Milling

Objective

To identify the design and methods of milling operations in the brewery

Why mill?

We have to convert whole malt corns into a condition suitable for:        Total starch conversion in the mash        Rapid and complete separation of wort from the mash solids.

Milling systems must take into account the different types of mashing and wort separation systems in breweries.Two processes take place during milling:

Breaking open of the corn.

This exposes the starchy endosperm

Particle size control.

This produces the husk and the starchy flour. Undamaged husk is important for wort filtration. The flour is worked on by the enzymes. The consistency of the flour is important. Too fine and the mash will stick. Too coarse and the enzymes will not be able to reach all the starch.

The art of milling is to produce a grist with just the right proportion of husk, fine flour and larger sized grits.Typically for a Lauter tun  the grist should consist of:Husk 15%Coarse grits 23%Fine grits 30%Flour 32%These figures vary for mash filters and infusion mash tuns.Only roller and hammer mills are used in breweries.All roller mills require the malt to be fed evenly onto the rolls. All variations of this type of mill are equipped with a set of deeply grooved feed rollers in addition to those used for size reduction.

Uneven feed onto the rolls leads to uneven grinding and variable wear on the rollers. Some mills also have a grooved, metal plate that orientates the corns lengthways as they contact the grinding rollers.Rollers are mounted on pivoted, threaded blocks. This allows adjustment of the roller gap. The gap is important in determining particle size.Checks across the whole width of the rolls using feeler gauges should be done at regular intervals. Modern mills are computer controlled. They automatically set roller gap to match grinding gaps.

The Four Roll Dry Mill

Small breweries often use a simple two-roll mill. Such equipment is low cost and simple to operate. However this type of mill will only function satisfactorily with well modified malt.

Introducing a second pair of rolls improves the flexibility of a mill. Some separation of flour is made before material enters the second set of rolls. This is because the presence of flour interferes with grinding grits. This type of mill is usually equipped with cylindrical sieves and beaters below the first rolls. These divert husk, fine grits and flour. This leaves only coarse grits to pass through the second set of rollers . Even so its use is normally restricted to breweries using mash tuns which require a coarse grind from well modified malt. 

 

The Six Roll Dry Mill

This type of mill is able to handle a wide variety of malts. It can produce both coarse mash tun and finer lauter tun grists. It is also capable of a finer grinds suitable for a mash filter. However lower capital cost hammer mills are normally used for mash filters. They are very much cheaper than a standard

mill. Also mash filter worts produced from roller milled grist tend to be cloudier than when hammer milled grist is used.

Three pairs of rolls allow optimum grinding for husk with coarse grits and fine grits. Paired screens below each of the first two sets of rollers divert the various fractions, either to the appropriate set of rolls or direct to the mill outlet. 

 

The Hammer Mill

The hammer mill is an impact mill widely used throughout the food processing industry. A hammer mill consists of a rotor made of two or more plates with pins to carry the hammers. Hammers are simply flat metal bars with a hole at one or both ends. They may have some type of edge preparation such as hard facing or carbide coating to provide better wear resistance. Milling in a hammer mill is a result of impact between the hammer and particles. There is also an effect between particles and screen when the material accelerated by the rotor flies out into the screen. All the moving parts are contained within a heavy casing. This is usually compact enough to withstand 10 bar shock pressure in the event of dust explosion. Particle size leaving the mill is mainly controlled by a sieve. Rotation speed and feed rate can effect some change. A faster in-feed tends to slightly coarsen the grind. Rotation speed varies with the size and type of hammer within the mill. Pivoted hammers typically rotate

within the range 2,500 - 5,500 rpm. They require only 1/2 to 2/3 of the power input needed to drive a fixed plate beater arrangement.Hammer mills have the following advantages: -

Simple and robust. Adjustment is not critical. They will handle variable quality in feed material.

The grist particle size depends on the size of the holes in the sieve.

Wet Milling & Steep Conditioning

Wet milling combines the mill and masher in one unit.Conditioning malt toughens and gives elasticity to the husk. It leaves the endosperm unaltered. Wet milling goes further. Greater amounts of moisture are absorbed. This is sufficient for the endosperm to become softened and semi dough like. This allows it to be squeezed out of the intact husk during milling. 

Equipment consists of a steep tank where the entire malt charge for a brew is soaked in water for up to 30 minutes. The amount of water is preset. The soaked malt then passes through a simple two roll mill. Here the endosperm is squeezed out. Because the husk is soft it is not damaged. The milled malt passes directly into the mashing system. Additional water is sprayed in to get the mash consistency correct. Steep water can be re-used as part of the mash. The mash either falls directly into the mash vessel or is pumped to it using a low shear positive displacement pump.The advantages for wet milling are: -

Good husk preservation Faster lauter tun run off or alternatively, plate loadings can be increased Higher extract recovery Simple mill Lowest air pick up (oxidation) of grist of any milling system Raw barley may also be milled. (this does cause excessive wear on the

rolls) No dust or explosion risk  There is no need for a separate grist case

The disadvantages are: -

Over-steeping can occur. However under-steeped malt will shatter in this mill.

Milling must take place as part of the mashing sequence. Motors and other equipment must be larger than normal to accommodate the high process rate.

The system must be flushed clean after every batch. This requires up to 6 hl water / tonne of grist. This can lead to thin mashes.

CIP  equipment must be fitted. Unless kept scrupulously clean the equipment can become a source of infection.

A variation of the conventional wet milling procedure described above is the steep conditioned milling system.During steep conditioning milling the malt is not left to soak in a hopper but is sprayed with hot water before it enters the mill. The malt moisture increases approximately 15% and the husk becomes very pliable. The endosperm remains dry. A simple two roller mill is used. The roller gap depends on malt quality but is normally within the same range as is expected for the bottom rolls in a six roller mill. The in-feed is regulated by a feed roller. The advantages and disadvantages of this system are basically the same as for

conventional wet milling. Since smaller amounts of water are used it is able to produce thicker mashes.

Figure based on an original Steinecker drawing.

Mill rolls

Mill rolls are made from hard steel. The grooves (also called  flutes) must be kept at a minimum depth. The grooves will require regrinding at intervals. This happens especially when raw barley is used. Raw barley is much more abrasive than malt.

The flutes do not run parallel but are rather twisted across the surface. The opposing mill rolls also rotate at different speeds. This is what produces the shearing action necessary to spit the husks.