ISO 22284:2020 pdf free.Leather- Raw skins – Guidelines for preservation of goat and sheep skins .
4 Characteristics of salt and auxiliary substances used in preservation processes
4.1 Characteristics of salt
For preservation by salting, clean salt should be used. Halophilic bacteria get acclimatised to salt media and can grow even in the presence of high salt concentrations. During bacterial growth, they produce pink, red or violet pigments on salted skins, called red-heat. Sea or lake salt tends to be more susceptible to contamination risk and development of red-heat. Hence, using rock salt is advantageous as it is free from chemical impurities.
Salt used in preservation processes should preferably contain 98 % sodium chloride (NaC1) of total dry weight and should not contain clumped particles. The impurities should not exceed the following limits otherwise the salt may negatively affect the quality of the finished product:
— Total calcium (Ca) and magnesium (Mg) compounds in salt: 1 %.
— Total iron (Fe) compounds: 0,01 %.
Particle size of salt is also important to achieve effective preservation. If salt particles are very fine, salt tends to form pasty patches with uneven coverage. If salt particles are very coarse, they can immediately fall off the skin while handling. Particle size also affects speed of dissolution; therefore, to ensure an appropriate dissolution, particle size of salt should be 2 mm to 3 mm.
4.2 Quantity of salt
To ensure proper dehydration of raw skins, the quantity of salt should not be less than 30 % of the fresh weight.
4.3 Auxiliary chemicals and preservation mixtures
Improvement in preservation could be achieved by the use of additional substances along with common salt.
The addition of sodium carbonate (Na2CO3) to the salt mixture prevents the formation of iron stains on skins, which can form due to existing impurities in salt. If iron impurities exceed 0,01 % of the total mass, iron stains occur in limed pelt, resulting in inferior quality.
Restricted pesticides or other toxic substances, such as pentachiorophenol (PCPJ, should not be used in preservation mixtures.
5 Preservation methods of skins
5.1 General
When an animal is alive, the animal skin serves as an anatomical barrier between external physical, chemical and biological agents and the internal environment. After the death of the animal, the skin becomes susceptible to microbiological spoilage because of its natural structure and excess water content. Preservation is necessary to maintain the quality and commercial value of the skin.
Before the preservation process, skins are conditioned by carrying out certain basic operations such as cleaning, draining and trimming in order to ensure effective preservation. Skins are subsequently cured, which can take 4 h to 5 h depending on environmental conditions. There are several ways to preserve skins to inhibit micro-organic putrefaction and autolysis.
Microorganisms need humid conditions to survive. Hence, the most important step for preservation of skins is decreasing the water content of fresh skins, which will prohibit bacterial growth and activity.5.2 Drying
Drying is one of the oldest and simplest preservation forms, in which the water content is lowered to a certain level where microorganisms are inactive and unable to spoil skins. Although dry conditions are hostile for many microorganisms, some could stay dormant until the environment becomes favourable again.
In this method, dehydration of raw skins is carried out by natural airflow at ambient temperature in the shade. Extreme conditions and irregular foldings should be avoided. Direct sunlight and high temperatures can cause flash drying of outer layers but the inner layers still retain moisture. Hence, the drying is not homogeneous. Cross-sectional transfer of water can take place in such cases, thereby leading to decay during storage.
Dry cured skins should not contain the following defects or formations which negatively affect the quality:
— overdried and crusted areas and irregular foldings;
— discolorations and livid areas;
— small or large tears caused by autolysis;
— insect infestations and digested areas;
— wetted areas or skin masses caused by improper storage;
— partial gelatinisation due to application of high temperature while drying.
If skins have one or more of these defects, they might be degraded until castoff depending on the degree of existing defects.ISO 22284 pdf download.