Sugarcane is a tall, perennial tropical grass cultivated globally for its sweet sap. The stalk, or culm, is the central focus of its commercial value as it functions as the primary storage unit for sugar. After harvest, this fibrous stem undergoes industrial transformations designed to extract the sugar content and repurpose the remaining material. The harvested stalk generates crystallized sucrose and secondary products used in energy production, livestock feed, and manufacturing.
Anatomy and Life Cycle of the Stalk
The sugarcane stalk is a stout, jointed cylinder that typically reaches a height of 10 to 13 feet when fully mature. It is composed of alternating nodes and internodes. Sucrose is stored within the soft, fibrous interior tissue called the pith, protected by a hard, waxy outer layer, the rind. The nodes mark the points where leaves were once attached.
During the growth phase, the internodes elongate and fill with sugary sap produced through photosynthesis. Sugar concentration increases as the plant matures, a process that usually takes between 12 to 18 months. Although the plant is perennial, the stalks are typically harvested annually or bi-annually, with new shoots, called ratoons, regenerating from the underground portion for the next crop cycle.
The Primary Process: Extracting Sucrose
The industrial process begins by cleaning and cutting the harvested stalks into smaller pieces called billets to facilitate juice extraction. These billets are fed into roller mills or a diffusion system, where pressure is applied to crush the fibrous material and squeeze out the juice. To maximize sugar recovery, hot water is sprayed onto the crushed fiber in a process called imbibition, washing out residual sucrose before the solid fiber is separated.
The raw juice is an opaque, dark liquid containing sucrose, water, and various impurities. This juice is treated with agents such as lime, which raises the pH and causes non-sugar solids to coagulate and settle out in a process known as clarification. The clarified juice is fed into evaporators where a significant portion of the water is boiled off, often under reduced pressure to prevent sucrose degradation.
This evaporation yields a thick syrup. The syrup is transferred to vacuum pans where it is boiled until it becomes supersaturated, allowing microscopic seed crystals to be introduced. This seeding initiates the crystallization of the sucrose, forming a dense mixture of sugar crystals and a viscous liquid known as massecuite. Finally, the massecuite is spun in centrifuges, separating the solid sugar crystals from the remaining liquid, which is known as molasses.
Valuable Byproducts Beyond Refined Sugar
The processing of the sugarcane stalk generates two major coproducts: bagasse and molasses. Bagasse is the dry, fibrous residue left after the juice has been fully extracted from the stalk. The primary use of bagasse is as a biofuel, combusted in high-pressure boilers to generate steam and electricity for the mill’s operations, a sustainable practice known as cogeneration.
This cogeneration often produces surplus energy to sell back to the public power grid, making the mill energy self-sufficient or even a power producer. Beyond fuel, the high cellulose content of bagasse makes it a suitable raw material for manufacturing:
- Paper and cardboard.
- Building materials like particleboard.
- Biodegradable packaging and disposable tableware, offering a renewable alternative.
Molasses is the thick, dark syrup left over after the final crystallization and separation of the sucrose. This liquid contains residual sugars, non-sugar solids, and trace minerals, making it valuable for other industries. Molasses is a common ingredient in livestock feed, where it serves as an energy supplement for cattle. It is also fermented to produce industrial alcohol, such as ethanol for biofuel, or distilled to make spirits like rum.
Composition of Raw Sugar Cane
Before processing, the sugarcane stalk is composed predominantly of water, which makes up approximately 63% to 75% of its total weight. The main commercial component is sucrose, the soluble sugar stored in the internodes, representing about 12% to 16% of the stalk’s mass.
The solid, insoluble fiber component, which eventually becomes bagasse, accounts for about 11% to 16% of the raw stalk weight. The remaining percentage consists of non-sugar solids, including small amounts of glucose and fructose, organic acids, proteins, and trace minerals.