SUGAR INDUSTRY RESEARCH INSTITUTE
OF THE
SUGAR INDUSTRY AUTHORITY OF JAMAICA
DEVELOPING AND INTRODUCING AN ALTERNATIVE
METHOD
TO DETERMINE
DEXTRAN IN SUGARCANE JUICE AND RAW SUGAR
GRANT CONTRACT 2010 / 247-866
EXTERNAL ACTIONS OF THE EUROPEAN UNION
Dextran –
Generic name for a group of high molecular weight polymers
(Av. M ~ 5 x106 Da)
Produced by the enzyme dextransucrase secreted by bacteria esp.
Leuconostoc mesenteroides.
Enzyme hydrolyses sucrose to fructose and glucose. Fructose is used by the
organism and the remaining glucose fragment is converted to various
polymers.
Polymers contain α – 1,6 linkages predominantly but also elaborate various
side chains linked to the main chain through α -1,2, α -1,3 or α -1,4 linkages.
Dextrans from deteriorated cane or from sugars produced from deteriorated
cane show high molecular weight range (M ~ 20 – 25 x 106 Da).
main chain
1,6-linkage
side chain
1,3-linkage
Dextran fragment
Whereas starch and cellulose are polysaccharides invariably present
in sugarcane, the presence of dextrans is a post harvest phenomenon
of sugarcane deterioration due to environmental conditions,
harvesting methods, kill-to-mill delays, bacterial infection.
Dextrans of differing molecular mass are reported to influence
processing operations differently in conversion of sugar cane
juice to sugar.
Dextran slows the rate of crystallization of raw sugar, leads to the
formation of irregularly shaped crystals and decreases the
exhaustion of molasses, i.e. it reduces the recovery of sucrose
by “dragging” sucrose (possibly equal to its weight) into the
molasses mother liquor. Its presence thus results invariably in
increased production costs.
Dextran, because it is a polymer, increases the viscosity of sugar
solutions, frequently slowing the passage of sugar solutions
through the refining process to a halt; this results in significant
losses of production time with corresponding increases in
production costs.
Thus, the presence of Dextran in cane juice is a major concern to
the sugar cane industry. For example, if cane growers are paid by
the miller for their canes based on the amount of sugar (sucrose)
present, dextran inflates the estimation of the amount of sucrose
present.
If there are high levels of dextran present in the juice extracted
from the cane, then:
1. The grower is paid for sugar that is not present with consequent
loss in revenue to the miller.
2. Raw sugar suppliers receive lower premiums or, as is the case
in U.S. markets, suffer penalties for supplying raw sugar
containing dextran at levels greater than an acceptable
minimum.
A method of determining the level of dextran present in cane juice,
process materials (syrups, molasses, raw sugar, etc.) is therefore
required. This would allow processors to monitor dextran levels
and, where possible, take corrective action to minimize its impact
on the quality of sugar delivered to our international markets. Such
a method must satisfy the following criteria:
(i)
(ii)
(iii)
(iv)
It must be rapid and quantifiable.
It must be accurate, reliable and reproducible.
It must be able to process and analyse within a reasonable
time frame the large number of samples required for
representative acceptance.
The equipment required should be robust and manageable
by intermediate level technical staff.
Several methods of dextran analysis have been published in the technical and
scientific literature but they do not show all of the above characteristics.
Those currently employed in the sugar industry are listed below:
1.
2.
3.
4.
5.
Haze test for sugars
Not specific to dextrans; repeatable but not accurate for juice samples
SCGC (Belle Glade Rapid Test)
Modified for juices but not specific to dextrans.
Roberts Copper Test
Specific to dextrans but technically difficult and time consuming (2.5 hr).
Rapid Test (SPRI)
Not specific to dextrans, not reproducible.
Midland MCA Sucrotest
Specific to dextrans; not reliable, prohibitively expensive for routine
analysis.
A method developed by a U.K. based firm, Optical Activity Ltd., in
collaboration with the University of Westminster, London, Dextran and
Sucrose Analysis (DASA), underwent preliminary field trials by the
Sugar Industry Research Institute, Jamaica. Results indicated that the
method had the potential to satisfy the criteria stated above but it
required further refinement and evaluation.
The method, based on Near Infra Red (NIR) polarimetry, evolved
from experimental studies which explored the possibility of monitoring
enzyme-mediated reactions (e.g. the hydrolysis of polysaccharides
such as dextran by substrate-specific enzymes such as dextranase)
using polarimetric techniques.
Dextran is an optically active dextro-rotatory compound.
Hydrolysis of dextran by acids or enzymes converts it to simpler
compounds (oligo- and monosaccharides) having polarimetric
properties different to those of the original dextran.
Thus, changes in dextran concentration can be monitored by
following the changes in the polarimetric properties of solutions
containing it when these solutions are treated with dextran-specific
enzymes and these changes can be measured and quantified by NIR
polarimetry.
Preliminary results:
Studies have focussed on determining the precision and accuracy of the
method at various dextran levels ranging from 100ppm – 5000ppm in sugar
solutions (26%).
Coefficient of variation,CV, (relative standard deviation) calculations indicate
that results obtained for dextran concentrations across the range 600ppm –
5000ppm are acceptable.
CV vs [Dex]/ppm
50
coefficient of variation (CV)
45
40
35
30
25
20
15
10
5
0
100
150
200
400
600
800
[Dex]/ppm
1000
1500
2000
4000
5000
Preliminary Results:
Cane juice samples
Cane juice samples have been collected from the main cane
growing areas across the industry.
Dextran values ranging from ~60ppm – 8000ppm were
recorded.
(Percentage accuracy of detection of dextran levels below
100ppm is low – instrument limitation)
Current activities:
Number of refinements to experimental procedures being explored:
- Reduction in the reaction time for dextran hydrolysis (~ 15 minutes)
to ~ 5 - 6 minutes to be in line with time needed for pol determination
on samples.
- Determining the stability and activity of the dextranase currently
used.
- Investigating the availability of dextranases with higher activity.
Begin dextran analyses of syrup and molasses samples.
Preparation of laboratory space to receive equipment on order (DASA
System and HPLC Equipment )
Acknowledgements
The Institute wishes to acknowledge the assistance of the
European Commission and that of the ACP Sugar Research Group
for the provision of financial support for the project and the ACPEU Coordinating Unit for its continuing administrative support in
facilitating the project.