Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
Jennifer asks via Twitter:
As we've discussed earlier when chatting buttercream, milk and milk products such as butter are emulsions. In general, an emulsion is a relatively stable combination of water and oil, perhaps with some other goodies such as proteins and sugars thrown in. In cream's case, the emulsion is helped along by the proteins, which wrap around the fat droplets to help them play nice in the water. Like M&Ms, the hard shell keeps the fat protected.
When you whip cream, what you're doing is pulling air bubbles into the cream while breaking the protein shell around the fat. When the fat is thus exposed, it will, like a celebrity on vacation exposed to the paparazzi, turn any way to avoid facing the water. Eventually the metaphorical celebrity will come in contact with another celebrity (more fat), or will find some open space where there's no paparazzi, which is the air bubble that you've introduced.
After enough whipping, and what you have is a mesh of celebrity fat globules all huddled together and connected, with some pockets of contained paparazzi water and other pockets of air. The protein and sugars are still around. The proteins you can think of as the bodyguards, while the sugars are, I dunno, attention-seeking friends who are desperate for media attention and, thus, will throw themselves in the middle of the paparazzi, which is the fancy way of saying that the sugar dissolves in the water.
Now that we've stretched this metaphor about as far as it will go (a lie, because I could go on with adding flavorings), let's see why whipped cream isn't all that stable. Because the heavy lifting in the foam is made from milk fat, we know that it's going to be temperature sensitive. Butter is much softer at room temperature than it is in the refrigerator, and that change in consistency carries over even moreso when it's not all clumped together but spread out in a foam.
What you need here is a stabilizer. Traditionally, what works best with whipped cream is gelatin. Let's say you are whipping 2 cups of cream. If you have a mass-market powdered, unflavored gelatin, you're looking to soak 1 teaspoon of gelatin in 2 tablespoons of water for 5 minutes. Then dissolve the gelatin in a double-boiler type setup, indirectly heating your gelatin with the steam coming off of a simmering pot of water. Let this cool to 98°F or thereabouts. Too cold, and it'll turn solid. Too hot, and it won't mix well with the cream.
Now start whipping your cream. You want the barest amount of whip here, some extremely soft peaks. While whipping, pour in the gelatin and finish whipping to its appropriate consistency. Hopefully you're using a mixer at this point, but I'm sure if you're the type to hand-whip cream, you can swing it.
There are other ways to stabilize the foam. As with the gelatin, in each case you're attempting to incorporate something that makes structure alongside the base mesh of fats. Gelatin is a pain because of the temperature management and liquids and such. You could also use 2 tablespoons of corn starch for your two cups of cream. This will not be as smooth, but it is far easier.
I'm told that adding some nonfat dry milk, four teaspoons for your two cups of cream, will give it some extra protein. There are some grocery-store items that stabilize whipped cream as well, and these will probably use starches to do the work, but have a better chance of being smoother than corn starch because they will have been specifically engineered to the task.
So, there you have it: firm up the structure with something that will dissolve in liquid without messing with the flavor, and you will have a stable foam. Your options are many, and should allow you pick and choose between ease, structure, and consistency.