The Book of Household Management by Mrs. Beeton

flesh undergoes very considerable changes. For instance, when the animal is young, the fluids which the tissues of the muscles contain, possess a large proportion of what is called _albumen_. This albumen, which is also the chief component of the white of eggs, possesses the peculiarity of coagulating or hardening at a certain temperature, like the white of a boiled egg, into a soft, white fluid, no longer soluble, or capable of being dissolved in water. As animals grow older, this peculiar animal matter gradually decreases, in proportion to the other constituents of the juice of the flesh. Thus, the reason why veal, lamb, and young pork are _white, and without gravy_ when cooked, is, that the large quantity of albumen they contain hardens, or becomes coagulated. On the other hand, the reason why beef and mutton are _brown, and have gravy_, is, that the proportion of albumen they contain, is small, in comparison with their greater quantity of fluid which is soluble, and not coagulable.