Collagen is a water insoluble
connective tissue animal protein which is made up
of water soluble alpha chains (MW ± 100 000) which cross-link to form a water insoluble polymer.
As the animal ages the initial collagen cross-links are stabilised by rearrangement of the double
bonds between alpha chains. With young animals, the
cross-links can be destabilized or
hydrolysed in water by mild heat (50ºC) to release
the alpha chains into solution but as the
animal ages the destabilization temperature rises and with relatively old animals it appears
that temperatures of up to 100ºC are unable to denature all the collagen and release the alpha
chains into solution. (Cole & Roberts 1997). This means that part of the collagen of skin and
bone may not be convertible into soluble protein.
Other than this there are components of skin
like the epidermis which is composed of keratin,
which can not be solubilised in water by heat.