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Effects of Dextran and Starch in Sugar Refinery

This article provides an overview of the adverse role of Dextran and Starch in the sugar production process, including their effects on the process house and measures to control them.

Dextran

Leuconostoc mesenteroides is a type of lactic acid bacteria that can break down sucrose into dextrose and levulose through a biological process. Dextrose and Levulose are monosaccharide isomers of Glucose and Fructose. Leuconostoc mesenteroides produces an enzyme called dextran sucrase during the break-down which causes the polymerization of dextrose into a polysaccharide called dextran.

C12H22O11 => C6H12O6+C6H12O6 => (C6H12O6)n

Sucrose => Dextrose + Levulose => Dextran

Dextran isn’t a specific molecule but rather a group of long chain polysaccharides which have a varying range of molecular weight from 2000 to 20000. Dextran is formed when many glucoses is attached together at ɑ 1,6 by glycosidiclinkages. It may also have branches at ɑ 1,2 or ɑ 1,4 or ɑ 1,4. Generally, it has more than 50% ɑ 1,6 linkages.

Image Source: Wikipedia

Dextran can form either during transportation and/or storage of raw sugar or in sugarcane itself during plantation. Sugarcane growth time may vary from 3 to 8 months depending on geo-location and sun-time. Longer growth time sugarcane is much more prone to dextran formation than short growth time plants. And there is chance of dextran formation if crops collection is delayed as it may form in mature sugarcane. And sugar industries try to maintain the safety factor of 0.25 or less for storage and transportation of raw sugar to prohibit dextran formation from happening. A small amount of moisture content (water) may reduce the protective film of molasses around the crystal to a dangerously low density, permitting microorganisms to penetrate sugar crystals themselves and start deterioration. Sugar refining industries use water content to non-sugar content ratio to calculate the safety factors.

SF= (W*100)/ NS = (W*100)/ (100-(S+W))

Where W is water, NS is non-sucrose and S is sucrose

In mature sugarcane, lactic acid bacteria (Leuconostoc mesenteroides and Streptococcus) may infect the exposed areas of the plant and start degrading sucrose into glucose. Dextran formation during plantation is affected by external factors like temperature, interval between crop collection and crushing in the mill and humidity. Dextran density in the plant implies the sugar quality can be obtained from the cane. Dextran gets extracted along with the juice and vitiate the raw sugar and poses adverse effect in sugar refining process.

Low Dextran formation doesn’t have any significant effect on raw sugar processing. Less than 2 mg/kg of dry solids are considered low dextran presence. To form every molecule of dextran, 4 molecules of sucrose are degraded. We can say, 0.25 mg/kg brix sugar loss from liquor is due to 1 mg/kg brix dextran yield. If we consider large scale, 1% dextran formation for every ton of sugar, we lose 0.25% of sucrose. For every 10000-ppm dextran formation we lose 1% of sucrose. Dextan is an extremely dextro rotatory substance, at least 3 times that of sucrose. So, it has an adverse effect on polarization determination. So, during testing in lab, lead agent is used to dissipate the rotatory effect to determine sugar Pol.

Effects of Dextran in Refinery

  1. Dextran is gelatinous in nature and thus its presence makes liquor viscous and makes subsequent clarification process inefficient.
  2. Dextran also makes it difficult to maintain pH. This can be problematic for reactions in carbonation as maintaining pH is an important parameter at this stage.
  3. Dextran presence has negative impact on heat exchange at Carbonation, Evaporation and Pan boiling stage as it reduces heat transfer capability of liquor.
  4. It makes pan boiling inefficient and can cause false graining and take much longer time than usual to finish a strike.
  5. Which causes extremely unwanted poor centrifuging rate as much more water and time is required after each batch is dropped from centrifugal to wash and prepare for the next feeding. Thus, it affects production capacity by reducing centrifuging capability.
  6. Because of being Gelatinous in nature, Dextran presence in centrifuged sugar tends to stick together which makes it difficult to dry. It may even form lump and a well percentage of refined sugar lump can be separated in screener and sent back for processing.  Which makes the process uncomical.

Starch

Starch is a polysaccharide formed when many sugar molecules are attached together in a long chain. In case of the number of sugar molecules in stach chain, its basic chemical formula is (C6H10O5)n. Stach is a 20:80 combination of Amylose and Amylopectin. Both of them are polymer of Glucose but the first one is linear while the second one is branched. Amylose is formed when Glucose is attached togethers with ɑ 1,4 linkage while Amylopectin is formed when Glucose is attached and creates a branched chain with ɑ 1,6 linkage. Unlike Dextran, Starch is a natural polysaccharide in Sugarcane, it forms in cane nodes during plantation and spreads into sugar juice during raw sugar production. Compared to undeveloped cane, full-grown canes consist of lesser amount of starch in their nodes. The percentage of starch present in raw sugar depends upon the harvesting time of sugarcane.

The presence of a high percentage of starch in raw sugar has disadvantageous effect on the process as it directly affects the filtration capability and eventually decreases the production capacity. 

Effects of Starch in Refinery

  1. It is believed that if the starch content in raw sugar exceeds 250 mg/kg, it can have a significant negative impact on refinery operations. The starch content in raw sugars can vary from 76 to 590 mg/kg. 
  2. A higher starch percentage has a negative impact on filterability and reduces the production capacity of a refinery.
  3. Starch rich liquor tends to take much more time for pan boiling and crystallization.
  4. Centrifuging time increased for high starch presence in sugar as more frequent washing is necessary then. And thus, it reduces centrifugal capacity.


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