Chemistry Reference
In-Depth Information
Starch slurry
(35% ds; pH 6.5; 40 ppm Ca
2+
)
α
-Amylase
105
o
C, 5 min
Gelatinization
Gelatinized starch
(<1 DE)
95
o
C, 120 min
Liquefaction
Liquefied starch
(>1 DE; 0.3% Glu; 2% Mal; 97.7% oligosac.)
Li
fi d
h
Saccharification
Fungal
α
-amylase
Glucoamylase + pullulanase
pH 5.5; 50 ppm Ca
2+
;
55
o
C, 48 h
pH 4.5; 60
o
C, 72 h
Glucose syrup
(99 DE; 97% Glu;
2% mal; 1% oligosac.)
Maltose syrup
(44 DE; 4% Glu; 56% mal;
28% maltotri; 12% oligosac.)
Fig. 14.4
Schematic overview of the various steps in industrial starch processing.
unfolds quickly and becomes inactive. Through a series of holding tubes the heated slurry is
kept for 5 min at 105
◦
C to ensure a proper gelatinization.
In the next step, the liquefaction (Fig. 14.4), the slurry is flash-cooled to a temperature of
95-100
◦
C at which it is held for 1-2 h. The
B. licheniformis
amylase has a somewhat higher
temperature stability compared with the
B. amyloliquefaciens
enzyme, thus determining
the maximum temperature that can be applied during the liquefaction stage. During the
liquefaction, the thermostable amylase hydrolyzes the
,1-4 linkages in both the amylose and
the amylopectin to produce dextrins. Amylases like Termamyl work by an endo-action, that
is they hydrolyze their substrate internally into smaller fragments. This reduces the amount
of interactions (i.e. hydrogen bridges) with an accompanying reduction of the viscosity.
The liquefaction process is allowed to proceed until the required dextrose equivalent (DE)
value is obtained. The DE value is defined as the number of reducing ends relative to a
pure glucose of the same concentration. Glucose has a DE of 100, while starch has a DE
of near zero. The higher the DE value, the shorter the dextrins present. The maximum DE
that can be obtained using these
Bacillus
amylases is around 40. Usually, the liquefaction
step is stopped when a DE of 8-12 is reached. Prolonged incubation leads to the undesirable
formation of maltulose (4-
O
-D-glucopyranosyl-D-fructose) that is resistant to degradation by
glucoamylase and
α
-amylases.
Depending on the type of product to be produced, the reaction is either stopped or
followed by a saccharification step. In this step, pullulanase, glucoamylase, β-amylase or an
α
α
-amylase are added to further degrade the liquefied starch into maltodextrins, maltose or
glucose syrups. Saccharification is mostly performed as a batch process. The liquefied starch
with a DE of 8-12 is pumped into a large stirred vessel, and after adjusting the pH to 4-5
and the temperature to about 60
◦
C glucoamylase is added. The temperature must be lowered
quickly to avoid retrogradation of the liquefied starch. Due to the pH adjustment the bacterial
amylase used during the liquefaction phase will be inactivated. When the desired DE is
