Hide and Skin – What’s In It For You?18 November 2009
By Amanda Michel of Leather WiseRaw hides and skins are the most expensive commodity that the tanner has to purchase, yet up to two thirds of this commodity is water! On average, freshly flayed hide or skin will contain around 65% water, but this will vary from one species to another. There is also some degree of variation according to location with the bellies tending to have a higher water content due to the larger gaps between the collagen fibre bundles.
The water does not just sit between the fibres bundles; it penetrates right through the hierarchy of the collagen structure down to the molecular level where it helps the skin maintain turgidity and structure. In the medical profession a test for dehydration is to pinch the skin. Fully hydrated skin should rapidly return to its original shape. Reluctance to regain shape indicates dehydration.
Maintaining water levels in the skin is vitally important during the life of the animal, but the water content of the skin can cause problems once the skin has been removed after slaughter. Putrefactive bacteria can multiply at an alarming rate when the right conditions are provided: warmth, moisture and nutrients – all to be found in abundance in a freshly flayed hide or skin. During life, the epidermis acts as a natural barrier against these bacteria, but once the skin is removed from the carcase the exposed flesh surface presents an easy route in. If the hides or skins cannot be processed immediately, reducing the moisture content to below the level that can support the growth of microorganisms is an easy and effective means of preventing bacterial damage. Effective means of reducing the moisture content in raw hides and skins can be:
• Air drying
The cheapest option is, of course, to air dry but this can only be implemented if climatic conditions allow. The temperature needs to be sufficiently warm to enable rapid evaporation of water from the skin to give a moisture content of between 10 and 15%, but if it is too warm the outer parts of the skin can dry too hard and trap moisture in the centre. Putrefaction can then occur in the middle layers of the skin. Also, if temperatures are excessively high, heat damage of the collagen can occur, especially in direct sunlight. Therefore, shade drying is preferable. Air drying is a relatively easy process to use for small thin skins, but not so well suited for thicker hides.
In the cooler wetter parts of the world, such as Europe, air drying would take so long that putrefaction would have taken hold before the moisture content was sufficiently lowered to inhibit bacterial growth. Therefore, salting or brining is the norm in many countries.
The application of common salt (sodium chloride) to the flesh side of cooled hides or skins draws out the water by osmosis. Typically, approximately 30% of the weight of the fresh hide or skin in salt is needed for adequate preservation. On a typical cattle hide this would be around 10-12kg. On a sheepskin approximately 1.5kg is needed. The salt can be applied mechanically or by hand, but it is most important that the salt is applied to all parts of the flesh side, including the creased areas that are inevitable at the edges.
Over a period of several days the water within the hide or skin will drain away. To prevent puddles forming that could dilute salt concentrations, hides should be just folded in half and raised on a pallet to allow the liquors removed by the salt to freely drain away. When draining is complete, a little additional salt can be applied to the flesh side and the hides folded into quarters for storage. It can take several days for the water to be drawn out and many more days for the salt to fully permeate the hide structure. Brining (immersing the hides in a concentrated salt solution) speeds the process up significantly; a full cure is normally achieved in 18-24 hours.
If insufficient salt is applied, the water is not allowed to drain away properly or the salt is unable to penetrate due to excessive fat/flesh and the degree of salt saturation may be insufficient for good storage. Salt saturation levels are calculated by determining the salt and moisture content of the hide or skin in the laboratory. The US National Hide Association1 offer the following guidelines:
Providing storage conditions are cool and dry, in this state the hide or skin can be stored successfully for many months without the risk of putrefaction, especially if biocides are added to prevent the growth of halophylic (salt loving) bacteria.
So, having made all this effort to reduce the water content of raw hides and skins, the very first job that the tanner has to do is replace it! Apart from the need to wash out the salt that may have been used to temporarily preserve the hide or skin, it is important that it is fully rehydrated so that there is sufficient water present within the structure to enable later process chemicals to be carried through. Usually, this is achieved by a short wash (dirt soak) which removes excess salt and dirt from the surface followed by a longer soak in which the hide is rehydrated. Water absorption can be encouraged at this stage by raising the pH to around pH 10, agitating the skins in the float or adding detergents or enzymes.
The rehydration of air-dried skins is a more tricky affair. Initially, soaking needs to be static since the dried collagen fibres are liable to break if flexed and, due to the initial lower moisture content, full rehydration may not be achieved for anything up to 48 hours. For this reason, it is essential that a good biocide is used in the soak liquor in order to prevent putrefaction taking place.
The soaking process also has other functions such as the removal of soluble matter such as non-structural proteins which will be discussed in more detail in later articles in this series. n