Guide to milling and mashing

Two parts of any distillery tour that are most likely (though not always!) to be glossed over or sped past, to get to the more “exciting” aspects of the whisky making process - where ethanol rears its boozy little head - are the stages of milling and mashing. 

But any whisky geek worth their salt will find the processes of milling and mashing as fascinating as any discussion about cut points of fermentation times. In this article, we’ll take an in-depth look at these stages and see how they can make or break a successful production cycle.

Milling

Before a single grain of barley can contribute its sugars, it must first be ground down to expose the starch. This process is called milling. 

The primary purpose of milling is to break down the barley grains into a coarse mixture known as "grist." The most common method is to use a “grist mill” or “malt mill”, which typically involves a series of rollers that crush the malted barley.

Cereal mills are used in many agricultural applications, with the type and specification of mill depending on the intended use of the processed grain. Animal feed production, for example, requires a roughly ground cereal; therefore, a basic two-roller mill will suffice.

In whisky production, four and six roll mills are the most common. Their purpose is to break open the endosperm of the barley to expose the starch granules to the endogenous enzymes (if you’re scratching your head, then read our guide on why we use barley to make whisky). 

How does a malt mill work?

Despite the perception of being rather a “blunt instrument”, mills are actually very precise pieces of machinery and so much more fascinating than ‘shoogle-box’ gimmicks and “it never breaks down!” clichés. 

Let’s take a look under the bonnet of the humble malt mill!

The four-roll mill (wonderfully drawn above) is the most common in single malt production. It has four main components:

• Feed roll – controls how quickly malt flows into the mill
• First pair – crack open the malt to reveal that starchy, enzyme rich goodness
• Screens – some particles are already fine enough and don’t need to be crushed again; they go straight through. It’s the speedy boarding queue at the airport, but for malt. 
• Second pair – further crush the coarse particles to the correct grade

Other mill types include a six-roll mill: used for less malted/modified malts and less common in scotch whisky, and a hammer mill: more common in grain distilleries where mash tuns are not present and good husks are not vital to the process. These produce a fine grind and maximise enzyme content.

The precise ratio of husks, grits, and flour (often referred to as the “mash bill") is crucial. Too coarse, and it will be inefficient for extracting the sugar, leaving valuable starches behind. Too fine, and it can make separating the wort difficult and time-consuming. 

The pairing of the ideal type of mill – set correctly – with the intended mash tun can also be the difference between producing beautiful, clear, bright worts and a pile of useless porridge.

Mashing

Mashing involves mixing the grist with hot water in a large vessel called a mash tun. The primary goal is to activate the naturally occurring enzymes (primarily amylase enzymes like alpha-amylase and beta-amylase) within the malted barley. These enzymes break down the complex starches present in the grist into simpler, fermentable sugars (mainly maltose, but also glucose, maltotriose, and dextrins). This sugary liquid is known as "wort”. 

The efficacy of these enzymes depends on temperature. Different enzymes have optimal temperature ranges for their activity. While some distilleries use a single-infusion mash (a single temperature for the entire mash), many Scotch whisky distilleries employ a temperature-programmed mashing approach. This involves holding the mash at specific temperatures for set periods, known as "rests," to favour the activity of different enzymes:

At Lagg, we use a more modern semi-lauter mash tun, with the use of rakes. We run the rakes while the wort is being transferred to the washback’s, which ‌produces a much cloudier wort by allowing better drainage of heavier particles in the malt. 

We want a cloudy wort for a more oily and heavy final product. This is in contrast to our sister distillery at Lochranza, where the goal is a clear wort for a fruiter and lighter whisky.

The relationship between mash and mill

The mill and the mash tun are like ‌Laurel and Hardy, or the Lennon and McCartney of the distillery. Both good on their own, but definitely better alongside the other. The type of mash tun, or to give it its Sunday name “wort separation vessel”, dictates the mill’s setup. Get it too coarse and the starch won’t be accessible to enzymes; get it too fine and you’ll create something between Scott’s Porridge Oats and cement. Get it right and you will achieve excellent, bright, high gravity worts – the key to the door that leads to a good fermentation and a fruity new make. 

So what constitutes right and wrong? At Lagg we use a semi-Lauter tun that uses 4 tonnes of grist per mash. This year, we have roughly been working to a ratio of:

• Husks – 24%
• Grits - 62%
• Flour – 14%

The presence of a quantity of husks in any mash is very important. They sink to the bottom of the vessel and act as a filter, also ensuring that the whole mash doesn’t sink to the bottom of the vessel and stick there until an unfortunate mashman (i.e. the youngest one) has to get in there with a shovel and dig it out!

Anyone reading this who has been on a distillery tour will most likely have scoffed at the warnings of the explosive atmosphere inside the still house. In truth, the mill room poses a greater risk‌. Any area that produces fine dust in any great quantity needs to be well ventilated and treated with real caution as the atmosphere itself becomes highly flammable. For this reason, most mills have strong magnets inside that trap any metal particles that could have become mixed in with the malt in processing and transportation. Should one of these create a spark in a poorly ventilated mill room, a large bang may well follow…

Nature’s will to grow the barley, the energy to harvest and the skill in malting would all go to waste were it not for a good, well-calibrated mill. Next time you walk through a distillery mill room, take a moment to acknowledge just how vital these rumbling giants are to the whole process.

Distillery tour

If you’d like to know even more about our process, come and join us for a tour of the distillery. Our guided tours take you through the whisky making process, from grain to glass. You can also enjoy a select tasting of our expressions, grab a bite to eat in our cafe, or enjoy a dram in the bar.