The centrifugal mill

The distinction matters more than it sounds. A hammer mill reduces fruit by repeated impact, and impact reduction produces a wide spread of particle sizes: a lot of fines from the material that has been hit many times, plus whatever squeezed through the screen last. A grating action removes material a layer at a time at a defined depth, set by the tooth geometry and the rotor-to-ring clearance, so most of the output is the same size.

For pressing, the fines are the problem. Fine particles migrate under pressure, fill the drainage channels in the cake and blind it. A mash with the same average particle size but fewer fines will press faster and to a drier cake. This is the main argument for a grating mill in a plant with a belt or horizontal press.

It also means the mill is more sensitive to fruit condition. Soft, over-ripe or frosted fruit does not grate cleanly; it smears. Firm fruit grates well. A hammer mill is far less fussy, which is part of why it remains commoner in cider than in dessert-juice work.

Because a grating mill produces less fine material and works the fruit for a shorter time, it typically extracts less phenolic material into the juice than a hammer mill run to the same average particle size, and aerates the mash less violently. Juice from a grating mill tends to be paler and to carry less suspended solid.

Whether that is desirable depends entirely on the cider. For a pale, aromatic, low-tannin product it is an advantage. For a traditional bittersweet cider where phenolic load is the point, it is a loss that has to be recovered by maceration after milling, or accepted.

Centrifugal grating mills are largely a plant-scale item. They are less common in small cider houses than hammer mills or scratters, partly on cost and partly because their advantage — press efficiency on large continuous presses — is not realised on a small batch press.

Some small mills sold for garden and craft use also work by a centrifugal grating principle, with a plastic or stainless grating drum inside a housing. These are genuinely the same idea at a very different size, and they share the same characteristic: even output, moderate extraction, and a strong preference for firm fruit.