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Equipment

The belt press

What is a belt press and why do large cider plants use one?

In short

Mash is fed onto a moving porous belt, a second belt closes over it, and the sandwich passes through a gravity drainage zone and then between rollers whose gaps narrow progressively. Juice is squeezed out through both belts and collected in trays below.

The press runs continuously as long as mash is fed to it, which is why it dominates in high-volume juice and cider production. Throughput is measured in tonnes per hour rather than batches per day.

It is highly dependent on mash preparation: fine, enzyme-treated mash of the right consistency presses well, and mash that is too wet simply extrudes out of the sides.

The zones of a belt press

A belt press is usually described in three stages. First a gravity or wedge zone, where free-run juice drains from the mash by gravity through the lower belt before any pressure is applied — a substantial fraction of the total on well-prepared mash. Then a wedge or low-pressure zone where the two belts converge and squeeze gently. Finally a high-pressure zone where the sandwich passes over and around a series of rollers of decreasing diameter and gap, applying both compression and the shear that comes from bending the cake around a roller.

That shear is important: bending the cake repeatedly around rollers of different sizes tears open new drainage paths that pure compression would have closed. It is the same principle as breaking up a basket press cake, applied continuously.

The belts run over rollers and then through a wash station, where high-pressure water sprays clean the weave before the belt returns to the feed. This wash is a continuous and significant water demand and it is also the point where belt condition is inspected.

Why the mash matters more than the press

A belt press has no cage to contain the mash; the only thing holding it in is the friction of the belts and its own consistency. Mash that is too fluid squeezes out at the edges, which loses yield and makes a mess. Mash that is too coarse does not form a coherent cake and the belts skid on it.

Commercial practice therefore treats the mash before pressing: pectinase to reduce viscosity and free bound juice, sometimes a press aid such as milled cellulose fibre to give the cake structure, and careful control of milling. The press manufacturer’s throughput figures assume this treatment, and a plant that tries to run untreated traditional pomace on a belt press will not see them.

Because the process is continuous, it also fits naturally with continuous downstream handling: mash pump, press, juice buffer tank, and on to clarification without a batch break anywhere.

Oxidation and juice character

A belt press is the most oxidising press in common use. Mash is spread in a thin layer, fully open to air, and juice runs across open trays for the whole run. In a large plant the mash may have been sulphited or the juice may be handled quickly, but the exposure is inherent to the design.

The other consequence is solids. High shear puts fine particulate and colloidal material into the juice, so belt press juice normally goes straight to enzyme treatment, settling or centrifugation. Producers who want the juice’s aromatic fraction intact generally do not choose a belt press.

Where the product is a large-volume mainstream cider destined for filtration, blending and carbonation, none of these are disqualifying, and the throughput and yield are decisive.

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