Have you ever whipped cream that was warm or overwhipped the cream? You likely found cream whipped in this nature was oily or tasted kind of like butter. To better understand what is going on while making whipped cream, one has to understand what is happening to the fat globules of heavy cream during the whipping. Mechanical agitation by the mixer beaters causes the membranes covering the globules to break and shear, forming smaller globules with sections of exposed fat (oil) droplet.
Whipping also introduces small air bubbles to the cream, and the interaction of air with the globule also helps to disrupt or break apart the membrane. While whipping, the air bubbles become smaller and the sections of exposed fat within the globules clump or coalesce to form cages surrounding the small air bubbles. The trapping of air increases the volume of the now “whipped cream.”
As the fat globule membrane is disrupted, some of the casein protein will coat some of the exposed fat surface, which supports the fat–air interface. Stop whipping too early, and the cream remains unconnected milk fat globules from which air will easily disperse. Whip too long, and the globules will break down into many smaller fat droplets with too little membrane to cover them. This will result in the beginning of butter, as the fat droplets are mostly naked oil and can readily coalesce into a solid mass of fat.