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Equipment

The centrifuge

What does a centrifuge do in a cider plant?

In short

A disc-stack separator spins liquid in a bowl containing a stack of closely spaced conical discs. Solids denser than the liquid are thrown outward, slide down the discs to the bowl wall and are ejected; clarified liquid leaves from the centre.

It replaces settling time. Juice that would take a day or more to fall bright, or lees that would take weeks to compact, can be separated in a single pass at a defined rate.

It removes yeast and gross solids but is not a sterilising operation; a centrifuged cider is bright, not stable.

What it is used for

Its role is nearly always to save time and to protect a downstream filter, rather than to achieve something no other process can. A small producer who has cold storage and patience can reach a similar clarity by settling; what they cannot do is reach it in an hour.

The costs and the risks

Centrifuges are among the most expensive pieces of equipment in a cider plant and among the most maintenance-hungry. The bowl runs at very high speed, the seals and bearings are precision components, and the machine must be balanced, serviced and inspected on the manufacturer’s schedule. It is not equipment that tolerates neglect.

The quality risk is oxygen. A poorly set-up separator can draw air into the feed or leave the bowl only partly full, and cider passing through a spinning bowl with air in it picks up dissolved oxygen very efficiently — the geometry is close to ideal for gas transfer. Machines used on finished cider are run hermetically or with the bowl and discharge under an inert gas blanket for exactly this reason.

The second quality risk is shear, though it is less severe than a rough pump: what a centrifuge does to lees particles is remove them rather than break them up. The bigger stylistic question is what removing them costs. A cider that would have matured on its lees loses that, and a producer who centrifuges for speed has made a decision about texture whether or not they meant to.

Centrifuge, filter or time

These three are alternatives for the same problem and they are not equivalent. Time — settling, cold storage, racking — removes solids gently and selectively and costs tank space and months. A centrifuge removes them quickly and fairly indiscriminately by density. A filter removes them by size and, in the case of depth media, by charge as well.

Most commercial operations use all three in sequence, and the interesting question is where each one is placed. Putting the centrifuge early, on juice, saves settling tanks and lets fermentation start clean. Putting it late, on finished cider, saves filter media and shortens maturation, at the cost of whatever the lees were contributing.

For a producer whose product depends on lees contact and colloidal body, a centrifuge on finished cider is the wrong machine at the wrong point, however convenient. For one making a bright, consistent, filtered cider at volume, it is close to essential.

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