Analisis Instrumental

Singleton et al.: A New Polarimetric Method for the Analysis of Dextran and Sucrose

A NEW POLARIMETRIC METHOD FOR THE ANALYSIS OF
DEXTRAN AND SUCROSE
Victoria Singleton1, 2, Dr. Jennifer Horn1, Prof. Chris Bucke 2 and Dr. Max Adlard2
1. Optical Activity Ltd. Cambridgeshire, England.
2. University of Westminster, London, England.
ABSTRACT
A new method for dextran quantification has beendeveloped and field-trialled in Jamaica, in
association with the Sugar Industry Research Institute. The method uses a near infrared (NIR)
polarimeter and a specific dextranase. The dextranase selectively breaks down the dextran into sugars
of lesser specific rotations without affecting any other substance present in the juice. The initial dextran
concentration is derived from the calibrationcurve of the change in observed optical rotation (OR) due
to enzymatic hydrolysis and output automatically by the polarimeter. Readings are not affected by the
molecular weight of the dextrans, the entire procedure takes less than 10 minutes to perform and it is
semi-automated. Use of a NIR polarimeter negates the need for lead acetate clarification. The method is
suitable for both juice and rawsugar samples.
Keywords: Dextranase, Near Infrared (NIR) polarimeter, Polysaccharides.
INTRODUCTION
Dextran is produced by microorganisms which infect the cane and feed on the sucrose;
therefore, the presence of dextran immediately indicates lost sugar. The bacteria are mainly
Leuconostoc species and are ubiquitous in the soil. They enter the cane at places of exposed tissue
caused bymachine harvesting, cutting, burning, growth, freezing, disease and pests. Any delay in the
kill-to-mill time allows the bacteria to proliferate and the dextran levels to soar, especially in wet muddy
cane.
The name dextran refers to a large family of glucose polymers whose structures and subsequent
properties can vary widely. Technically the molecular weight (Mr) can range between 1500 and severalmillion; therefore, a dextran of say 1 million Mr has potentially thousands of possible structures due to its
branched nature. This massive variation in structure poses a huge challenge for any analyst trying to
detect the molecules especially against a substantial background of saccharides with similar structures
and properties.
Consequences of Dextran
Dextran is highly dextrorotatory,approximately three times that of sucrose, and, since the
farmer is largely paid on the basis of the polarimeter reading, there is an obvious need for assaying for
dextran in the core lab. This would allow correction of the falsified reading and identification of the
sources of dextran contamination entering the factory. The problems associated with dextran
contamination in both the factory andthe refinery are well documented in the literature and so are briefly
summarised below in Table 1.

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Journal American Society of Sugarcane Technologists, Vol. 22, 2002

Table 1. Summary of the detrimental effects of dextran in terms of the resulting losses.
Production losses
Increased viscosity leads to
reduced throughput due to:
-poor filterability
-reduced evaporation rate-reduced flocculation rate
-slow mud settling

Sucrose losses
As dextran formed in cane

Direct financial losses
False pol reading leads to
overpayment to farmer

To molasses (melassigenic effect)
In trade of raw sugar as
part of dextran penalty
system using unreliable
tests

Poor crystallization (elongation)

Most dextrans are insoluble in alcohol making sugars and syrups containingit unsuitable for the
production of alcoholic beverages. The two most important factors in the purchase of raw sugar are the
polarisation and the crystal size distribution. Both of these are dramatically affected by the presence of
dextran. The affination rate (removal of molasses from the crystal surfaces) is greatly reduced, leading to
further losses of sucrose to the molasses. It is for...