7.2.5 Deliming
In enzymatic leather processing, this stage is eliminated since no lime is used.
However, lowering of the pH of the hide/skin by soaking in acidified water is
necessary since proteolytic dehairing is commonly done under alkaline environment.
Lowering of pH is critical in preparing of the pelt for the subsequent operation since
alkalinity has detrimental effect on the tanning process. The conventional deliming
process is executed to eradicate the liming agent from the pelts by lowering the pH to
between 8 and 9 [38]. The process involves gradual lowering of the pH using weak
acidic solutions, fresh water or ammonium chloride salt. Acidification liquids which
may still contain sulphide generate poisonous hydrogen sulphide gas. The process
may also involve increase in temperature, removal of residual chemicals and
degraded skin components. The quality of the final leather determines the extent
of deliming. After deliming, the hides and skins are ready for vegetable tanning.
Chrome tanning requires delimed hides and skins to be further processed through
pickling and bating [5]. Once the skins and hides have been delimed, they must be
taken to the next process immediately since removal of alkali provides favourable
conditions for putrefying bacteria to thrive.
7.2.6 Bating
Application of protease in dehairing eliminates bating process since the hair roots are
fully removed. However, the traditional chemical dehairing method leaves the skin
surface with some attached hair roots and epidermal pigments which are undesirable
for certain types of leather. Removal of these remnants is effected by the bating
process. In the current bating process, commercially available proteolytic enzymes
are used. Bating is stopped by lowering the temperature and pH and diluting the
enzyme solution when the appropriate level of softening is achieved. This assessment is hard to standardize although it is based on the empirical evaluation by
experienced personnel [1].
7.3 Tanyard Operations
The tanyard operations are commonly carried out in a section of the plant known as
the tanyard. The operations are often carried out in the same processing vessels, with
changes of float and chemicals. In chromium tanning, the vessels are usually rotating
drums.
Alkaliphilic Enzymes and Their Application in Novel Leather Processing. . .
211
In enzymatic leather processing, this stage is eliminated since no lime is used.
However, lowering of the pH of the hide/skin by soaking in acidified water is
necessary since proteolytic dehairing is commonly done under alkaline environment.
Lowering of pH is critical in preparing of the pelt for the subsequent operation since
alkalinity has detrimental effect on the tanning process. The conventional deliming
process is executed to eradicate the liming agent from the pelts by lowering the pH to
between 8 and 9 [38]. The process involves gradual lowering of the pH using weak
acidic solutions, fresh water or ammonium chloride salt. Acidification liquids which
may still contain sulphide generate poisonous hydrogen sulphide gas. The process
may also involve increase in temperature, removal of residual chemicals and
degraded skin components. The quality of the final leather determines the extent
of deliming. After deliming, the hides and skins are ready for vegetable tanning.
Chrome tanning requires delimed hides and skins to be further processed through
pickling and bating [5]. Once the skins and hides have been delimed, they must be
taken to the next process immediately since removal of alkali provides favourable
conditions for putrefying bacteria to thrive.
7.2.6 Bating
Application of protease in dehairing eliminates bating process since the hair roots are
fully removed. However, the traditional chemical dehairing method leaves the skin
surface with some attached hair roots and epidermal pigments which are undesirable
for certain types of leather. Removal of these remnants is effected by the bating
process. In the current bating process, commercially available proteolytic enzymes
are used. Bating is stopped by lowering the temperature and pH and diluting the
enzyme solution when the appropriate level of softening is achieved. This assessment is hard to standardize although it is based on the empirical evaluation by
experienced personnel [1].
7.3 Tanyard Operations
The tanyard operations are commonly carried out in a section of the plant known as
the tanyard. The operations are often carried out in the same processing vessels, with
changes of float and chemicals. In chromium tanning, the vessels are usually rotating
drums.
Alkaliphilic Enzymes and Their Application in Novel Leather Processing. . .
211