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Milling: the basis of the process

09 January 2018
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What you should do and know to ensure proper malted barley milling and obtain good-quality beer

By Michael Trommer

Composition of the grist and volume–filterability ratio

Grinding is the basis for the gain of the wort and the volume of the grist. It is critical for good wort filtration and good mashing performance, as well as for determining the amount of liters of wort at the end of the process and the intrinsic quality of the finished beer, without acidity.

  • An amount of 100 grams of malted barley occupies a volume of approximately 180 ml.
  • If the malted barley is ground in different ways, its volume changes, as shown below:
    • coarse grind – approximately 280 ml;
    • fine grind – approximately 210 ml.

Therefore, it is possible to notice that the finer the grist, the smaller the volume occupied. In order to ensure that, the gap between the rollers must also be smaller. This is extremely important for the volume of spent grains inside the lauter tun.

As stated above, 100 grams of ground malt correspond to a volume of 180 ml. This volume remains between 150 and 200 ml as spent grains in the lauter tun.

The finer the grist, the smaller the distance between the rollers. Consequentially, it is also smaller the volume of grist and the height of spent grains in the lauter tun. To obtain very fine grist, a filter press and a vacuum filter need to be used.

In case of dry and conditioned milling, the results are as follow:

Volume of grist for 100 g:

Dry – 260 ml;

Conditioned – 320 ml.

Volume of spent grains in the lauter tun:

Dry – 208 ml;

Conditioned – 230 ml;

Wet – 320 ml.

Conditioning the malt before milling makes its volume increase 10%, while in wet milling the malt volume increases 50%.

Grist determination

An empirical analysis visually assesses husk break and the composition of the grist and flour. Samples must be taken from several points under the rollers to check the consistency of the mill’s settings. The samples should not contain whole grains nor “overcrushed” grains (dust).

Empirical analyses do not provide reliable results. In virtue of that, the use of sieves in the assessment is necessary.

The selection of grist by the sieves may be performed through the methodology presented in the book MEBAK II, on page 16. The sieves are differentiated by wire diameter, and mesh aperture and width. The Pfungstadt plainsifter (a series of five sieves, one on top of the other) has the following structure:

  • A sample of 100 g of husk in a graduated cylinder presents volume above 750 ml, taken from the sieve #16 of the Pfungstadt.

The sieve set plus the grist must be agitated at a speed of 300 rpm per minute for 5 minutes.

Sampling

The sample should not be taken from the grist case, because the elements that make up the grist end up mixing, due to the weight difference (specific weight). Multiple samples should be taken from under the rollers, and they should not be greater than 100 to 200 grams each.

Every mill presents sampling points under the rollers, as well as one point for the grist, before it falls into the grist case.

Mills

The malt grains are crushed with the help of rollers. Those rollers can be smooth or slotted (with slots made of high-duty cast iron). A different speed between the two rollers has the advantage of increasing attrition, ensuring a best exposition of the endosperm, which removed from the amide in the husks.

Two-roller mills

It is the simplest set for grinding. The mill has two identical smooth rollers (one fixed and one adjustable), typically with a diameter of 250 mm, rotating at the same speed.

Even with a well-regulated mill, the quality of the grist depends on the quality of the malted barley. With this type of mill, only good malt, thoroughly dissolved after a successful malting process, offers good wort filtering speed and satisfactory mashing yield. In order to reach these optimal results, the brewer must check if the following conditions are observed:

  1. a) homogeneous feed throughout the roller surface;
  2. b) a ratio of 15 to 20 kg of malted barley per centimeter squared of roller, per hour;
  3. c) roller speed at 160 to 180 rpm per minute.

In case of less-dissolved malted barley, the use of a mill with more rollers is necessary.

The performance of a two-roller mill can be further improved with slotted rollers, one rotating at a higher speed than the other, as well as conditioning the malted barley with water at a temperature of 35° to 50° C, before it goes into the mill.

Fraction

Sieve number

Wire diameter (mm)

Mesh aperture (mm)

Husks

16

0.31

1.270

Coarse grind

20

0.26

1.010

Fine grind I

36

0.15

0.547

Fine grind II

85

0.07

0.253

Flour (powder)

140

0.04

0.152

Flour-like

Floor

-

-

Fraction

Lauter tun

Mash filter

Husks %

18

11

Coarse grind %

8

4

Fine grind I %

35

16

Fine grind II %

21

43

Flour %

7

10

Fine flour %

11

16

Four-roller mill

The first stage of malt grinding occurs in the upper set of rollers: the coarse break, that is, the pre-break.

