Enzymes for Starch
In the early 19th century, it was discovered that by boiling
starch with acid it could be converted into a sweettasting
substance, which consisted mainly of glucose. Since then acids
have been used widely for breaking down starch into glucose. This
technique does have a number of drawbacks.
The DE (dextrose equivalent) value is used as an indication of
the degree of hydrolysis of syrup. The DE value of starch is zero
and that of dextrose is 100. In the last 25 years, as new enzymes
are available, starch hydrolysis technology has move away from
acids to enzymes.
Enzymatic starch conversion, depending on the enzymes used, syrups
with different compositions and physical properties of starch.
There are three basic steps in enzymatic starch conversion: liquefaction,
saccharification and isomerisation.
Firstly, there is a liquefaction process. A starch suspension
containing 30-40% dry matter is first gelatinised and liquefied.
By using heat-stable bacterial alpha amylase, 'maltodextrin' is
obtained which contains mainly different oligosaccharides and
dextrins. Maltodextrins are only slightly sweet and they usually
undergo further conversion.
In most starch conversion plants, starch liquefaction takes place
in a jet-cooking process. The heat stable alpha amylase is added
to the starch slurry after pH adjustment, and the slurry is pumped
through a jet cooker. Live steam is injected here to raise the
temperature to 105?C, and the slurry is then passed through a
series of holding tubes for 5-7 minutes, which is necessary to
gelatinise the starch fully.
Then the temperature of the partially liquefied starch is reduced
to 90-100?C by flashing, and the enzyme is allowed to react further
at this temperature for 1-2 hours until the required DE (Dextrose
Equivalent) is obtained.
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Saccharification is the second step in the process. Depending
of the desired end product, a glucoamylase or a fungal alpha amylase
is used further break down the Maltodextrins. The glucoamylase
can hydrolyse starch completely to glucose along with, a little
maltose and isomaltose. A pullulanase is a de-branching enzyme
that can also be used to aid saccharification. Fungal alpha amylases
can also be added in order to produce syrups with a higher maltose
content, which means high fermentability and a relatively high
degree of sweetness.
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Further going one step ahead, a proportion of the glucose can
be isomerised into fructose, which is about twice as sweet as
glucose. An immobilized glucose isomerase is used.
Maps offers a range of amylases and glucoamylase for starch conversion
depending on the desired end product.
||Heat-stable alpha amylase for starch liquefaction
||Alpha amylase for starch liquefaction
||Glucoamylase for starch saccharification
||Fungal alpha amylase for starch saccharification