Egg proteins
Résumé
Hen egg was categorised by Baldwin in 1986 as a polyfunctional ingredient, as it
can simultaneously realise several technological functions in the same
formulated foodstuff. Its emulsifying, foaming, gelling, thickening, colouring
and aromatic properties make it still today a universal basic ingredient for the
domestic kitchen and the food processing industry. Whereas egg yolk is well
recognised for its emulsifying properties, egg white (or albumen) is a reference
in terms of foaming and both parts are used as gelling ingredient in many foods.
Yolk takes part in the formation and the stabilisation of emulsions. In spite of
the intensive use of yolk in formulated foodstuffs, and since the invention of
mayonnaise three centuries ago, the role of its major constituents is not clear
because of its complex structure. Yolk is a mixture of proteins and lipids
forming natural assemblies at various scales. These natural assemblies contri-
bute to the nano- and the microstructure of yolk. Thus, an understanding of the
emulsifying properties of yolk lies in the comprehension of these various levels
of structure.
The exceptional foaming properties of the albumen are also the base of
traditional recipes among which meringues act certainly as reference. Indeed, the
extreme simplicity of their formula (albumen and sugar, possibly added with
flavours) allows albumen to express in an optimal way its foaming properties.
However, the technological parameters influence the final quality of foam
obtained, and three types of meringues (traditional meringue, Swiss meringue
and Italian meringue) can be distinguished, depending on whether whipping is
achieved in the presence or absence of sugar, and at ambient or warm
temperature. But there is also a great number of other products in which
previously foamed albumen is added, which are either fat-free formulas (angel
food cake) or lipid-containing formulas (spoon biscuits, `sponge' cake, blown).
In such products, the complexity of the phenomena is extreme, foaming and
emulsification taking place simultaneously, which makes the control of the
physico-chemical and technological parameters of these operations very delicate.
Concerning the gelling properties of albumen and yolk, they are related to the
heat-gelation capacity of egg proteins. Then, these properties imply a cooking
step during the food processing. The heat gelation of egg proteins completely
conforms to the model of heat gelation of globular proteins. The corresponding
mechanisms have been extensively studied, on egg proteins as well as on other
ones, and the key technological parameters have now been identified. However,
the addition of other ingredients in mixture with egg (polysaccharides, for
example) complicates the understanding of the egg gelation behaviour, and
developments with more complex models are still needed.
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