Understanding Whisky Production Unit 3: Raw Materials Processing
Chapter 7: Wort Separation and Cooling: Mash Tuns, Lautering, and Heat Exchange
Traditional mash tuns, semi-lauter rakes, Meura mash filters, sparging regimes, clear versus cloudy wort, and counter-current cooling.
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Having coaxed barley enzymes into dissolving starch into sugar, the distiller faces the mechanical problem of draining the sweet liquid (wort) away from the spent grain husks (draff).
In traditional infusion tuns and modern semi-lauter vessels with rotating knives, the grain bed acts as its own sieve. Water is applied in several successive sparges at rising temperatures to wash out every stray molecule of sugar. Whether the resulting runoff is sparklingly clear or milky and turbid will dictate the spirit’s destiny: clear worts leave yeast to produce bright, delicate fruit esters, while cloudy worts carry grain lipids that yield nutty, cereal, and oily profiles.
Wort Separation Overview
Depending on the type of whisky being produced, the distiller has two options after mashing-in is complete:
- Separate the wort from the spent grain prior to fermentation.
- Keep the wort and grain together for an ‘all-grain-in’ fermentation.
All-grain-in fermentations are common in grain whisky distilleries and American distilleries that make bourbon. These distilleries commonly use hammer mills to create incredibly small grain particle sizes. These small particle sizes make it difficult to separate grain from liquid, although it is not impossible, as we will see later in this chapter. These whiskies also use many un-malted, huskless grains.
Distilleries that separate the grain from the wort generally produce whisky from 100% malted barley that has been milled to larger particle sizes. These mashes contain a large amount of husk material left from the barley after milling. Husks act as a natural filter medium to pass liquid wort through. This filter removes other unwanted grain solids in the process. Let us take a closer look at the equipment and processes involved in wort separation.
So what do we need in order to separate grain from our sugar-filled wort? We need a filter of some sort to let the liquid pass through and hold back the unwanted grain solids. The husks from the barley help, but they can easily be carried into our wort if we are not careful. Ideally, we would have a perforated false bottom in our wort separation vessel, to keep all the large grain particles from entering our liquid stream. The remaining grains will also be holding a lot of residual sweet liquid, so we will need a way to add additional water to rinse these grains of their sweet liquid. To do this, we use a technique called sparging.
Wort Separation Technology - Mash Tun
Traditionally, we use three types of wort separation vessels in the distilling industry. A new, fourth type of vessel, known as the mash filter, is beginning to gain attention in some sectors. The traditional wort separation vessels are:
- The mash tun
- The semi-lauter tun
- The lauter tun
Let us briefly explore how they work and consider how they differ from each other.
Mash tun
The previous lesson introduced us to the mash tun. If our mash tun has a false bottom covered by a set of perforated/slotted plates, then we have a way to separate our wort from our grain. In order to sparge, or rinse our grains (sparge), additional hot water is pumped in through the hydrator or a mash mixer fixed on top of the mash tun. Older styles of whisky mash tuns had paddles, or some other kind of grain agitation device, to properly mix the water with the grains.
Wort Separation Technology - Lauter Tun and Semi-lauter Tun
The two other most common types of wort separation vessels are the lauter tuns. for our purposes, the semi-lauter tun and full-lauter tun are incredibly similar to each other. We can thus discuss them together.
Both utilise a series of rakes that move in a circle around the vessel to mix the mash effectively with the hot water. Contrastingly, the rakes in full-lauter tuns also have the ability to move up and down. Both types of vessels sparge the grains through a spray or sprinkler system put in place near the ceiling of the inside of the vessel. This allows for a gentle spray of hot water onto the grains. Most importantly, perhaps, modern lauter tuns typically have a shallower depth than the mash tun. This is important as it affects the rate of wort run off.
Key Operating Insight
Welcome back. So what were your thoughts? A shallower grain bed, in theory, allows for a faster wort run-off. This sounds efficient for a whisky distillery, doesn’t it? We are going to now discuss why this is the case, and consider the inherent, potential dangers involved if run-off is too fast or too slow.
Wort Separation Process - Mash and Lauter Tuns
So, our mash is properly mixed and has been sitting at the right conversion temperature for the adequate amount of time. We are confident that our starch has gone through the proper amount of conversion, and we now have a very fermentable wort just waiting to get into the fermenter with some yeast.
Our grain is sitting on top of a series of perforated plates with a false bottom underneath. The rich, sugary wort is heavily concentrated in the false bottom. If we drain this liquid out and do nothing else, we will end up with an incredibly high sugar wort that may not ferment well. It would also not be very efficient, because there would be a lot of fermentable sugar left in the grains sitting on top of our false bottom. Fermentable sugar is converted to alcohol, and alcohol is what makes money for our distillery. Therefore, we do not want to leave very much of it behind.
