Whipped cream has been on my mind a lot lately. A classmate and I just gave a presentation about our experimental studies on the physics of these aerated dairy emulsions for an Applied Physics class that we're taking. People have made whipped cream since at least the mid 1600's, but it has only become more common in the past hundred years thanks to centrifugation (to separate out cream from whole milk), refrigeration (to shorten the whipping time and make stiffer foams), and pressurized nitrous oxide dispensers. Even more recently, scientists have begun to examine its micro-scale characteristics.
From a physical perspective, whipped cream is more complicated than most typical foams, which are just a dense packing of air bubbles held together by surfactant (like soap). The air bubbles in whipped cream are all coated with fat molecules, which is why it tastes so good, and why you need to start with about 30% fat content in the cream. These fat molecules are initially contained within a coat of proteins, but somehow need to escape to attach to the entrapped air.
The physics of mechanically whipping cream is fairly well understood. The whisk introduces large air pockets into the cream, which break into smaller air bubbles. At the same time, the whisking breaks down protein-coated fat globules in the cream, which allows naked fat molecules to adhere to the air bubbles. If the temperature is slightly above freezing, then the blobs of fat only partially coalesce, leading to a more rigid structure.
However, whipped cream that comes out of a can or nitrous oxide-powered dispenser follows a completely different process. The gas is initially dissolved in the fat globules, but comes out of solution when the pressure is released and the whipped cream is dispensed. With the traditional method of making cream, large air pockets are fragmented into small air bubbles. With aerosolized whipped creams, dissolved gas expands to form the air bubbles. We wanted to know how the resulting creams differed.
To be more quantitative about the mechanical properties of the various types of whipped creams, we put a small amount of each in a device called a rheometer. The device consists of two parallel circular plates that can rotate relative to each other in a controlled way. By placing the whipped cream between the plates, we can measure how viscous (like honey) or elastic (like a rubber band) the material behaves. In one experiment we rotated the plates back and forth over a range of frequencies and saw how the mechanical properties of the whipped cream changed. I'd be happy to discuss the results in more detail, but the basic idea is that additives in store-bought whipped cream make the whipped cream less elastic, but more stable.
We also did some preliminary microscopy measurements. My favorite was the confocal fluorescent microscopy, in which we added a lipid-soluble dye that revealed the location of all the fat globules. In the image below, you can see the fat globules clustering around an air bubble (the field of view is about 0.2 mm).
These early experiments are far from rigorous, but they do suggest all types of further studies. There is no shortage of variables to control: the amount of surfactant, the types of fat molecules (e.g. saturated vs. unsaturated), the protein composition (pasteurization can have a major effect), the air bubble size distribution, etc. Moreover, chefs are finding all types of novel uses for the refillable whipped cream dispensers, such as making espumas of seafood, mushrooms, and vegetables, as well as single-serving cakes.
On the agenda this month: the science of chocolate.