DEVELOPMENT OF AN ALTERNATIVE METHOD FOR
DETERMINING DEXTRAN IN PROCESS MATERIAL FOR
ACP SUGAR PRODUCING COUNTRIES
Earle V. Roberts
Sugar Industry Research Institute
of the
Sugar Industry Authority of Jamaica
Background
The sugar cane industries of ACP countries face several
challenges to maintain access to existing international markets,
especially in Europe and the USA. as a result of the decision by
the EU to implement changes to the Sugar Protocol which
governs trading arrangements between the two groups of
countries. Although the most serious of these was the reduction in
the price of raw sugar paid to ACP countries by the EU, the need
to meet increasingly rigorous specifications in quality demanded
by sugar refiners in the EU (defined as “fair average quality”)
poses similar difficulties.
There are several parameters which are embodied in the
designation of fair average quality:
Polarisation (a measure of the
Amount of sucrose present)
Safety Factor (Moisture)
% Ash
Grain size
Colour (Whole Raw)
Colour (Affined Raw)
Dextran
98.0˚
(minimum)
0.30%
(maximum)
0.35%
(maximum)
8%
(maximum)
on 355micron screen
4500 I.U. (maximum)
1200 I.U. (maximum)
200 m.a.u. (maximum)
Acceptable sugar quality is largely dependent on the quality of
the raw material (the sugar cane) received for processing.
One of the impurities present in raw sugar is dextran, a
polysaccharide produced by the action of bacteria, in particular
Leuconostoc mesenteroides, which is present in soils.
The higher temperatures associated with the growth of sugar
cane under tropical conditions favour bacterial growth.
Consequently, dextran formation poses a more serious
problem for sugar cane producers than for beet growers.
Dextran repeating unit
The presence of Dextran in raw sugar is a major concern to the
sugar cane industry for several reasons, e.g:
-
cane growers are paid by the miller for their canes based
on the amount of sugar (sucrose) present and dextran
inflates the estimation of the amount of sucrosepresent. If
there are high levels of dextran present in the juice
extracted from the cane, then the grower is paid for sugar
that is not present with consequent loss in revenue to the
miller.
-
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.
-
because it is a polymer, dextran 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.
-
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 low 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 specificto 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 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.
The method, if successfully developed,
-
would be useful for identifying and monitoring the
occurrence and/or development of dextran in sugar
processing operations and could result in
considerable reduction in production costs.
-
would permit the implementation of corrections to
current payment systems for cane where the
presence of dextran causes inflated polarimetric
readings resulting in overpayment for contaminated
cane.
More importantly, it will allow processors to institute quality control
measures to minimise the levels of dextran in raw sugar exported to
refiners thus eliminating or at least reducing the possibility of
shipments being rejected or being subjected to penalties for
containing high levels of dextran. The elimination and/or reduction
of dextran in raw sugar exports would also increase the fair
average quality demanded by refiners, paving the way for payment
of increased premiums for improved quality.
In addition, NIR polarimetry has the advantage that, unlike
polarimetric methods currently used in sugar laboratories, it does
not require the use of clarification reagents, some of which are toxic
(basic lead acetate) or expensive (Octapol). The successful
development and implementation of the DASA method would have
clear operational cost benefits (reduction of laboratory expenses)
as well as environmental benefits (reduction of health hazards and
of disposal problems associated with toxic chemicals).
The successful development and implementation of the
DASA method would have clear operational cost benefits
(reduction of laboratory expenses) as well as environmental
benefits (reduction of health hazards and of disposal
problems associated with toxic chemicals).
It will be necessary to validate both the DASA method and the
analytical results determined for dextran in cane juice and raw sugar
against an accepted reference analytical method.
It is proposed to use as a reference method the method developed
by P. G. Morel du Boil, Proc. S. Afr. Sug. Technol. Ass., 74, 317-327,
(2000), which is based on the use of enzyme hydrolysis with
dextranase followed by High Performance Anion Exchange
Chromatography with Pulsed Amperometric Detection (HPAEC-PAD).
The method gives accurate, reliable and reproducible results but is
too expensive and time consuming for routine use.
Finally, given the general nature of the problem of dextran control
in sugar cane production, it was recommended that the project be
given some regional character by including other participating ACP
institutions as collaborating Project Executing Agencies in the project.
The laboratories of the Belize Sugar Industries Ltd. (BSIL), Belize and
GUYSUCO Sugar Research Institute, Guyana were invited to participate
in the project.
An overall budget of €1,186,258 has been approved for the project.
Project Personnel:
Technical:
1 Senior Investigator (Senior Scientific Officer), SIRI, Jamaica
1 Investigator (Scientific Officer), SIRI, Jamaica
1 Investigator (Scientific Officer), Belize
1 Investigator (Scientific Officer), Guyana
Administrative Support Staff, SIRI, Jamaica
1 Biometrician for Statistical Analysis, Part Time (5 Months).
It is proposed to implement the project in January 2012 and it
will have a life time of 3 years.
Activities will undertaken in the first year, 2012 - 2013 at SIRI,
Jamaica only.
If the results obtained indicate feasibility of the DASA method
for routine dextran determination and initial validation of the
method is achieved, activities will then be extended to the
other Executing Agencies in Belize and Guyana (2013 - 2015).
Activity
2012
2013
2014
Year
Period
I
II
III
IV
V
VI
VII
VIII
Phase 1, Jamaica only
1.1 Appointment of Technical Staff,
1.2 .Preparation of Laboratory Area
---------------
2.1 Training of Technicians
----------
3.1 Collection and Analysis of Samples
----------
-----------
3.2 Technical Services for Raw Sugar Analysis
-----------
------------
3.3 Laboratory Services for testing and
validating analyses
3.4 Evaluation and assessment of results
-----------
----------
------------
-----------
------------
3.5 Additional Analyses
------------
Phase 2, Jamaica, Belize, Guyana
2.1 Training of Technicians in Belize, Guyana
-------------
3.1 Collection and Analysis of Samples
------------
-----------------
3.2 Technical Services for Raw Sugar Analysis
-------------
---------------
3.3 Laboratory Services for testing and
validating analyses
3.4 Final Report
3.5 Capacity Building
--------------
---------------------------------
IX
Current Status:
1. Human Resources:
1.1.1.
Senior Technical Investigator appointed, 01/02/12
Technical Investigator appointed,
01/02/12.
3. Equipment and Supplies:
3.3.3. Dextran Analysis System, Jamaica:
Pro forma Invoice invited from Optical Activity, UK. 12/03/12
3.3.7. High Pressure Anion Exchange Chromatography (HPAEC) System
European Manufacturers of Equipment not yet identified
3.5.3.Laboratory Area
to house Equipment:
Construction Plans prepared and approved by
Manchester Parish Council.
Tender document prepared and applications
for construction invited.
Final date for submission – April 13, 2012.
Preliminary Studies:
Good Correlation obtained between Pol values for
sucrose solutions with varying dextran levels and dextran
values measured on a DASA system (borrowed).