THE PHYSICAL CHEMISTRY OF RAW HIDE AND CURING PROCESS
a) Autolysis of hide /skin:
The hide of a live animal contains 62-78% water. Death causes dramatic change in metabolic process. O2 and nutriton is cut out, removal of metabolites from the cell is stopped. Toxic accumulation results. Due to inactivation of coenzymes Enzyme controlled processes stop.
The process of self digesting(autolysis) of the cells starts ie. intercellular enzymes cathepsins (peptide hydrolases). Autolysis does not cause change in flayed hide at r.t. even at 24 hours. Autolysis of salted hides depend on temperature and amount of salt . The higher the temperature, the higher the autolytic process. However, the rate decreases with increasing salt concentration.
Common preservative like boric acid or sodium carbonate do not inhibit autolysis at all. The yellow “salt” spots on hide arise from autolytic activity (not from bacterial activity) due to effect of alkaline phosphates in presence of calcium sulphate.
The secondary process accompanying autolysis is action of putrefactive bacteria for which autolysis products offer an excellent medium.
b) Bacterial growth on flayed hide:
For bacterial growth certain humidity is required. Usually 30-35%, for molds it may be 12 -15 %.
Minimal temperature of possible growth is usually 50C higher or equal to to the freezing temperature of medium. The majority of bacteria find their optimal living conditions at neutral or slightly alkali pH, the majority of molds – at acidic ones (Approx. pH 5).
Na2CO3 and naphthalene as antioxidant or trichlorobenzene is used for best prevention of bacteria.
c) Aging:
Changes in collagen occur due to aging (on storage). Crosslinking in collagen is increasing (observed by phenomena: Ts, acid and base swelling and trypsin action) in vivo and post mortem.
d) Hides and skins curing by salting and dehydrating:
The main problem in preservation of skins is to remove significant part of water and saturation of remaining water with salt, usually NaCl. Also important are use of bactericides.
Cooling of raw skin to -25 to -300C may be used as well but may easily be mechanically damaged (broken).
Saturation of the system with salt:
Flesh cattle contains 1.38 % of NaCl (calculated on hide substances). Dry salting, spraying dry salt on flesh side and flesh to flesh stacking on brining (in saturated salt solution )., in both cases there is osmotic penetration of salt into hide.
Salt penetration at r.t. takes about 48 hours. Concentration remains lowest in middle layers. The rule is to use coarser salt for hides, finer salt for skins. A great amount of Mg and Ca sulfates in salt (approx. 2 %) promotes appereance of “salt” spots. This is due to activation alkaline phosphatases in autolytic process. The salt spots do not arise when brine is used for preservation.
This is due to precipitation of Ca+2 and Mg+2 compounds.
Among the efforts to replace salt by other chemicals that are less contaminating to waste water and used in smaller amounts is formaldehyde, a powerfull crosslinking agent and kills almost every microorganism. Authors recommend 0.25 % formaldehyde as preservant. In this concentration the leather is slightly firmer than usually obtained. This difficulty may be overcome by post-tanning tereatment. Excess formaldehyde may cause difficulties in unhairing. The amount proposed increases Ts from 64 to 680C. Addition of some (7 %) salt makes hides mellower and with flatter grain. A Ts increase to 75 0C is then observed.
Water removing:
The aim of curing is to remove water from tissue to such an extent that no irreversible changes in the collagen properties should take place.
When liquid is removed from the pores the porous body changes its shape. Interfibriller pores, during shrinkage (walls approaching each other) may be torn or be closed completely (due to high tension that may be occur in capillaries).
Curing of skin by drying is applied to fur skins as a primitive, uncontrolled way of preservation in hot climate.
Dehydration of hide by methyl alcohol or ethyl alcohol followed by ethyl ether is a process different from drying . This process is used in industry ( USA and CHECK) as a process of quick introducing of tanning agents into hide, followed by quick tanning by water addition.
Freeze – drying:
A modern way of preserving skins (used for most precious fur skins). Skins are dried after freezing. Evaporation occurs in high vacuum and liquid state is omitted. The substantial difference between air drying and freeze drying is that during air drying the surface of the skin is the surface of evaporation as well whereas in freeze drying, the evaporation occurs on the fiber surface and among fibers where the ice crystals are present. Another difference is lack of any motion of liquid. Goods dried in this way are very stable, and after moistening they take the shape equal to that of a fresh one. Almost no changes in chemical and physical proporties are observed .
