include α-amylases [98, 99], β-amylases [100], maltogenic amylases [101], branching
enzymes [102], debranching enzymes [103] and amyloglucosidases [104]. These may
be used separately or in combination [103]. Maltooligosaccharide-forming amylases
have also been shown to reduce the staling of bread as they produce maltotriose
(G3), maltotetraose (G4) and maltopentaose (G5), which retain water molecules,
thereby preventing starch-starch interactions and inhibiting starch recrystallization
[105, 106]. However, excessive use of α-amylase results in stickiness in bread [101].
This is circumvented by the use of intermediate temperature-stable (ITS) α-amylases,
which are inactivated before the completion of the baking process [10].
4.5 Alkaline-Active Raw Starch-Degrading Amylases
for Pulp and Paper Industry
In the pulp and paper industry, the primary role of amylases is in the production of
suitable starch for coating and surface sizing of the paper. It is important at this point
to distinguish between internal sizing and surface sizing during the production of
paper. Internal sizing refers to the process of adding hydrophobic substances to
the pulp slurry in order to produce paper that is fluid resistant, while surface sizing
involves the application of a viscous solution to the surface of the paper [107]. Paper
sizing is primarily done to protect the paper against mechanical damage, enhance the
stiffness and strength of the paper and improve the erasability of the paper [10].
Earlier, internal sizing was conducted at acidic conditions using rosin- and
aluminium-based compounds. Since the 1980s, there has been a significant shift
towards using sizing chemicals such as alkylketene dimer (AKD) and alkenylsuccinic
anhydride (ASA) at alkaline conditions [107]. This is because paper produced from
sizing at acidic conditions was found to deteriorate very rapidly, while at alkaline
conditions, a more durable product is produced.
For surface sizing and coating of paper, starch has been found particularly
useful, and the process is well described in a review by Gupta et al. [10]. Starch is
added to the paper in the size press, and the paper picks up the starch by passing
through two rollers that transfer the starch slurry. The temperature of this process lies
in the range of 45–60
C. A constant viscosity of the starch is required for reproducible results at this stage. The viscosity of the raw or native starch is too high
for paper sizing and is adjusted either chemically or by partially degrading
the polymer with α-amylases in a batch or continuous process. The hydrolysis
conditions depend on the source of starch and the α-amylase used. Since alkaline
conditions are preferred for the internal sizing process, it is therefore advantageous to use alkaline-active amylases for the production of starch with suitable
viscosity. Furthermore, α-amylases that are capable of hydrolysing raw starch
are preferred, as this will reduce production costs by avoiding expensive chemically
modified starches. Raw starch-degrading amylases have been reported from
alkaliphiles. The enzyme from Bacillus sp. IMD 435 was reported to hydrolyse
Starch-Modifying Enzymes
235
enzymes [102], debranching enzymes [103] and amyloglucosidases [104]. These may
be used separately or in combination [103]. Maltooligosaccharide-forming amylases
have also been shown to reduce the staling of bread as they produce maltotriose
(G3), maltotetraose (G4) and maltopentaose (G5), which retain water molecules,
thereby preventing starch-starch interactions and inhibiting starch recrystallization
[105, 106]. However, excessive use of α-amylase results in stickiness in bread [101].
This is circumvented by the use of intermediate temperature-stable (ITS) α-amylases,
which are inactivated before the completion of the baking process [10].
4.5 Alkaline-Active Raw Starch-Degrading Amylases
for Pulp and Paper Industry
In the pulp and paper industry, the primary role of amylases is in the production of
suitable starch for coating and surface sizing of the paper. It is important at this point
to distinguish between internal sizing and surface sizing during the production of
paper. Internal sizing refers to the process of adding hydrophobic substances to
the pulp slurry in order to produce paper that is fluid resistant, while surface sizing
involves the application of a viscous solution to the surface of the paper [107]. Paper
sizing is primarily done to protect the paper against mechanical damage, enhance the
stiffness and strength of the paper and improve the erasability of the paper [10].
Earlier, internal sizing was conducted at acidic conditions using rosin- and
aluminium-based compounds. Since the 1980s, there has been a significant shift
towards using sizing chemicals such as alkylketene dimer (AKD) and alkenylsuccinic
anhydride (ASA) at alkaline conditions [107]. This is because paper produced from
sizing at acidic conditions was found to deteriorate very rapidly, while at alkaline
conditions, a more durable product is produced.
For surface sizing and coating of paper, starch has been found particularly
useful, and the process is well described in a review by Gupta et al. [10]. Starch is
added to the paper in the size press, and the paper picks up the starch by passing
through two rollers that transfer the starch slurry. The temperature of this process lies
in the range of 45–60
C. A constant viscosity of the starch is required for reproducible results at this stage. The viscosity of the raw or native starch is too high
for paper sizing and is adjusted either chemically or by partially degrading
the polymer with α-amylases in a batch or continuous process. The hydrolysis
conditions depend on the source of starch and the α-amylase used. Since alkaline
conditions are preferred for the internal sizing process, it is therefore advantageous to use alkaline-active amylases for the production of starch with suitable
viscosity. Furthermore, α-amylases that are capable of hydrolysing raw starch
are preferred, as this will reduce production costs by avoiding expensive chemically
modified starches. Raw starch-degrading amylases have been reported from
alkaliphiles. The enzyme from Bacillus sp. IMD 435 was reported to hydrolyse
Starch-Modifying Enzymes
235