In this stage of the grinding, only one break takes place. With the use of vibrating sieves, part of the amide comes out of the grain via mechanical agitation, while the rest of it remains in the husks. There should not be whole grains after this stage. In order to ensure proper grinding, the following are required:

  1. a) constant and reduced mill feed with malted barley of 20 kg/cm² per minute;
  2. b) rotation speed of 160 to 180 rpm.

The second pair of rollers, located under the pre-break rollers, must have a smaller gap to continue the grist break.

The grist milled between the first and the second set of rollers increases the volume in 50 percent. Due to this increase of volume, the bottom rollers need to rotate at a higher speed — between 240 and 260 rpm.

Mills with larger capacity have a higher rotation speed, as do smooth roller mills: pre-break rollers (first set) at 200 rpm and bottom rollers at 300 rpm.

Technically, it is not an advantage to grind all the grist twice – only the part that needs that. The elements of the grist that do not need a second grinding are removed from the process. The removal occurs with the help of vibrating sieves inside the mill. In order to prevent grist (dust) accumulation, the sieves must have rubber balls and a filter for cleaning the dust and dampen the noise from the trepidation. An eccentric shaft makes the sieves move.

In the case of four-roller mills, after the grain pre-break, the part that consists of fine amide grind and dust pass through the sieves, while the husks and coarse grind are submitted to further, finer grinding, through the bottom rollers.

This mill has capacity of 25 kg/cm². The set of pre-grinding rollers (first set) must have a speed of 180 to 200 rpm, and the second set, 200 to 220 rpm, in order to avoid damaging the husks. For that reason, the second set of rollers is smooth. The result of the grinding depends on the quality of the malt.

Six-roller mill

Six-roller mills have three stages of grinding and generally two separate sets of sieves.

In six-roller mills, the feeding occurs through slots or a flap. After the pre-break, the sieves divided the ground malt into three fractions. The fine flour is taken out of the grinding system, for not needing regrinding. The husks with grist inside are caught by the upper sieve and are carried to the second set of rollers, called husk rollers. In this stage, the husks are not crushed – they only release any grist stuck inside. The vibrating sieve separates flour and grist, and the former is removed from the grinding process. Similarly as the previous stages, the coarse grind is carried to the third set of rollers, for intensive grinding.

The area of the sieves is carefully calculated. They are mounted at an angle (slope) to ensure homogeneous sieving action.

The sieve boxes, which serve to separate the grain elements and send them to the following stage (inside the mill, or out of the grinding process and into the finished grist case), are activated by a single eccentric shaft. This shaft creates a wave, which moves all of the sieve boxes.

Grinding results obtained with the six-roller mill

 

Pre-break

Husks

Grind roller

Total grist

Husks %

60

55

0

18

Coarse grind I %

9

11

10

8

Fine grind I %

12

16

46

38

Fine grind II %

8

8

22

17

Flour %

1

1

5

5

Fine flour %

10

9

17

14

Every grain mill requires an oxygen-free environment, and breweries use carbon dioxide, which is produced by the beer fermentation process itself. Before the grains are fed into the mill, they need to pass over a magnet, which will retain any metal that may impair or reduce the quality of the final product. Before grinding, it is recommended to perform the cleaning of the grains, using sieving or exhaust system.

As previously mentioned, conditioning of the malt with water is good, but not essential. A fine spray of water at 35°C to 50°C is used to wet only the grain husk and make it more elastic. The water spray must not exceed 50° C, so as not to activate the enzymes.

Conclusion

If the brewery does not have an adequate grinding system, it can purchase the best malted barley, but will not be able to achieve its goal of producing beer that meets the desired standards and within expected cost.

Grinding is the basis of the entire brewing process. Therefore, it is essential to pay close attention to this stage and take special care in this initial step. Since brewery are typically very dusty and hot, brewers do not pay due attention, and are guided only by laboratory results, forgetting that many problems that occur in the following stages of the brewing process may have originated in the first stage of the process: malt cleanliness and grinding.

Michael Trommer

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Michael Trommer

Michael Trommer, descendente de pais alemães que vieram para colonização do norte do Paraná e posteriormente migraram para São Paulo.

Com 28 anos de experiência profissional na área de bebidas (cervejas e sucos). Contudo, sua maior atuação foi na indústria cervejeira, onde atuou na área de produção, laboratório, desenvolvimento de produtos maltados e não maltados, padronização de processos (sistema da qualidade) e implantação de manutenção preventiva e preditiva. Presta serviços para empresas de pequeno, médio e grande porte do segmento de bebidas.

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