Stale Cane: The Dextran Problem (1997, Vol. 20 No. 4) - SIA
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Stale Cane: The Dextran Problem (1997, Vol. 20 No. 4)
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STALE CANE: THE DEXTRAN PROBLEM
by Michael Hylton
Staling of sugar cane
(i.e. deterioration) is defined as the loss of recoverable sugar between the
time of burning or cutting and the time of grinding (“kill to mill”).
The loss of recoverable
sugar results in a lower cane price and considerable difficulty in processing
the cane into sugar.
The staling of cane is
caused by a common soil bacteria known as Leuconostoc. As staling
takes place, Leuconostoc bacteria enter the cut ends or via
“wounds” in the cane stalk. The bacteria enter the juice-rich interior and
their activity results in the conversion of sucrose to polysaccharides known as
dextrans.
There is evidence that
dextran formation in cane is greater when cane is burnt and left standing
instead of being cut after burning. Hot, wet and humid weather is ideal for the
development of dextrans.
CANE DETERIORATION
The degree of cane
deterioration is difficult to measure in absolute terms. A number of indicators
are often used: apparent purity, JRCS and dextran levels in cane juice.
Juice Purity
Juice purity under
normal conditions can be a good indicator of cane quality.Its measurement
however, can be affected by the products of cane deterioration such as
dextrans. Dextrans can rotate polarized light more strongly than sucrose itself.
Thus if staleness is measured by changes in juice purity derived from
polarimeter measurements, the cane can appear to be even improving in quality
when it is in reality rapidly losing sucrose.
Recoverable Sugar
The Jamaican Recoverable
Cane Sugar (JRCS) takes into consideration the total components of cane and is
therefore, a good indicator of cane quality although it can be affected by
purity readings.
Dextran Levels
While the level of
dextrans in juice would be the best indicator of the extent of cane
deterioration it is a difficult substance to measure accurately in a timely
manner.
Various methods are used
for the analysis of dextrans. These utilize a property of dextrans that is
different from those of sucrose and invert sugars:-
1. Alcohol Haze method - depends on the insolubility of polysaccharides in alcoholic solutions.
2. Roberts method - combines the alcohol precipitation of polysaccharides with the selective precipitation of dextrans by a copper reagent.
3. Enzymatic technique - makes use of the measurement of viscosity before and after treatment with the enzyme dextranase.
A Rapid Haze test was
developed in Louisiana to screen dextran levels in canes delivered to the
factory. Although not as accurate as the Haze method, it takes a relatively
short time to be completed and indicates trends in juice dextran levels.
Dextran determination by
the Rapid test on cane allowed to stale over a period of 165 hours shows a
rapid increase in dextran levels after 60 hours see Fig. 1.
STALE CANE COST
Stale cane is very
costly to the cane grower because of:
1. Loss of cane weight
2. Lower cane quality
3. Lower cane payment
4. Lower relative factor
5. Reduced quota
Loss of Cane Weight
The loss of cane weight
due to drying out varies with variety, degree of damage and weather conditions.
Experiments carried out in Barbados recorded weight losses averaging 0.75% per
day for green cane and 1.5% for burnt cane, both exposed in windrows in the
field. Assuming similar results in Jamaica, quite apart from sucrose loss, a
farmer could easily lose approximately $16.00/tonne cane/day of staling.
Lower Cane Quality
The quality of a
farmer’s cane is a function of the juice quality and the fibre content. The
quality of cane is determined by measuring the JRCS. As cane stales there is a
loss of sucrose accompanied by an increase in impurities.
Work carried out by SIRI
indicates that within the first three days of cutting or burning there is a
loss of more than 1 unit of purity for each day’s delay before processing,
accelerating thereafter to over 4 units/day by Day 6, Table 1.
Under experimental conditions in the cane yard rate of decline in JRCS was
relatively gradual from 11.67 on Day 1 to 9.41 on Day 6. Experience tells us
however that under field conditions the rate of deterioration is much more
rapid so that by Day 6 a figure much below 9.41 would be expected.
Lower Cane Payment
The price a farmer
receives for cane is based on the average quality of all cane supplied to the
particular factory. Then, his own cane quality is taken into account and
compared with the average quality of all suppliers to that factory at the time
of delivery.
As indicated in Table
1 cane staling experiments have shown JRCS declining by an average of
0.45 unit per day of delay in processing. Using a sugar price of $18 000/tonne
and a JRCS of 10%, a farmer stands to lose approximately $80.00/tc for each
day’s delay. Stale cane will result in low JRCS, hence a low price to the
farmer.
Lower Relative Factor
A farmer’s relative
factor is the ratio of his JRCS to the average JRCS of all suppliers to that
factory. It is intended to ensure that a farmer is not at a disadvantage
because of the time of year in which his cane happens to be delivered. A
relative factor which is less than 1 will result in the farmer receiving a cane
price lower than the average. A farmer who delivers stale cane will usually
receive a relative factor which is below that average as often his JRCS is much
less than the average JRCS of all suppliers.
Reduced Quota
The processing of stale
cane tends to reduce the factory’s throughput and hence the quantity of cane it
can order on a daily basis. Delivering stale cane to the factory therefore
slows the rate at which farmers can deliver their quota thus causing an
extension of the crop. Extension of the crop in turn forces the factory to
operate outside of the period when sugar content of cane is at satisfactory
levels and lowers the overall average cane price to all suppliers.
COST TO MANUFACTURERS
Stale cane not only
lowers cane price to the farmer but also creates several processing problems
for the factory. Some of the areas in which the factory is affected include:
• Reduced grinding rate
• Lower juice purity, higher tc/ts
• Poor clarification
• Increased pan boiling, crystallization and curing times
• Lower sugar quality
• Increased sugar loss in molasses.
Stale cane increases the
cost of processing. Juice extracted from stale cane is usually of a lower than
normal pH. To obtain good clarification, factories have to use increased
quantities of lime resulting in higher chemical cost.
While stale cane affects
many areas of factory operations, one in which it is most felt is the pan
floor. Dextrans in cane juice slow down the boiling process and leads to the
formation of needle-shaped crystals. The poor, irregularly shaped crystals and
the highly viscous molasses require longer than normal centrifugal time for
separation. Increased boiling and centrifugal times lead to a reduction in
grinding rate, a longer cropping period and higher cost of production.
Sugar produced from
stale cane has high dextran levels which attract penalties in our overseas
markets. In 1996 the Industry paid penalties of J$23 million for sugar shipped
to the U.S.A.
Stale cane is a serious
threat. Let us work together to save our industry.