Lesson 7 - Milling Technology SAQ

** What mill would you recommend for the following breweries and why?

•  Small pub brewery making 3,000 hL per year, using an isothermal mash tun

•  Large brewery making 1,500,000 hL per year, using lauter tuns in an area of water scarcity

•  Medium sized brewery specialising in export beer making 150,000 hL per year, using a mash filter

Response:

Small pub brewery making 3,000 hL per year, using an isothermal mash tun

With isothermal mash tuns, both mashing and wort separation are undertaken in the same vessel and wort separation relies on the majority of the spent grains being suspended in the water above the vessel’s false floor. Wet mills hydrate the grist too thoroughly to enable the grain to remain buoyant. While it is feasible to use any dry mill, only either a two- or four-roller dry mill are easily able to make grist with a particle size large enough to remain buoyant in a mash. Of these two options, the two-roller mill has the lowest cost and so is the best option. If the brewery has a large budget, the four-roller mill will give a better extract efficiency, but even with this saving on malt, the payback period for the mill is likely to be very long.

Large brewery making 1,500,000 hL per year using lauter tuns, in an area of water scarcity

The best mills for lauter tuns are wet mills or six-roller dry mills. The size of the brewery and water scarcity both mean that six-roller mills will be the best option. All wet mills use more water and produce more effluent than dry mills, so in an area of water scarcity, they would not be suitable. The six-roller mill could be used with a conditioning screw to improve husk volume and hence lautering. Oxidation of the grist could be minimised by purging with CO₂ or nitrogen, and is best practice during grist storage and mashing in. Even with these adjustments, it is likely that oxygen pick-up will be greater than would be the case if a steep conditioned mill was used.

Medium sized brewery specialising in longer shelf life export beer, making 150,000 hL per year using a mash filter

The mills suitable for mash filters are hammer mills, underwater disc mills, and the DISPAX-REACTOR®. The brewery produces longer shelf life beer, which means that the flavour and colloidal stability must be excellent. Oxidation of lipids in the malt produces the precursors of stale beer flavour compounds, which develop over time in packages. The ability of the beer to withstand oxidative damage is also reduced if oxygen is introduced earlier in the process. One aspect of the fine comminution achieved by all these mills that could detract from flavour stability, is damage to the acrospire that can increase levels of LOX in the mash. This could mean that lipids are oxidised at a faster rate than is the case with the grist produced by other mills. The control of oxygen is therefore of increased importance for mash filter grist. If it’s not there it can’t oxidise! For hammer mills, the Carbomill provides the best control of grist oxidation but this control is inferior to that of the options that mill underwater. Also, the consumption of nitrogen and carbon dioxide is less for underwater disc mills and the DISPAX-REACTOR®, making these the best option. The choice of which one to purchase probably depends on the recommendation of the mash filter supplier. There are two main mash filter suppliers, and each one sells one of the underwater options. Performance guarantees for the equipment are likely to be tied to also using the mill they supply.