Lesson 7 - Milling Technology SAQ

**Briefly describe the design and operation of a six-roller dry mill and a steep conditioned wet mill. Explain why you would select each.

Response:

Six-roller mills

Six-roller mills have three sets of crushing rollers. Oscillating or inclined screens between the rollers separate the components of the grist by size. The number and location of the screens will depend upon the design of the mill. The first pair of rollers breaks up the kernel, the second pair mills out the endosperm and the third pair comminutes the coarse grits. The exact operation of the mill depends on the arrangement of the screens. In some models, there are screens between the first and second pair, and the second and third pair. In other models, screening is only undertaken between the second and third pair.

The screens in the mill help to prevent over-comminution of the husk particles and allow the second and third sets of rollers to focus on breaking up endosperm fractions. The third set of rollers is intended to deal with the hard ends encountered with under-modified malt. The six-roller mill provides a good level of extract efficiency while limiting damage to the husk to aid wort separation.

Reasons to select a six-roller mill

Six-roller mills are best suited to working with lauter tuns and are able to process more poorly modified malts than wet and dry two- and four-roller mills, which in turn yields greater levels of extract. They are particularly well suited to very large breweries. They have a lower energy requirement than hammer or wet mills at 2.2 kWhr/t of malt, but they do not provide as much extract as a hammer mill, underwater disc mill, or a DISPAX REACTOR®.

Steep conditioned wet mill

The steep conditioned wet mill is typically a two-roller mill attached to a steeping chamber fed by a malt storage hopper. Malt is fed from the storage hopper into the steeping chamber (sometimes called a chute or cylinder), where water is sprayed onto its surface. A feed roller regulates the flow of malt to ensure that the chamber is not overfilled and that water can access the grain easily. The speed of this feed roller can be adjusted to increase or reduce the residence time of the malt and hence the steeping time. Excess water is drained from the chamber as necessary. This is usually recovered and used as mashing water. Conditioning time is determined by the speed at which the grain flows through the mill. A control system and gears ensure that the rate of malt entering the system matches that of the mash leaving the system. A feed roller above the crushing rollers meters the grain onto the crushing rollers, distributing it evenly across them. After crushing, the grist is further hydrated with sprays of mashing water to produce the mash. The mash is then pumped to the mash conversion vessel. After milling, the mill is flushed with water to remove as much of the malt/grist residue as possible.

The steep conditioned wet mill is designed to preserve the husk, providing excellent rates of wort separation and/or increased loading in lauter tuns. Hydrating the husk enables this while allowing a finer comminution of the endosperm than is practical in the equivalent dry mill.

Reasons to select a steep conditioned wet mill

Steep conditioned mills cause less husk damage compared to dry mills, so are ideal for lauter tuns in medium to large sized breweries. The use of water prevents explosions and can help with the control of grist oxidation. It is also possible to add lactic acid to the mill to help limit the activity of lipoxygenase enzymes (LOX) and achieve the target mash pH. It is also possible to use hotter water to denature LOX.