Talking Rot!

Putrefaction Because of the barrier the epidermis on the outside of the skin presents, bacteria are unable to penetrate the skin from this side. But once removed from the carcase, the exposed flesh side of the skin is vulnerable. Bacteria need warmth (ideally 35°C), moisture and nutrients to proliferate. Therefore a freshly flayed hide or skin would be regarded as heaven to any self respecting bacterium.  In ideal conditions, bacteria can proliferate at an alarming rate because they reproduce exponentially, ie 1 bacterium becomes 2, then 4, then 8, then 16 etc.  In ideal conditions some strains of bacteria can reproduce every 20 minutes, so within a couple of days the population can be enormous (more than 8,000 million). Proteolytic bacteria exude enzymes that digest proteins such as the collagen that hides and skins are largely composed of. At the very least, the resulting damage to the collagen structure results in weakness of the leather, grain damage, or even holes in some cases. Other types of bacteria can also cause us problems, eg those that live on fats (lipolytic). If the fats in the hide or skin are broken down by these bacteria, the fat molecules are broken down into glycerol and free fatty acids. These fatty acids can react with chrome used in the tannage to give chrome soaps or later give rise to fatty spues on the leather. But how can you be sure that the problems you are experiencing are due to putrefaction? Well, if the raw hides or skins have not yet gone into work, the classic symptoms of advanced putrefaction may present:

• Malodour
• Hair slip
• Red heat

But if the putrefaction is at a less advanced stage, or processing of the hides/skins has begun, a more sophisticated approach is needed. Over the years scientists have evaluated several methods of identifying putrefaction in raw hides ranging from the relatively simple gelatin film test where liquid squeezed from the hide is placed onto the gelatin side of photographic film. The proteolytic enzymes produced by any bacteria that may be present dissolve away the gelatin.  The numbers of bacteria in a given weight of hide can be estimated by diluting a macerated sample in sterile water and spreading it onto an agar plate. The colonies of bacteria that grow can then be counted. However, this is very time consuming.  More high-brow techniques investigated include the use of the ‘electronic nose' to sniff out rotting collagen more easily than our own olfactory senses. But by far the most reliable method of detecting putrefaction in hides or leather is the microscope. Using this technique, the presence and location of the bacteria can be determined as can the amount of potential damage caused; the destruction of skin's own cells and breakdown of the hair within the follicle indicates that the enzymes produced by putrefactive bacteria are likely to have penetrated the skin structure and caused damage to the collagen. Causes and prevention of putrefaction The most common method of temporarily preserving raw hides or skins until they can be processed into leather is by salting or brining. This partially dehydrates the hide making it unfavourable for bacterial growth. Putrefaction in salted or brined hides or skins can occur due to:

• A delay between flaying and curing/processing the raw skin
• Washing the hide or skin after flaying; this will spread bacteria from the hair or wool and accelerate putrefaction
• Not using enough salt in curing
• Poor distribution of the salt onto the hide or skin, eg not spreading out to the edges or into the creased areas
• Lack of penetration of the salt due to heavy fat/flesh deposits
• Insufficient draining of the salted/brined hide allowing liquors to pool and reduce salt concentrations
• Poor storage conditions, eg exposed to the rain allowing salt to be washed off or in direct sunlight creating excessively warm conditions
• Prolonged storage, particularly under warm conditions.

Salting hides To achieve a well cured hide, it is recommended that the hide should be cooled immediately after flaying and kept cool until the hide is salted. Cooling with the flesh side uppermost should be avoided as this can allow the flesh surface to dry out making penetration of the salt difficult.  As soon as possible, the hide should be treated with clean salt.  Between 35 and 40% of the green hide weight should be applied either mechanically or by hand. Ideally salt crystals of 2-3 mm size should be used; larger crystals are slower to dissolve and finer crystals can lead to caking. Pure dried vacuum (PDV) salt is a popular choice. Solar pan or sea salt should not be used as it could be heavily contaminated by halophilic (red heat) bacteria. Contamin-ation of hides with halophilic bacteria can also occur through the use of recycled salt from contaminated hides. Drum salting will produce a more rapid absorption of salt, whilst piling will call for at least seven days in salt for the required level of saturation to be achieved. A significant amount of liquid will drain from the hides, so they should be raised on pallets to allow good drainage. Hides should be folded in half along the backbone with the flesh side innermost whilst draining; folding into quarters can cause draining liquors to pool in the fold lines. When salt penetration is adequate, the salted hides should be opened individually and some additional salt applied before the hides are palletised for storage or shipment. Hides are usually folded into quarters with the flesh side innermost and stacked onto pallets. If wrapped, the use of clear polythene should be avoided to minimise heat build up when exposed to direct sunlight. The cured hides should be stored in cool, dry conditions, out of direct sunlight until processed. Brine curing The principle purpose of brine curing is the same as salting; to dehydrate the hide to a level that makes it unfavourable for bacterial growth. In the brine curing process, the hides are immersed in a saturated brine solution. Being in liquid form, rather than solid, the salt is able to penetrate the hide more quickly thus achieving an adequate level of saturation in a shorter time (about 24 hours). Brine curing can take place by simply immersing in a vat of brine but some degree of mechanical action improves penetration, so raceways or hide processors are more usually employed. As moisture is drawn out of the hides by the brine, it inevitably becomes diluted.  Therefore, the brine solution should be regularly ‘mended' by adding more salt to maintain a saturated solution. Brine solutions often become heavily contaminated with dung and blood from the hides and, therefore, can be heavily loaded with bacteria. Halophylic bacteria, eg red heat bacteria, can also build up in brine solutions unless they are regularly changed or an efficient biocide is used. When brining is complete, the hides should have excess brine liquor removed, either mechanically or by hanging to drain. To ensure safe storage of salted or brined hides, the level of cure should be checked periodically, especially if problems with putrefaction are suspected. It should be remembered that no matter how fresh or well preserved the raw hides or skins are, putrefaction can still occur if they are not soaked properly at the start of processing. Ideally, following a short dirt soak to remove free salt from the surface of the hide and blood or dung contamination, a more extensive soak should take place in the presence of a good bactericide.    The effectiveness of soaking bactericides can be monitored by using dipslides. These are plastic strips that have a coating of agar. After dipping into the soak liquor they can be incubated overnight and then the number of colonies of bacteria that grow on the agar estimated. References 1. The US National Hide Association (1979) - Hides and Skins, Jacobson, 3rd Edition