Whipped cream’s fantastic fluffiness is built by fat-covered bubbles
Whipped cream might make a lot more sense if we just called it “foamed cream.” Sure, there is a lot of whipping or whisking involved, but that hardly describes what’s happening inside the collection of milk fats and water that can cause it to turn from a thick, yummy liquid into a fluffy, nearly-sculptable foam. The cream isn’t solidifying thanks to crystalization like freezing water— if anything, stiffened cream is something closer to a non-Newtonian fluid like Silly Putty. Unlike those fluids though, whipped cream is sort of a one way road- you have to get it right the first time, as this process can’t be reversed and reattempted.
Whipping up some fat-lined foam
The key to this structural pickiness is the composition of milk fat, as whipped cream is technically an emulsion. Starting with whipping or heavy cream from the store, you have a liquid with some proteins, sugars, water and a lot of tiny globules of fat. Those globules are small enough to flow and feel like liquid in your mouth, but they actually have a bit of structure to them. The inside of each globule are fatty acids, mostly consisting of the hydrophobic triglyceride. To avoid interacting with water molecules in the milk, the triglycerides are contained in an outer shell of phospholipids. This is a stable arrangement, but it’s also too runny to put on your pumpkin pie or strawberry shortcake.
When you start whisking your cream, the whisk introduces little air bubbles into the liquid cream. Normally, these bubbles can escape by floating to the top of the cream, but as you keep whisking something else happens to trap them. The fat globules are big enough that they get hit and broken apart by the whisk, and while the phospholipids hardly care, the triglycerides still want to avoid interacting with water. So to find an attachment point, they glom onto the air bubbles floating through the cream, creating little fatty-acid shells around them. The high fat content of the cream serves up plenty of triglycerides to do this job, and eventually the triglyceride-lined bubbles are numerous enough to provide a structure to the cream. This is the moment when you not only get peaks in your cream, but it starts to grow in volume as well since there’s now air trapped inside.
What happens beyond the bubbles
If you keep whipping though, you can start to break up these precious triglyceride-bubbles too. The air pockets will be destroyed, as the triglycerides are left with nothing to grab onto but each other. You’ll get further separation between your fat and water, eventually leaving you with butter. It’s obviously not a total loss, but possibly not what you meant to put on your dessert.
So why does properly whipped/foamed cream eventually liquefy again? As you might expect, those triglyceride-lined bubbles aren’t that strong, and heat only serves to soften them further. Inversely, if you’re whipping cream, make sure it’s cold so that it will be as strong as possible. If you really need to prolong your cream’s fluff-time, cornstarch or powdered sugar can be added, giving the concoction more structural elements to work with, like a tastier version of many slime recipes.
What does the whipping in cream from a can?
If you’re in a bigger hurry, use the canned stuff, that uses pressured nitrous oxide to inject bubbles into cream. As that cream is forced through a tiny nozzle, the original fat globules are sheared open, recreating the effect of whisking in an instant. Try not to inhale the nitrous oxide though, as it’s not good for your brain, or for the ozone layer either.
Source: Cream Science: On Whipping, Butter, and Beyond by Claire Lower, Serious Eats