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.
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 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.
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.
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:-
Alcohol Haze method - depends on the insolubility of polysaccharides in alcoholic solutions.
Roberts method - combines the alcohol precipitation of polysaccharides with the selective precipitation of dextrans by a copper reagent.
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:
Loss of cane weight
Lower cane quality
Lower cane payment
Lower relative factor
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.
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