Taking the strain

Butcher’s Strain
After slaughter, hides and skins are removed from the carcase either entirely by hand or, more commonly, by machine. There are many advantages to mechanical flaying; hides and skins can be removed much faster and machine flayed hides and skins have fewer flay cuts and are often a better shape than hand flayed. It is also a more hygienic method of hide removal since there is less contamination of the carcase with dirt on the hide. However, mechanically removed hides and skins do suffer significantly more from butcher’s strain than hand flayed hides or skins.
There are several designs of mechanical hide pullers. Most, but not all, are operated by an electrically driven winch that pulls the hide, usually in a downward direction via a floor mounted pulley (see diagram1).
During the pulling of the skin from the carcase, the weakest point will break first. Usually this is at the fatty connective tissue between skin and meat, which is fine. However, sometimes the grain layer of the skin can be equally weak and tears and breaks can occur at the skin surface. Because the skin in the belly area is much thinner and weaker than other areas, it is much more susceptible towards this type of damage, although other areas can also be affected.
The problem arises because the grain layer of hides or skins commonly used for leather
manufacture is inherently weaker than the underlying corium layer.  This is because the grain layer contains many structures such as hair follicles, sebaceous glands, blood capillaries and sweat glands. Consequently, the amount of strength-giving collagen in the grain layer is
considerably less than it is in the corium layer which contains relatively few structures other than collagen fibre bundles. Unless there is something seriously wrong with the set-up of the machine, it is unlikely that the damage will extend into the corium layer.
Domestic sheep have been selectively bred over centuries to produce fine wool for our use. This means that sheepskins tend to have many more hair follicles per unit area of skin than other types of animal typically used for leather manufacture. This inevitably results in a reduction in the amount of collagen in the grain layer.
For this reason, the grain layer of sheepskin is noticeably weaker than that of most other animals. Consequently, butcher’s strain is most commonly seen on domestic sheepskins.

Prevention
When using mechanical flaying devices, correct set-up of the hide or skin puller in terms of speed etc is obviously essential. But it is the preparation of the skin prior to removal that has been found to be of the utmost importance.
The hide or skin must be loosened by hand at strategic points, particularly in the belly and around the legs. Studies2 have shown that if the hand preparation of the hide or skin is not done correctly before the machine pulls it off the carcase, the incidence and severity of butcher’s strain increases significantly. 
Flaying is best done as soon as possible after slaughter whilst the carcase is still warm. Once it has cooled, considerably more force is required to remove the hide or skin and this can lead to increased damage.

Damage to leather
Other than complaining to your hide or skin supplier, there is little that the tanner can do about the problem of butcher’s strain as the damage is usually too deep to be easily disguised by finishing.
Detection early on in processing is advantageous as this permits sorting badly affected skins appropriately. Only the worst butcher’s strain is visible by eye on wet part-processed material (wet-blue or pickled pelts). However, the damage can be highlighted by pigmented materials, eg jeweller’s rouge, carbon black or Sortassist3.
Depending on the wool density it is also possible to detect butcher’s strain on the flesh side of raw sheepskins if they are held against a bright light. References
1. Meat and Livestock Australia
2. www.leathercouncil. org/fair/news3.htm
3. Sortassist - a pigment dispersion that can be decolourised available from Clariant