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Glucose Isomerase:
With the development of glucoamylase in the 1940s and 1950s
it became a straightforward matter to produce high percent glucose
syrups. However, these have shortcomings as used in the sweetener
industry. D-glucose has only about 70% of the sweetness of sucrose,
on a weight basis, and is comparatively insoluble. Fructose is
30% sweeter than sucrose, on a weight basis, and twice as soluble
as glucose at low temperatures so a 50% conversion of glucose
to fructose overcomes both problems giving a stable syrup that
is as sweet as a sucrose solution of the same concentration.
One of the triumphs of enzyme technology so far has been the
development of 'glucose isomerase', which in turn led to the
commercialization of high fructose corn syrups. Now it is known
that several types of bacteria, can produce such glucose isomerases.
They are resistant to thermal denaturation and will act at very
high substrate concentrations, which have the additional benefit
of substantially stabilizing the enzymes at higher operational
temperatures. The vast majority of glucose isomerases are retained
within the cells that produce them but need not be separated
and purified before use.
All glucose isomerases are used in immobilised forms. Although
differerent immobilisation methods have been used for enzymes
from differerent organisms, the principles of use are very similar.
The corn syrup is then converted to fructose in a batch process
to make 42% fructose syrup.
For many purposes a 42% fructose syrup is perfectly satisfactory
for use but it does not match the exacting criteria of the quality
soft drink manufacturers as a replacement for sucrose in acidic
soft drinks. For use in the better colas, 55% fructose is required.
This is produced by using vast chromatographic columns of zeolites
or the calcium salts of cation exchange resins to adsorb and
separate the fructose from the other components.
High fructose corn syrups are classified according to the
fructose content (i.e. 42%, 55%, 90%).
Sweetness Scale and other Sweetners
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