The way we do this is through rinsing the grain with sparge water. In a brewery, this process is somewhat continuous; a gentle spray of water is pumped onto the top of the grain bed, over a period of one to two hours. However, with most malt distilleries, the approach is a little different.
The process of wort separation begins by allowing the hot wort to begin running off from the bottom of the mashing vessel. This liquid has a high sugar concentration; it is sometimes referred to as ‘first runnings’ or ‘first waters’. Some distilleries will recirculate the first waters by pumping them from the bottom of the mash vessel back to the top of the grain bed. This helps to set the grain bed as a stable filtration medium. It also clarifies the wort.
First waters have a high sugar concentration. The technique of recirculating first waters for clarification is referred to as “vorlauf”. Clarified wort means that there are no small bits of grain material left in the wort.
Once the first waters are properly clarified, they are redirected to a wort chiller (more on this later) and pumped into the fermenter where yeast is quickly added. This has to be done carefully. If the first waters are pumped too quickly, the grain could settle and become compacted on the perforated plates. This can cause wort flow and filtration issues. However, pumping the first waters too slowly can slow down the overall plant efficiency. We must therefore maintain a balanced run-off rate. As mentioned above, grain bed depth has a big effect on this. In general, lauter tuns have shallower grain bed depths than the traditional mash tuns. This makes run-off times much faster.
As the first waters are being moved through to the fermenter, the distiller will begin sparging the grain bed with hot water at 75˚C. Some distilleries will turn the rakes in the lauter tun on very slowly, moving them through the grain bed and increasing drainage. If mixing is part of the procedure, then the distiller may want to do another vorlauf in order to clarify the wort.
Other distilleries do not want to disturb the grain bed. They thus opt to spray the water gently over the static grain bed, without mixing. This process must be done carefully as there is a risk of creating “channels” in the grain bed. Channels allow water to directly bypass much of the grain thereby leaving sugars behind.
Eventually, the drain valve will be opened and directed to the wort chiller and the fermenter. This is referred to as the second waters.
Second waters have a lower sugar concentration than the first waters.
Finally, the distiller will add another volume of water to the mash at 80˚C (referred to as the “third waters”). The whole process of mixing (optional), recirculating (optional), and draining is repeated. However, these ’third waters’ are not then sent to the fermenter. They are instead pumped into a holding tank, where they will be used as part, or all of, the mashing liquid for the next mash.
Third waters contain very little fermentable sugars. However, the saving and recovery of what little there is, is still worth the effort so long as the process is done hygienically preventing unwanted microbial spoilage.
In the next section, we will discuss a relatively new technology used for wort separation - the mash filter.
Wort Separation Process and Technology - Mash Filters
The mash filter is a relatively new technology in the world of whisky distilling. Mash filters have become more popular over the past few decades in the beer brewing sector. However, their typically high price tag along with some distillers’ desire to maintain tradition, has somewhat slowed their adoption in the distilling industry.
A mash filter is exactly what the name implies: it filters the mash. However, the method it uses to do this, and subsequently the way we need to design distilleries in order to implement its usage, could not be more different from the traditional mash/lauter tun system.
Earlier in the lesson, we mentioned that in systems that use a hammer mill to crush the grain, wort separation could prove difficult, due to the small grain particle size. Mash filters are designed to easily handle these small particles. Indeed, operations looking to install a mash filter also install a hammer mill. It does not matter if the husk is intact if a mash filter is used because the mash filters have sheets that act as filters to produce clear wort. Whereas, in the mash and lauter tun, the large husk particles settle to form a layer that helps to filter the wort.
Another difference between these operations is that the mashing vessel does not need a false bottom or perforated plates, since wort separation happens within the mash filter (though starch gelatinisation and conversion still happen in the mash vessel).
The diagram below shows a modern mash filter. The theory behind its operation is simple. The mash (grain and liquid) is pumped into the filter from below. As there is a lot of liquid already present in the grain, sweet wort will almost immediately begin to flow into the wort collection channels. Eventually, however, this flow rate will slow as the grain releases increasingly less liquid. At this point, the membranes will be inflated, which puts pressure on the grain and squeezes out any remaining liquid.
Next, the distiller will allow sparge water to flow into the mash filter, re-wetting the grains and rinsing them of their residual wort sugars. Once again, the filter membranes will be inflated to squeeze the grains and remove residual wort. At this point, the grains will have had so much liquid removed, that they will be typically close to a cake consistency. The mash filter frames can be opened up, and the spent grain can then be dumped out.
In the next section, we will discuss the final step of transferring the wort to the fermenter: wort cooling.
