10.2 Milling
The purpose of milling is to reduce the size of the malt particles and to expose the endosperm to attack by enzymes during the mashing process, so that the greatest conversion of starch to fermentable sugar is achieved in the shortest possible time. Particle size is important for the performance of the mash separation system, and for lauter tun performance it is considered beneficial to preserve the husk portion of the malt grain. For mash filters, this is not critical as the filter bed is comparatively thin and the pressure differential across the filter back is comparatively high.
For a lauter tun, poorly modified malt should be milled more finely to give enzymes access to the starch which can be shrouded behind cell wall material which has not been broken down adequately in the malting process. However, the smaller particle size impacts negatively on the rate of run-off of wort. Conversely a well-modified malt can be milled more coarsely to permit faster separation without sacrificing extract performance.
10.2.1 Dry milling
There is a wide variety of dry mills incorporating two, three, four, five, or six rollers, but in larger breweries today they are almost exclusively of the six-roll variety arranged in three pairs with the gap between each successive pair being less than the preceding pair. The first pair is intended to crack open the malt grain and remove the husk which is then separated on a vibrating sieve and arranged to bypass any subsequent milling operation. The top rolls may be fluted and rotate at typically 300±400 rpm with a diameter of 250±300 mm, and, the gap between them for dry malt should be 1.6±1.8 mm. The successive pairs are similar but the gaps are 0.8±1.2mm and lastly 0.5±0.8 mm. By adjusting the gaps and analysing the results through a set of sieves, whilst monitoring brewhouse performance for yield and throughput, it is practical to optimise mill settings. Furthermore, different settings can be utilised for varying recipes of raw materials, and the modern versions of six-roller mills are capable of the gaps being set remotely from the control system as a recipe-derived parameter.
10.2.2 Conditioned dry milling
It is desirable to preserve the malt husk for fast separation in the lauter tun, but the husk is a very friable material which shatters easily, and conditioned milling was developed to overcome this by adding a small amount of water or steam to the malt through a screw conveyor just before milling. Typically, 1% of the malt by weight is added and this is taken up entirely by the husk without penetrating the kernel; the moisture content of the husk is raised to perhaps 10% by weight making it flexible. The kernel must remain substantially dry and brittle so that the necessary size reduction can be carried out effectively. Conditioned dry milling has been reported to reduce cycle time with poorly modified malt by up to 10 minutes in a 3-hour lauter tun cycle, or alternatively can run with a deeper grain bed and therefore longer brewlength for the same cycle time (Andrews, 1996).
10.2.3 Steep conditioned milling
In steep conditioned milling the malt is fed into a conditioning chamber prior to milling where it is sprayed with warm mashing liquor at 50±70 °C for a controlled time, typically 60 s, such that the uptake of moisture by the husk is considerably enhanced to perhaps 25% by weight to make it very pliable and to preserve it through subsequent milling operations. The mill normally consists of a pair of rollers with the gap between them set at 0.3±0.5 mm. Immediately after milling all the balance of the mash liquor is added and the mash is pumped to the mash conversion vessel (Herrmann, 1998). The main benefit is that the husk is almost entirely preserved and the lauter tun bed therefore has an enhanced porosity. This allows a much higher specific bed loading, typically 20% more than with dry milling, and therefore the lauter tun can have a smaller diameter and lower capital cost.
The main reported disadvantages are fourfold. Firstly that the power consumption is typically four to six times higher than for a dry mill for the same brewlength. This is because it is essentially a combined mill and mashing device and mashing of necessity should be carried out in an absolute maximum of 30 minutes, whereas the grist from a dry mill can be milled over 2±3 hours and held in a grist case for mashing later.
Secondly, air is entrained in the milling and mashing process which can lead to lipoxygenase activity with pale lager malt, resulting in possible undesirable flavours in the beer. This can be overcome by flooding the milling and mashing chambers with carbon dioxide or nitrogen. Thirdly, the turbidity of the wort is higher, which is a function of the fine mill gap setting. Fourthly, the cleaning of the mill and mash chambers must be rigorous as any residue of mash can lead to infection (Wilkinson, 2001).
10.2.4 Hammer milling
Hammer milling is a dry process used exclusively for the preparation of grist for use in a mash filter. The machine consists of a perforated cylinder or sieve, and inside this is a rotor mounted with freely swinging `hammers'. The malt is introduced to the centre of the cylinder and is pulverised between the rotor and the sieve so as to reduce it to powder. There is no requirement to adjust the particle size of the grist. The power requirement is very high, 100kW being typical in a one million hectolitre per annum brewery. Consequently the vibration and noise outputs are high and require special housings to absorb them. Maintenance must be regular and replacement of the sieve is not uncommon after 300±400 batches, with hammers being replaced after 1000 batches.
10.2.5 Submerged disc milling
Recently a fine milling process for mash filters analogous to wet roller milling has been pioneered (De Brackeliere, 2000). Thus milling and mashing are carried out at the same time between two counter-rotating stainless steel ribbed discs with a gap of 0.3±0.5mm between them. The ensuing mash is pumped to the mash conversion vessel. Very little data is yet available on the benefits or otherwise of this system, but it has been developed by Meura, the leading producer of mash filter
