Foodstuffs - Foamed
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Confectionery frappe
A frappe is the starting material for a number of foamed confectionery products such as nougat. The young frappe consists of a mixture containing some or all of the following: foamed ovalbumin, sugar, and liquid or solid fat. The foaming is normally performed in a pressure beater and the resulting bubble sizes are normally small and of uniform diameter (10-75um).
Bubbles are also spherical with very few polyhedral examples. As the fully-formulated product matures, the bubble-size distribution alters and a population of larger bubbles develops as a result of the degradative processes of disproportionation, creaming and coalescence.
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 first image shows the typical structure of strongly-beaten foamed egg white. In the second image, both bubble shapes are visible.
Ice-cream
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 between about 5-10um. The second image shows that, at higher magnifications, fat globules between 200nm and 2um (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).
In image three, etching (sublimation) shows the ice crystal spaces, and the fourth image shows deep-etched (sublimated) ice-cream preparation.
Whipped double cream
Whipped double cream is another example of a fat-stabilised foamed emulsion. The first image shows that, at low magnification, milk fat droplets in the continuous phase are visible as 2-10um diameter particles (marked by arrows). Bubbles of a variety of sizes between 10-100um are also obvious. Bar: 100um (inset 50um).
At higher magnification, the second 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.