Wort Cooling Process and Technology
Distillery yeasts do not like things to be hot. In order for them to survive and do the job of fermenting our sugary wort, they have certain temperature requirements that need to be met. Too hot, and they may not survive. Too cold, and they may become inactive. Temperature demands are largely yeast strain dependant. However, in general, distillery yeasts work best between 20-30˚C.
In order to reach these temperatures, we need to take our wort (which is around 60˚C) and cool it to a temperature that our yeast will be happy with. This can be achieved by running the wort through a heat exchanger on its way to the fermenter.
Several designs of heat exchangers are available, but the most commonly used type in the distilling industry for wort that does not contain grains, is the plate heat exchanger. (The diagram below gives us an example of this.)
Hot wort enters the heat exchanger and moves in counterflow to a cooling agent, which is usually cold water across the plates. The design of the heat exchanger prevents the two liquids from coming into contact with each other. Since they are moving in counterflow, the heat from the wort is quickly transferred to the cold water. The cool wort exits the heat exchanger at the required temperature and moves to the fermenter. The newly heated water exits; some distilleries may collect it in order to utilise the heat energy for a different process.
Another type of heat exchanger is the shell and tube. Here, the cooling liquid flows through the shell around the tubes, removing the heat from the wort that flows inside the tubes. These are usually found in the larger grain distilleries where the cooling load is higher than that which could be handled by a plate heat exchanger. Furthermore, a plate heat exchanger will block with solids if the distillery practises an ‘all-grain-in’ system, so in these circumstances a shell and tube heat exchanger is used.
If you have experience in working with beer, then you might be wondering where the oxygenation of the wort comes in. Brewers will often directly add oxygen gas into the cooling wort on its way to the fermenter, as oxygen is important for yeast health. Very few distilleries do this. The reasons are numerous.
Firstly, we do not boil our wort, in order to preserve enzyme activity for our secondary conversions. This means that a small amount of oxygen will remain still dissolved in the wort.
Secondly, the wort will usually pick up some air during transfer to the fermenter. We do not need much oxygen for the yeast, so this little bit of air pick-up can be very beneficial.
Thirdly, distilleries often use new yeast, either dried or in a yeast cake (more on this in an upcoming lesson). This yeast has been grown and propagated aerobically, which means that its oxygen needs have largely been met.
Most whisky distilleries do not boil or oxygenate the wort.
At the end of the process, we hope to have produced a good quality wort ready for fermentation. Yet what makes a good quality wort? Let us consider this in the next section.
Wort Composition
Having a good quality wort is critical to making a good quality whisky. Let us take a moment and consider what constitutes a good wort.
For the next several days, the wort is going to be the home for your distillery’s yeast. The yeast will do its job and convert your wort sugars into alcohol. We therefore want our wort to provide a comfortable environment for the yeast. We have already spoken about temperature. We will now look at some of the other factors needed to ensure wort quality.
Yeast needs more than just sugar to survive. It also needs vitamins and minerals for proper growth and cell function. Just as importantly, yeast needs nitrogen sources for the production of cellular proteins. We refer to this as FAN, which stands for Free Amino Nitrogen.
Fortunately, sufficient nutrients and nitrogen are not something we usually need to worry about in all-malt worts. What we do need to consider, is making sure that there is enough sugar in the wort for our yeast to consume.
At the end of mashing, our grains’ starches have been broken down into fermentable sugars. Maltose (composed of two glucose molecules) makes up the bulk of the wort sugars at about 70-75%. Maltotriose (composed of three glucose molecules) comes in second place at roughly 10-15%. Glucose comes in third at about 10%. There are also tiny percentages of fructose and maltotetrose.
However, this is not the whole story. There is still a fair amount of dextrins and larger carbohydrates in the wort left over from mashing. Yeast is not interested in these molecules in their current state, because they are not easy to consume. However, there is a fix for this. Remember, we did not boil our wort like beer brewers do. This means that we have not inactivated any of our mash enzymes by adding high amounts of heat. Since those mash enzymes are still somewhat active (though often slower at lower temperatures), they continue to work in the cooled wort, breaking down larger carbohydrates into fermentable sugars. One of the most important enzymes in this process is limit dextrinase which cleaves amylopectin branch points allowing for amylases to break apart more maltose and glucose molecules. Indeed, some estimate that between 10-15% of our wort’s fermentable sugars come about from this process. This process is what we call secondary conversion; it is very important to the overall efficiency and yield of a distillery.
Key Takeaways
Counter-current plate heat exchangers rapidly chill the boiling-hot wort down to around 18°C to 20°C before pitching yeast, recovering the heat to warm the water for the next mashing cycle.
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