Juice Extraction by Milling

The objective is to get as much of the cane’s sugar as possible out of the fibre and into the juice, because sugar left in the bagasse is sugar the factory will never see again. Once the fibre goes to the boiler, whatever sugar it still holds is burnt with it. Extraction efficiency at the mills therefore sets a ceiling on everything downstream: no amount of skill in the boiling house recovers sugar that never left the stalk.

The principle is straightforward expression. Cane arriving at the mills has already been cut and shredded so that its cells are ruptured and the juice inside has a path out; the mill rolls then compress the fibre mat and force that juice out through it. A single pass cannot do the job, because as the fibre compresses it holds juice in the structure that remains. So milling is arranged as a sequence — a tandem — with the cane passing through several mills in turn.

Between those mills, the process does something more subtle than squeezing harder. Water, or dilute juice from a later mill, is applied to the fibre before the next pass. This dilutes the juice remaining in the fibre so that the next squeeze removes more of the sugar with it — the added liquid moves counter to the cane, so the most dilute liquid meets the most exhausted fibre. The mill is not simply pressing repeatedly; it is washing sugar out of the fibre while pressing. That is why extraction can approach completeness in a way that pressing alone could not.

Two streams leave the milling tandem, and they have quite different standings in this model.

Mixed juice

The sugar-bearing liquid, carrying suspended soil, fibre, and colloidal material. An intermediate stream: it goes forward to clarification and then to the boiling house, and is not a product entity in its own right.

Bagasse

The crushed fibre leaving the last mill, still moist and still holding some sugar. A by-product, and the factory’s principal fuel.

Bagasse deserves more than a passing mention, because it is the reason cane sugar factories have the energy profile they do. Burnt in the factory’s boilers, it raises the steam that drives the mills and heats the evaporators, and it generates the electricity the plant runs on. Well-configured factories produce more power than they need and export the surplus. Bagasse also has uses beyond the boiler — as a fibre feedstock for pulp and board, and as a feed component — so where a factory has an alternative outlet, burning it is a choice among uses rather than the only option.

Neither stream is finished. The juice is a long way from sugar: it must be clarified, concentrated, crystallised, and separated before anything crystalline exists. Those are separate methods.

Milling cannot make sugar. The sucrose in the juice arrived in the cane, and the factory’s job from here is to lose as little of it as possible. What milling decides is how much of that sucrose is captured in the juice rather than carried away in the fibre — and, because the process adds water to improve extraction, how much water the boiling house will subsequently have to evaporate. That is a real trade: recovering more sugar by adding more water means more energy spent boiling it off later.

Sugar losses in cane processing begin before the mill. Cut cane deteriorates: once the stalk is severed, its sucrose begins to be lost to biological activity, and the loss continues for as long as the cane sits. This is why cane factories run continuously through the harvest and why cut-to-crush delay is watched so closely. It is a loss the mill cannot recover, only avoid by not waiting.

• Sucrose retained in the bagasse and burnt with it
• Sucrose lost to deterioration between cutting and crushing
• Sugar carried away with soil and trash entering with the cane
• Juice spillage and leakage across the tandem
• Added water raising the evaporation load downstream

The energy story of cane milling is genuinely unusual, and worth stating carefully. The factory’s main fuel is its own by-product: bagasse burnt on site raises the steam and generates the power the plant consumes, and surplus electricity is exported in many producing countries. This is a real instance of circularity — a residue stream carrying the process that made it — and it is a factual description of how cane factories are configured, not a claim that cane sugar is environmentally preferable to any alternative. Such a comparison would need a comparable evidence base and none is offered here.

Against that, milling is water-using and generates effluent carrying soil, fibre, and sugar, all of which need treatment. Bagasse combustion is a combustion source with the emissions and air-quality controls that implies. Both are matters for the applicable environmental regulator.