Grinding Time

Grinding Time

We investigate the process and the significance of milling

Production | 19 Jul 2013 | Issue 113 | By Ian Wisniewski

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Milling the malt (ie. crushing the malted barley) is typically considered entirely pragmatic, compared to distillation and aging which are hailed as creative, and instrumental in forming the character of the resulting malt whisky.

However, each stage of the production process is equally important, in the sense that it must be concluded successfully, otherwise each successive stage will be compromised.

The malt is initially passed through a dresser, effectively a revolving drum fitted with mesh that acts as a sieve, allowing malt to pass through but catching extraneous items such as twigs, pieces of straw and larger stones from the harvesting process.

The malt subsequently passes through a de-stoner, which operates on a similar principle to a dresser and catches smaller stones. This is vital as stones can damage the rollers of the milling machine. Additionally, stones striking the rollers, or each other, can create sparks which are potentially very dangerous.

That's because this process creates a lot of dust, which could be ignited by a spark and start a fire or even cause an explosion.

This also explains why dust extraction units are an integral part of the process.

The malt is then conveyed to a weighing machine positioned above the mill. A slide at the base of the weighing machine opens to release specific amounts of malt.

"Our weighing machine tips 50 kg batches of malt into the mill at regular intervals. Each tip of malt is known as a coup, and this is an optimum amount for the mill to process at one time. You can't rush it through," says Callum Fraser, distillery manager, Glenfarclas Distillery.

A mill is typically equipped with two pairs of rollers, one pair positioned at the top of the mill and the second lower down.

"Wooden rollers were once the most traditional option. Steel rollers came in during the 1950s, and these are now the norm. Our steel rollers are three feet long, with an eight inch diameter, the weight is unbelievable. The rollers are also very resilient, I've been here 35 years and we've only taken out the rollers once. They get slightly dented after years of milling, so we sent them to a steel mill to smooth out the surface," says Willie Cochrane, Isle of Jura's distillery manager.

Batches of malt are channeled to the first pair of rollers which crack open the husk. Each roller rotates at the same speed but in opposite directions for greater efficiency.

Exiting the first pair of rollers the malt falls onto sloping plates that channel it in a uniform flow to the second pair of rollers, which are much closer together than the first pair, and so grind the malt even finer.

"The first pair of rollers are set at 50 thousandths of an inch apart, and the second pair are 30 thousandths of an inch apart.

"Setting the gap between the rollers is straightforward, using a dial on the mill which indicates the distance between them," says Andrew MacDonald, distillery manager at Glenmorangie Distillery.

Malt leaves the second pair of rollers milled into three separate grades: the husk, grits (or 'middles') which are medium-ground, and the much finer ground flour (or 'fines'), collectively called grist.

A typical specification for the grist is 20 per cent husk, 70 grist and 10 flour. This is an optimum ratio for mashing, when the malt is placed in a mash tun (a large, deep vessel) and hot water is added to facilitate the conversion of starches within the grist into sugars, resulting in a sugary liquid that drains through perforated plates at the base of the mash tun.

The level of husk and flour are vital factors to promote the maximum conversion of starches into sugars, in the minimum amount of time.

Flour is the easiest for the hot water to convert into sugars. However, the percentage is always low as higher levels of flour would create a sticky mass that slows down, or even prevents, the sugary water from draining through the deep 'bed' of grist in the mash tun.

Husks promote the drainage of sugary water through the grist. However, higher levels of husk can also accelerate the drainage rate to the extent that the hot water isn't in contact with the starches for long enough, which would reduce the level of conversion from starches to sugars. Larger pieces of husk can also cause serious problems if they cause a blockage in the drainage plates at the base of the mash tun.
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