U – for Underback

What does the word Underback mean?

Literally translated, the English term Underback means a buffer ("back") that is connected at the lower end ("under") of a lauter tun.

https://youtu.be/G2qySDggBBI

What does an Underback look like?

An underback is a simple pipe at the height of the lauter tun, also known as the mash tun. This pipe, through which the wort flows, is connected to the mash tun below its false bottom. In addition to a simple pipe, the underback can also be a larger tank, as is often found in Scottish distilleries.

What is an Underback used for?

The underback is used to control the lautering process in the distillery, ensuring that the wort drains optimally through the false bottom in the mash tun. In the mash tun, crushed barley malt is first mixed with hot water, before the solid components are separated from the liquid components after a certain time via a false bottom located in the lauter tun, which is classically referred to as lautering. The liquid components – the wort – leave the mash tun and enter the underback. For an optimal lautering process, the liquid level in the underback should be at the same height as in the lauter tun. The underback thus corresponds to the principle of communicating vessels. During lautering, however, the false bottom in the mash tun can become so clogged by the compacting filter cake (spent grains) that the wort drains only slowly, or eventually not at all. In this case, the liquid levels in the underback and in the mash tun are no longer at the same level but at different levels.

What happens if the false bottom clogs?

A visually noticeable difference in level in the underback thus indicates a sub-optimal lautering process in the mash tun and requires readjustment of the wort flow rate. If, on the other hand, the false bottom is completely clogged by the compacting spent grains in the lauter tun, the production team can loosen it by switching on a raking machine, thereby restoring its permeability for the remaining wort.

Can lautering be controlled in other ways?

The underback is the original, analog way of controlling lautering in the mash tun. The modern, digital variant is control by means of so-called differential pressure measurement, which is also installed in the St. Kilian mash tun. Using sensitive pressure sensors, the difference between the pressure above and below the screen, which arises as a result of compaction of the spent grains during lautering, can be measured and indicate a necessary intervention by the production team. Compared to the classic underback, this digital measurement is the simpler and also more convenient method.