Egg
Also known as hen's egg, egg white, albumen, egg yolk, Ei, oeuf, uovo, huevo, jajko
A binder that sets with heat, which confines it to cooked products. White and yolk do opposite jobs: the white gels and firms, the yolk emulsifies and enriches, and a recipe that names one means it.
Egg appears in the cooked charcuterie and nowhere else, and the reason is that its usefulness depends entirely on heat. Egg proteins coagulate somewhere between about 62 and 70 C (144 and 158 F) depending on which protein and which part of the egg, and that setting is the binding. In a product that is never cooked, egg contributes moisture and a spoilage substrate and no structure at all.
The two parts are different ingredients. Egg white is almost pure protein and water, and on heating it forms a firm gel that holds a farce together and gives a springy, sliceable set. It is the choice where a maker wants structure without richness or colour, which is why the pale Spanish morcilla blanca and several white puddings use white alone. Egg yolk is about a third fat and carries lecithin, one of the most effective natural emulsifiers, so it stabilises fat in suspension, enriches, softens the texture and adds colour. Whole egg does both jobs at a compromise. A formula specifying egg white where another specifies whole egg is making a deliberate choice about firmness against richness.
In practice egg belongs to the terrines, pâtés and galantines of the French tradition, the white puddings and morcillas blancas of Spain and Britain, the boudin blanc family, the Polish and eastern European liver and offal preparations, and the cereal-bound puddings. It is a common component of a panade, the bread-and-liquid paste that lightens a coarse forcemeat.
The practical cautions are three. Egg sets hard if overcooked and a terrine taken well past its endpoint goes rubbery and weeps, so the binding that egg provides is also the thing that punishes a careless cook. Egg is a major declarable allergen. And raw egg carries a Salmonella risk that matters wherever the product will not reach a proper cooking temperature throughout: this is one more reason egg has no place in an uncooked preparation, and a reason to prefer pasteurised egg in anything served to vulnerable eaters.
Quantities vary enormously with the job. A terrine may carry one or two eggs per kilo of forcemeat as its principal binder. A morcilla blanca may use egg white at fifty or sixty grams per kilo. A panade uses far less, since the bread is doing most of the work.