are heated they coagulate. As they are heated and begin to coagulate, the molecules unfold and bond with one another."
My personal rule is that if it contains perishable ingredients, it is all considered perishable. Safer in cool weather, unsafe in hot weather.
IF I left something out, and something went wrong and someone got sick, the "scientific reasoning" won't save your busness and your reputation.
It is much better to err on the safe side than have to pick of the pieces afterwards.
I use dry coffee creamer in place of milk in my icing and always make icing day before. I always bake cakes day before they firm up and easier to handle 
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