The Link between Malt and Fermentation
The Link between Malt and Fermentation
After we've dealt with the production of malt in the previous article, we now want to discuss the further processing of the malt into a fermentable liquid, the so-called wort.
The germinated and dried malt can only be stored for a limited period of time. The storage life of the malt depends on the season during which it is stored. Of course it can be stored much longer in summer than in a cold and rainy Scottish winter. The dried malt is not completely dry. The residual moisture and the burst grain make it the perfect nutrient medium for mildew.
The malt silos of a distillery are closed airtight but, depending on the air humidity, malt can only be stored for 4 to 12 weeks in a silo. Whether the malt is produced on traditional malting floors or in an industrial malting, it is transported into the malt silo either with old-fashioned drag conveyors or with modern conveyor screws or a blower system. Every malt silo is filled at the top and emptied at the bottom. Gravity supports emptying, and this loading method guarantees that the malt isn't stored for too long but exchanged constantly. Only with several silos, the sequence of filling and loading must be watched so the content of a single silo isn't stored for too long.
The malt is extracted with simple taps and closed conveyors. The next station is not the malt mill but first a mechanical sieving machine that removes the last stones and unmalted, hard grains with the help of a swinging system and gravity. It's astonishing what combines fail to accomplish and how many stones are still in the malt.
Eventually the malt is ground in conventional gristmills with four rolls.
However, these mills don't grind the malt into fine flour but into coarse grist. The grains are thoroughly broken but the grist isn't as fine as flour so that during the later, liquid stage of the process the sieves and other tools don't stick together.
Grist is still predominantly made up of starch. The malting process has produced the enzyme amylase in the grain, which now splits the starch into various sugars. This process takes a lot of time, during which the mash cools down from 75°C to 60°C.
In order to convert the starch into sugar and then extract it with water, the mash tun was invented. In the beginning of whisky production in the 18th century, the mash tun was filled with grist, and boiling water was poured over the grist. Then the heavy mash had to be stirred by hand with shovels in order to accelerate or improve the extraction of sugar in the water. The mash tuns used to be made from wood like the wash tuns, but the hot water and the constant stirring wore the tuns out quickly.
With the advent of mechanisation the mash tuns were made of cast-iron segments that were bolted together at the seams. A mechanical metal stirring unit with a central bevel gear was installed to make stirring easier. That way the sugar could not only be extracted faster but also more efficiently. Less sugar was left in the husks of the grain, and the whisky became more affordable.
The mash has to be stirred for some time to extract the malt sugar from the grist. In earlier times, a lot of water used to evaporate during this process, and with the water steam the temperature fell constantly, which further impaired the extraction process. Only in the last century, when energy costs rose, nearly all mash tuns were equipped with a lid made of sheet metal (e.g. copper) in order to limit the loss of energy.
Today a multi-stage extraction process is used by all distilleries. The mash tun is filled with several tons of grist, and then hot water from the in-house boiler is added via the mash mixer. The ratio of grist to water is approximately 1:4 so that a hot mush similar to porridge is created. The water of this first filling is not so hot (60-70°C). There's another peculiarity about this first filling, which will be explained below. This first water remains in the mash tun for about an hour while the mash is being slowly stirred before it is pumped out.
Emptying the mash tun takes much longer than filling it. With modern, flat lauter tuns the sugar water can be let out through a fine sieve without any leftover husks getting into the collection container, the underback. Old, tall mash tuns often take up to 6 hours to be emptied in order to get huskless water (wort) for the fermentation.
When the sugar water, now called wort, is pumped out, a heat exchanger extracts heat from it and cools it down to approximately 20°C for the upcoming fermentation. The energy that is won back this way is later used to heat the second water. Since already a considerable part of the sugar has been extracted from the grist, the second filling is heated to over 70°C in order to make the extraction more effective. Almost always a third filling with 80-90°C is carried out, and in rare cases even a fourth one. All distilleries share the peculiarity of the last water. Since it contains only little sugar it is used as first water for the new filling without being reheated.
The modern lauter tuns, which are more and more widespread, aren't only large but also fast. Their low height and the sieves at the bottom allow for a much faster discharge of the sugar water via the bottom into the underback than what could be achieved with old traditional mash tuns. With the old tuns you have to wait a while, so the husks float to the surface of the sugar water, and the water can be discharged slowly. The yield of the modern lauter tuns was improved, too: Large circling arms with knives attached allow for a much better mixing of the grist with hot water. The shape of the bottom of a mash tun is important if you don't only want to get the sugar water out after the last filling but also the leftover husks. A curved bottom makes discharging the non-soluble remains of the grains easy.
These remaining husks are called draff and contain large amounts of proteins, trace elements and mineral nutrients. They are made into feed pellets for livestock breeding. The high water content makes the draff heavy and hard to transport. That's why the water is extracted from it in large evaporation facilities. The town of Rothes, for example, has its own evaporation facility where all local distilleries bring their draff.
Some distillieries (Glenfarclas and Mannochmore/Glenlossie) built their own evaporation facilities in the past. This way the animal feed can be directly collected by local farmers. This partnership between farmers and distilleries has existed for centuries and was first described by Alfred Barnard in 1887 for 35 of the distilleries he had visited.