Soaking

Soaking

Soaking is first important operation of leather processing. Hides and skins received into a tannery are in the four conditions, as green or fresh, as wet salted, as dry salted or as dried. It is advisable to carry out soaking for all types of skin and hides to obtain best quality leather. Soaking cleans hides and skins by removing dirt, blood, flesh, grease, dung etc. and most importantly, re-hydrates them to bring skins as far as possible back to state of green hides. Soaking agents fall into three categories, like Chemical Agents, Surface-active agents and Enzymatic agents.

Enzymatic agents are biocatalyst. Specific protease and lipase enzymes enhance water uptake by dissolving intrafibrillary proteins that cement fibres together and disperse fats and oils together with dirt and other contaminants present on skin.

Maps offers a range of protease and lipase for soaking which work in different pH conditions.

Palkosoak	A mixture of protease and lipase for soaking in alkaline pH conditions
Palkosoak ACP	A mixture of protease and lipase for soaking in acidic pH conditions

1.1 Soaking

Two effects have to be achieved in soaking of cured hides: cleaning up of the surface of the hide and rehydration of the interior of the hides. Manure, urine and blood generally contaminate the typical hide. Animal skins are naturally contaminated with soil, dust and sand from normal activities of the animals during grazing and on the feedlot. Furthermore large amounts of salt have to be removed some of which adheres to the outside of the hide and the rest that is well penetrated in the entire cross section of the hide.

Salted hides, fresh hides and chilled hides all require several washings with fresh water under strong mechanical action for proper cleaning. Drums, mixers and paddles can be used and the wash float should be changed at least twice with fresh water after 30 to 60 minutes of drumming.

The effect of washing can be followed by observing the color of the wash float. The first wash float is brown and appears to have a high content of solubles and solids. The color comes from blood and from manure. With each change in float it becomes clearer and more colorless. Picture 1 shows the soaking process in a paddle.

Full rehydration of the fiber structure is a very most important precondition for all of the beamhouse operations. Liming cannot substitute for insufficient soaking. Salted hides need about 5 hours of drum soaking and dried hides and skins may need 24 hours or more. The hide has to reach as nearly as possible the state of suppleness it had when it was freshly flayed from the animal.

Soaking in the final rehydration float is currently enhanced by adding either 0,3% sodium hydroxide,

(NaOH), or surface active agents and sometimes proteloytic enzymes. In many cases small amounts of bactericides are added.

The addition of NaOH has to made properly. The purpose is only to adjust the float to a slightly alkaline pH. Addition of too much alkali and/or poor mixing may result in hair immunization. Elevated temperatures up to 32 °C speed up the soaking. The warmth helps to make the fiber structure more flexible however water uptake is somewhat less than it would be at 15 °C.