Cryo-SEM: foamed ovalbumin
Such protein-stabilised foams degrade by gravitational separation (drainage), where liquid is lost from the plateau borders and lamellae. As a result of drainage the disperse phase volume fraction increases and the shape of the bubbles changes from circular to polyhedral.
The left-hand image shows the typical structure of strongly-beaten foamed egg white. In the right-hand image both bubble shapes are visible.
Cryo preparation is the optimal method for preparing foams for SEM observation.
Despite starting as a liquid, ice-cream is a solidified foamed emulsion containing air as the primary disperse phase. The air content of ice-cream varies between products but is generally about 50%, with a total fat content of 10-12%. The texture and rheological properties of ice-cream are directly related to structure and in particular, a) the percentage and dispersion of fat, b) the ice crystal size and, c) the size of air bubbles.
At low magnification air bubbles of a variety of diameters are visible dispersed among a frozen emulsion - shown in the first image. The ice crystal diameter is in the region of 5-10 um. The second image shows that, at higher magnifications, fat globules between 200 nm and 2 um (marked by arrows) are visible both at the gas/ice surface and also in the eutectic boundary between ice crystals. In addition large angular lactose crystals are visible (larger arrows).
The third image shows an air bubble with a large ice crystal and the fourth the boundary between the fractured face and original (unfractured) surface.
Cryo-SEM: whipped double cream
Whipped double cream is an example of a fat-stabilised foamed emulsion. The left-hand image shows that at low magnification, milk fat droplets in the continuous phase are visible as 2-10 um diameter particles (marked by arrows). Bubbles of a variety of sizes between 10-100 um are also obvious. Bar: 100 um (inset 50 um).
At higher magnification image shows that fat can be seen associated at the air/liquid interface with a continuous layer of liquid fat at the surface (marked by small arrows). In places (marked by larger arrows) fat droplets can be seen to be coalescing. Over-beating would result in the emulsion in the continuous phase inverting.