The Biological Role of Starch in Sugarcane
Starch, a complex carbohydrate, is a polysaccharide composed of long chains of glucose units. In plants, starch serves as the primary way of storing energy, and this is also true for sugarcane. It is produced during photosynthesis and stored for later use, primarily in leaves and the growing point, but also in the fibrous stalks.
Starch acts as a long-term energy reserve, unlike the simple sugar sucrose which is for immediate use. The amount of starch in cane varies based on maturity, variety, and growth conditions. Sugarcane starch granules are very small, about 1–3 µm, making them easily dispersed in juice during milling.
- Photosynthesis: Converts light to chemical energy (glucose).
- Energy Storage: Excess glucose becomes starch in leaves and stalks.
- Conversion: Enzymes break down starch to simpler sugars when needed or during ripening.
- Granule Size: Small size facilitates dispersion during milling.
The Negative Impact of Starch on Sugar Processing
Starch poses significant problems for the sugar industry during milling and refining when its granules are released into the juice. Sugar mills use methods to mitigate these issues.
- Increased Viscosity: Starch thickens juice and syrups, slowing filtration and evaporation.
- Inhibited Crystallization: Starch interferes with sucrose crystallization, reducing yield.
- Reduced Filtration: High starch impairs filterability, slowing clarification.
- Adsorption: Starch on sugar crystals distorts shape and color, lowering raw sugar quality.
Starch vs. Sucrose: A Comparison in Sugarcane
| Feature | Starch | Sucrose |
|---|---|---|
| Classification | Polysaccharide (Complex Carbohydrate) | Disaccharide (Simple Sugar) |
| Molecular Formula | $(C6H{10}O_5)_n$ | $C{12}H{22}O_{11}$ |
| Function in Plant | Long-term energy storage | Short-term energy and transport |
| Water Solubility | Insoluble (granules) | Highly Soluble |
| Molecular Structure | Long chains of glucose units, linear (amylose) and branched (amylopectin) | Glucose and fructose units bonded together |
| Effect on Processing | Interferes with filtration, clarification, and crystallization | Desired end product; key component of table sugar |
Methods for Starch Removal During Processing
To manage starch, sugar producers use various techniques, including traditional and enzymatic methods. The aim is to remove or break down starch into smaller molecules.
Enzymatic Hydrolysis
This is a common method in modern sugar mills using enzymes like alpha-amylases to break down starch.
- Target: Alpha-amylases break down starch's $\alpha$-1,4 glycosidic linkages into smaller molecules.
- Application: Enzyme is added to heated juice, causing starch granules to swell and become susceptible to breakdown.
- Result: Smaller fragments don't interfere with crystallization and are easier to remove or end up in molasses.
Clarification and Filtration Techniques
- Hot Lime Clarification: Heating juice with lime helps precipitate and remove some starch.
- Flocculants: Additives bind with starch and solids for easier removal by settling and filtration.
- Centrifugation: Early centrifugation can remove solid starch granules.
What Does This Mean for the Final Consumer Product
While important for manufacturers, starch has minimal impact on the end consumer of most refined sugars. The refining process removes almost all starch and other impurities.
Unrefined brown sugars may contain trace starch, which can distinguish cane sugar from beet sugar (which has no starch). These trace amounts are negligible for consumers and don't affect taste, quality, or dietary value.
Conclusion
In conclusion, starch is a natural component of sugarcane and creates processing challenges for the sugar industry. Its negative effects necessitate removal strategies like enzymatic hydrolysis. However, refined sugar contains negligible residual starch, making its impact on the final product insignificant for consumers, who can enjoy their sugar without concern for starch content.
