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Equipment

The continuous screw press

What is a continuous screw press and when is it used for cider?

In short

A continuous screw press has a helical screw turning inside a perforated barrel. As mash is conveyed along the screw the pitch narrows or the shaft thickens, so the free volume falls and the mash is compressed against a restriction at the discharge end.

It runs continuously, needs no cloths or racks, and takes up little floor area for its throughput. What it does not do is treat the mash gently.

In cider it is more often found doing rough work — recovering juice from pressed cake, handling large volumes of low-value fruit, dewatering pomace — than producing juice for a quality-driven ferment.

How it develops pressure

The pressure in a screw press is generated by resistance, not by a ram. Mash is pushed towards a cone, a spring-loaded plate or an adjustable gate at the outlet; the tighter that restriction, the higher the pressure in the compression zone and the drier the cake, but also the greater the torque, the heat and the shear. Operators trade cake dryness against juice quality directly, by turning a handwheel.

The screen is a perforated or slotted barrel, sometimes in sections of decreasing aperture. Blinding is the main operational problem: fines pack into the perforations and reduce drainage until the machine is stripped and cleaned.

Shear, solids and character

The screw grinds as well as squeezes. Skin, core, pip fragments and stalk are all worked hard, and the juice picks up a heavy load of fine solids along with material that a gentler press would have left in the cake. That includes phenolic compounds from skin and pips, and it can include the bitter and astringent fractions that a maker would rather not have.

The temperature also rises, because mechanical work goes somewhere. Warm, aerated, high-solids juice is the least favourable starting point for a controlled ferment, and it is why screw press juice is typically clarified aggressively before use or blended into a much larger volume.

Pips are worth a specific mention: crushing them releases amygdalin-derived compounds and a distinctly bitter character, and any press that mills the seed rather than passing it whole risks this. Screw presses are the most likely of the common types to do it.

Where it earns its place

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