Hide and Skin – What’s in it for you? Inter-fibrilliary matter24 November 2009
By Amanda Michel of Leather WiseThis is the last in a series of articles that have discussed the composition of the hides and skins that we use for making leather and what the implications are of that composition for the tanner. In this article inter-fibrilliary matter is discussed
After water, skin comprises predominantly of the fibrous protein collagen. This collagen is arranged in a highly organised fashion that forms the fibre structure that gives leather its unique properties; three amino chains twist together to form a helix called a collagen molecule. Several collagen molecules twist together to form micro-fibrils. Microfibrils twist together to form fibrils (typically 100 nm in diameter). Fibrils group together to form fibres (1 micron in diameter) and then the fibres group together to form fibre bundles. This complex structure is very similar to that of a rope. The fibre bundles then interweave with one another to create a very strong, yet highly flexible substrate.
The spaces between the minute collagen fibrils are filled with a variety of substances including the soluble proteins albumin and globulin (1% of raw hide), mineral salts (0.5% of raw hide) and glycosaminoglycans (GAGS), that are also known as mucopolysaccharides, (0.7% of raw hide).
There are predominantly two glycosaminoglycans found in skin; hyaluronic acid and dermatan sulphate. Hyaluronic acid is a large molecule comprising of a chain of repeating disaccharide (sugar) units. These chains form a tangled mass that has the ability to trap water which makes it highly viscous and gel-like. These chains of hyaluronic acid and the water trapped by them fill the spaces between the collagen fibrils which helps to keep the skin hydrated and turgid. If you imagine a sponge that has been dipped in water; the water soon drains out leaving you with a dry sponge again. However, if you were to dip the sponge in warm liquid jelly (the type you might use for dessert) and let it set, the water would then be trapped inside the sponge and not be able to drain away. Hyaluronic acid has a similar effect on the water as the jelly, except it doesn’t taste so nice!
This water-retaining ability of hyaluronic acid is a vital function in life and it helps to keep us wrinkle free which can only be a good thing! But for the tanner it is a nuisance because its viscosity inhibits the diffusion of dissolved processing chemicals through the structure of the hide or skin. Therefore, it is important that it is largely removed at the early stages of processing, ie during soaking.
Dermatan sulphate also contributes to water retention to a certain extent, but it is thought that its principle role is maintaining the physical structure of collagen. Dermatan sulphate has a protein core to which strongly negatively charged polysaccharide side chains are attached. The protein core of dermatan sulphate attaches to the collagen fibril via eletrostatic charges. In life, dermatan sulphate plays an important role in skin healing processes, but like hyaluronic acid, can impede the penetration of process chemicals if not properly removed early on in the leather making process. Dermatan sulphate will not be removed by soaking hides or skins in plain water unless proteolytic enzymes are also used which help break down the protein core. However, it is removed during the liming process and this process will be enhanced by the use of alkali stable enzymes in the lime. Measuring the amount of dermatan sulphate and hyaluronic acid in the raw and limed hide is a useful means of determining how efficient the beamhouse process is.
Albumins and globulins are water soluble non-structural (globular) proteins of which there are many different types. Albumin is one of the most common proteins found in blood where its role is to maintain osmotic pressure, but we are probably more familiar with it in egg white (egg white actually contains many different types of albumin). Although only found in small quantities in skin, these non-structural globular proteins contribute to maintaining osmotic pressure and water retention as well as interacting with hormones etc that are vital in regulating biochemical reactions in the body.
It is very important that the various interfibrilliary substances are thoroughly removed before the leather is tanned because they tend to act like a glue when the leather is dried; they stick the fibres together making the leather very firm. The salting of raw hides and skins will start this process since some of the soluble substances will drain away with the water that the salt draws out. But it is the soaking process that is the key stage for their removal. Providing there is sufficient mechanical action, much of the interfibrilliary matter can be removed by soaking in plain water. However, proteoglycans are not removed with water alone, but their removal, and the removal of other interfibrilliary matter, can be aided by the use of soaking auxiliaries.
Increasing the pH of the soak can aid the removal of non-structural proteins as it causes the hide or skin to absorb more water. However, it is very important that the pH does not exceed 10.5 since this could cause immunisation and subsequently poor removal of the hair. The addition of surfactants and proteolytic enzymes will further enhance removal of interfibrilliary matter. However, if soaking auxiliaries are used, it is important to check that they are compatible with the biocide being used otherwise one may negate the effect of the other.
This concludes this series of articles explaining the principle components that are found in hides and skin and how something that performs a vital
function in the life of the animal can create enormous problems for the tanner. It can now be appreciated what the tanner has to struggle with in order to turn what is essentially a waste
product into a beautiful and versatile material.