cells of the hair follicle and the cells of the Malpighian layer is found to be more
efficient, and it happens without disturbing the native state of skin. The loosening of
hair and its removal from the skin are accomplished after swelling and subsequent
protease attack on the outermost sheath leading to breakdown of the inner root sheath
(parts of the hair that are not keratinized). In enzymatic dehairing, the epidermal layer
is removed from the dermis without damaging the hair and the grain layer because
they are easily degraded and dissolved. Another component of raw skin (or hide) is
the subcutaneous layer (connective tissue and fat) which is removed by fleshing. That
layer, however, is of no relevance to dehairing except when applying the painting
method [43]. Application of proteases facilitates faster dehairing process without
damage to the fibrous collagen and hair or wool. It is worth noting that fully
developed keratin in hair, nails and the upper part of the epidermal layer is highly
resistant to chemical or biological attack, due to stable disulphide bonds (–S–S–),
except from sulphide which breaks down the disulphide bonds.
Enzyme concentration, pH and temperature also play a significant role in enzymatic dehairing. pH range of 9.0–12.5 provides optimal conditions for alkaline
protease activity and effectively aids in skin swelling and loosening of hair and
removal. Dehairing is accomplished between 3 and 12 h when temperature range is
30–40
C. At temperature below 30
C, the duration of enzyme exposure and concentration needs to be increased for complete dehairing [15]. Further, below 15
C no
appreciable dehairing within practicable time is observed. Studies have further
shown that about 2% (w/w) of crude enzyme was sufficient for dehairing but 3%
(w/w) of the enzyme was preferred because at this concentration even the tough hair
at the neck areas was completely eliminated [44].
Enzymatic hair loosening and removal provide the necessary conditions for hair,
wool, fur and scales recovery while still in good condition. Techniques commonly
employed in enzymatic dehairing processes include paint, dip and spray methods. In
the paint method, the enzyme solution is mixed with an inert substance like kaolin; a
thin paste is made after adjusting to the prerequisite pH before applying on the flesh
side of hides or skins. The skins are then piled, covered with polythene sheets and
kept till dehairing takes place. In the dip method, the hides or skins are submerged in
the enzyme solution at the right temperature and pH in a pit or rotating drum till all
hair loosen and fall off [3]. In the spray method, a concentrated solution of enzyme is
sprayed on the flesh side at a high pressure forcing its entry into the skin [3]. Difficult
areas to dehair such as backbone and neck can be sprayed with more amount of
enzyme, thereby accelerating the process. After dehairing, the enzyme may be
reused in the second or third pile of skin or incorporated in the soaking solution
till its activity diminishes. The major benefits of enzymatic dehairing include:
• Remarkable reduction or even total elimination of toxic sodium sulphide in
dehairing.
• Applying enzyme can reduce the total pollution load of a tannery by over 80%.
• The process makes it possible to recover valuable hair, wool, fur and scales while
still in good saleable condition.
Alkaliphilic Enzymes and Their Application in Novel Leather Processing. . .
209
efficient, and it happens without disturbing the native state of skin. The loosening of
hair and its removal from the skin are accomplished after swelling and subsequent
protease attack on the outermost sheath leading to breakdown of the inner root sheath
(parts of the hair that are not keratinized). In enzymatic dehairing, the epidermal layer
is removed from the dermis without damaging the hair and the grain layer because
they are easily degraded and dissolved. Another component of raw skin (or hide) is
the subcutaneous layer (connective tissue and fat) which is removed by fleshing. That
layer, however, is of no relevance to dehairing except when applying the painting
method [43]. Application of proteases facilitates faster dehairing process without
damage to the fibrous collagen and hair or wool. It is worth noting that fully
developed keratin in hair, nails and the upper part of the epidermal layer is highly
resistant to chemical or biological attack, due to stable disulphide bonds (–S–S–),
except from sulphide which breaks down the disulphide bonds.
Enzyme concentration, pH and temperature also play a significant role in enzymatic dehairing. pH range of 9.0–12.5 provides optimal conditions for alkaline
protease activity and effectively aids in skin swelling and loosening of hair and
removal. Dehairing is accomplished between 3 and 12 h when temperature range is
30–40
C. At temperature below 30
C, the duration of enzyme exposure and concentration needs to be increased for complete dehairing [15]. Further, below 15
C no
appreciable dehairing within practicable time is observed. Studies have further
shown that about 2% (w/w) of crude enzyme was sufficient for dehairing but 3%
(w/w) of the enzyme was preferred because at this concentration even the tough hair
at the neck areas was completely eliminated [44].
Enzymatic hair loosening and removal provide the necessary conditions for hair,
wool, fur and scales recovery while still in good condition. Techniques commonly
employed in enzymatic dehairing processes include paint, dip and spray methods. In
the paint method, the enzyme solution is mixed with an inert substance like kaolin; a
thin paste is made after adjusting to the prerequisite pH before applying on the flesh
side of hides or skins. The skins are then piled, covered with polythene sheets and
kept till dehairing takes place. In the dip method, the hides or skins are submerged in
the enzyme solution at the right temperature and pH in a pit or rotating drum till all
hair loosen and fall off [3]. In the spray method, a concentrated solution of enzyme is
sprayed on the flesh side at a high pressure forcing its entry into the skin [3]. Difficult
areas to dehair such as backbone and neck can be sprayed with more amount of
enzyme, thereby accelerating the process. After dehairing, the enzyme may be
reused in the second or third pile of skin or incorporated in the soaking solution
till its activity diminishes. The major benefits of enzymatic dehairing include:
• Remarkable reduction or even total elimination of toxic sodium sulphide in
dehairing.
• Applying enzyme can reduce the total pollution load of a tannery by over 80%.
• The process makes it possible to recover valuable hair, wool, fur and scales while
still in good saleable condition.
Alkaliphilic Enzymes and Their Application in Novel Leather Processing. . .
209