The essential nature of proper preservation

What is the best way of preserving a hide or skin? It obviously depends where you are in the world but what is most important is that once an animal dies it is quickly subjected to decay. The purpose of curing is to temporarily prevent this decay between offtake and processing. By definition, the process must be temporary, but even then it often affects the quality of leather.^1,2 One of the fundamental properties of the skin is to act as a barrier to micro-organisms, but once the animal is dead, bacteria are swift to penetrate and once inside multiply rapidly on the nutritious environment of blood, fat and tissue. And it is the proliferation of bacteria that preservation tries to overcome. Your average bacterium needs three things to proliferate: a certain amount of heat, water and a feeding source. The aim of preservation is to deny it some or all of these. Obviously, denying them the hide in the first place - the meal - would be ideal and this idea was taken up by David Bailey, formerly of the USDA, when he irradiated hides and made them sterile. He associated this idea with concerns that have grown over the past few years about the contamination of meat, especially from E.Coli O157:H7. But more of that later. If the water is either too alkaline or acid, or not there at all, most bacteria cannot survive or replicate, hence the use of acid (pickling) or drying to preserve. The addition of salt to a hide also reduces the availability of water to most bacteria and extracts water from many types of bacteria thus killing them. (There are, of course, exceptions such as halophilic bacteria) Now, when I was at college the standard method was to sprinkle on coarse salt, add a smidgen of beta-naphthol, fold the hide into quarters, sprinkle on more salt and store in a dry place. While salting hides and skins has been around for hundreds of years, either through dry salting or via brining - mainly used in the US - environmental concerns and the need to process hides more quickly means things are changing. After all why would a tanner want to store hides, which are valuable collateral, if he can process them? The usual reason is that the slaughtering is not necessarily near the tannery and transportation is slow or infrastructure poor. Also there is a vast market in trading hides and skins, so the preservation must be good enough to stop the hide from putrefying when in transit. That is actually asking a lot of the preservative because the cargo could be travelling from temperate climates to tropical ones and the changes in heat and humidity will affect the bacterial counts within the hides³. The disadvantage of salt curing is that it uses sodium chloride, which is a difficult component of the effluent to treat. Dr Bailey came up with an interesting alternative to common salt a few years ago, which is to use potassium rather than sodium chloride. Potassium chloride has similar physical and chemical properties to common salt, but is very different in one respect. While sodium chloride has a negative effect on the growth of plants when applied to the soil, potassium chloride is required for proper plant growth. Bailey carried out a number of trials involving around 4,000 hides and came to the conclusion that there was virtually no difference in the final leather properties. If chemicals don't appeal there is ice. Not, perhaps, the best solution for a hot country, but there are systems that take granular or flake ice from any type of ice making machine and deliver it to a blender. There, it is broken down and mixed with a weak brine solution (OK, a small amount of chemicals!). The blender turns the ice to a slush which can then be pumped over hides. The ice from the blender is pumped to a storage tank which is insulated and fitted with a stirrer to keep the ice in motion inside the tank. Ice can be stored in the tank for a week. The system is in industrial use⁵ and as hides are dumped into a bin, a valve over the bin, computer controlled, opens and delivers a short, variable, burst of liquid ice. The ice, being in liquid form, covers the whole of the surface. When the bin is full, a signal is sent to give a longer burst of liquid ice to cap the bin. The throughput of the automated hide grading and icing system is around 250 hides per hour. Normally there would be one man to clamp hides, two trimmers and a fourth man on a forklift to load the stock conveyor and move the bins of graded hides. Hides usually arrive from the abattoirs while they are still warm so the quicker they can be cooled the better in terms of grain quality. Liquid ice has proved to have eight to ten times shorter chilling time than conventional ice and the distribution over the hide is much better. For customers who don't want such a sophisticated system, fluid ice systems can be supplied with a simple manual valve attached to a hose. The valve is opened by an operator who holds the hose and fluid ice is poured onto the hides. Fluid ice has been used for years to rapidly chill fish on trawlers and in fish processing plants, so its application in the hide industry makes sense. A US tanner told me that instead of these 'short termist views' a much more interesting project that he was involved with was the irradiation of hides at the packing plants. 'Graham, this is the direction to go, no salt, and good preservation.' Another Bailey inspired idea, initially the use of gamma radiation was investigated but as he, Bailey, said: 'from a practical point of view, in the age of consumerism, it is unlikely that a new facility using spent nuclear fuels will ever be built', so he turned to electron-beam (e-beam) radiation. The thing about this system is that it works. The irradiated hide has the properties of a fresh hide, does not contain salt, and can be stored for extended periods of time. The question of costs, which is always raised when e-beam is discussed are real, but in countries such as the US, where it is feasible to install the equipment at the end of a meat processing plant, the long-term economics make sense. No pollution. The irradiated hides have to be bagged to keep them sterile, but they can be transferred to the tannery in this bag, which can then be re-used. I would imagine that a country such as China would also be interested. In conclusion, the problem is that the 'novel' technologies for rawstock preservation mostly involve high-tech solutions, and as Table 1 shows, the problems in terms of preserving the rawstock are often in third world countries, where collection rates are appalling. The term 'novel' is, I would argue, therefore a function of the location. In the Sudan 'novel' could well mean collecting and drying hides immediately, under cover, off the ground, and in a draught. The method is cheap, simple to operate, and would bring real benefits to the country. While in the mechanised world e-beam will be the way forward, if Unido, Esalia and the myriad of other NGOs in Africa and India could get together and build structures ⁷ and infrastructure, novel rawstock preservation would be a breeze - to the benefit of us all.