In this Distiller’s Dispatch our Distillery Manager Jacob is going to take us deeper into the science behind mashing - the process that breaks down the starches in grain into simple sugars that can be fermented into alcohol. For a simpler explanation see Jacob's previous blog post, and stay tuned for the next one which will explain how this process is relevant to the specific barley varieties that go into our mash here at North Uist Distillery. 

What The Grain Contains

Each grain that is harvested contains starches, or complex carbohydrates made up of long chains of glucose molecules in the form of linear chains (amylose), and branched chains (amylopectin) in a ratio of roughly 1:3 

Along with its store of starch, the grain also contains a set of enzymes which will break down these long chains. Enzymes are biological compounds which will react with the  carbohydrates and chop them into their smaller units. Similar to the grain itself, commercial brewing and distilling yeast can only utilise single glucose units or double units, called maltose, so this enzyme-assisted step is crucial. 

The Role of Enzymes in Breaking Down Barley

The most important enzymes are α-amylase; β- amylase and limit dextrinase, each reacts in a different way and requires its own specific conditions. 

• α-Amylase will indiscriminately chop the “stem” of these long chains, but not the branches of amylopectin. Its ideal temperature range is 60-70c.

• β- Amylase will chop up the branches of amylopectin chains, but not the branch point itself. It prefers a lower temperature range with activity peaking at 50c and dropping off sharply over 60c.

• Limit Dextrinase is able to break the branch point bonds on amylopectin molecules. Optimal activity ~60-65oC, dropping off sharply over 60oC.

Between these enzymes the complex carbohydrate chains can be broken into smaller units more easily utilised by the yeast during fermentation. In order to allow optimum activity for each enzyme and avoid heat damage, the mash is very carefully controlled at a specific temperature, typically 63-64c, however this can be shifted slightly to favour one or other enzymes depending on characteristics of the grain.

For example, older varieties which tend to produce a less fermentable wort due to a higher proportion of amylopectin to amylose, suit a lower temperature rest favouring β- amylase and limit dextrinase.

The Anatomy of a Barley Grain

This simple cross section shows you the anatomy of a barley grain. The important elements while we consider mashing are the husk - the protective outer layer of the grain, and the endosperm - the grain's store of carbohydrates. 

The Equipment and Processes of Mashing

1. Milling

Before mashing the distiller requires the starchy endosperm of the grain to be exposed, which is achieved mechanically by milling the malt into a grist.  To assess the efficacy of milling we define three categories: 

• Husk - the delicate protecting skin of the grain which plays a crucial role in the structure and porosity of the grain bed during the mash. An essential quality for the rapid and effective extraction of sugary worts from mash. 

• Grits - these are the coarse particles of broken up endosperm. By mechanically breaking down the endosperm of the grain we provide more surface area allowing better access for the native enzymes to travel to and act upon their target compounds. 

• Flour - finer particles of endosperm that again, provide more surface area for enzymatic reactions to occur. The ratio of coarse grits to fine flour will have important implications on the effectiveness of worts collection - too much flour and the mash can gum up, too much grits and some of the available sugar will be left in the mash resulting in a lower efficiency.  

Brewers and Distillers use a roller mill, as this type of mill tends to crack the grain rather than crush it. We assess the resulting grist using a “shaker box” made up of  a set of sieves which separates the grist into the above three portions. Typically we aim to achieve a  ratio of 10% husk, 20% grits, and 70% flour. However this must be continuously monitored and adapted to mash parameters, grain variety and malting specifications. 

2. Mashing

The grist is slowly conveyed into the 'Grist Mixer' which creates a vortex with the hot water feed and directs the resulting slurry into the mash tun. The distiller will carefully manage the speed of both feeds to ensure the grain is fully hydrated and free of any dry lumps or dough balls. The water temperature is managed such that the temperature of the slurry is optimal for the enzyme activity outlined above. Too cold and the enzymes will be sluggish to react, too hot and their activity will be reduced to zero - both scenarios result in an unfermentable wort.

The ratio of hot water to grist is also important: a thicker mash will provide more temperature protection to the enzymes, but reduce the speed of reaction and vice versa. We tend toward a thicker mash to protect the more heat sensitive enzymes, and mitigate the slower activity by extending the mash time. 

We have a semi lauter mash tun, which uses rotating rakes to help mix the mash and a perforated false floor to allow sugary worts to be collected leaving behind the spent grain or draff. Once the mash is in and settled the distiller will begin a process known as 'vorlauf', or recirculating. This involves slowly circulating the worts from the bottom of the tun back to the top of the grain bed, creating a filter bed of the mash and clarifying the worts. The length of the vorlauf will determine how clear the worts are, a crystal clear wort will create a cleaner flavour profile, while cloudy worts can contribute more biscuity/ grainy characteristics.

Once complete the worts are directed through a heat exchanger to cool it down to a more favourable temperature for fermentation, then conducted to our wooden washbacks. The distiller will slowly drain off a proportion of these worts to the washback before beginning to 'sparge' the grain bed, a process of rinsing it with progressively hotter water to wash through any remaining sugars to be collected into the washback. Gravity measurements of the worts determine the concentration of sugar and hence the effectiveness of the mash.  

Once the sparge is complete and all the worts have been collected we load the spent draff out of the tun to be collected by local crofters to feed the cattle that next door to the distillery.

The next step of the process is fermentation, but in the next couple blog posts we will talk more about the different barley varieties we use in the distillery, some of the challenges associated with their use and the different characteristics they contribute to our whisky. 

- Jacob

Distillery Manager