Wheat Milling Part 3 (Dampening and Tempering, The Second Cleaning Section)

By : Novozymes and Buhler

5. Dampening and Tempering

Dampening and tempering of wheat helps :
  • Make the bran tough and elastic
  • Mellow the endosperm
  • Achieve the correct flour moisture content
  • Maintain constant grinding conditions

During dampening, the grain enters the dampener through the front-end inlet. Together with the separately added  water, the grain undergoes an intensive intermixing and water penetration by the bi-rotor whirling system. The thoroughly dampened grain is discharged from the machine through the tangential outlet to the tempering bins.
Cross Section of dampener (Picture is courtesy of Buhler AG, Uzwil, Switzerland)
Selection of the moisture content of wheat at the 1st break depends on various factors. If we have to produce flour with low ash content, it would be better to run with higher moistures. This is especially the case with the hard wheat varieties. Here, we have a rather brittle bran layer and a harder endosperm and it is rather difficult to avoid too much bran breakage. By increasing the moisture content of the wheat, this problem can be solved to a certain extent. Conditions are just the opposite for the soft wheat which has tough bran and a soft endosperm. In other words, the harder the wheat, the higher the 1st break moisture should be.

The 1st break moisture, however, has to be selected mainly in relation to the maximum permissible moisture level of the flour. And this again needs to be considered in relation to the moisture loss (evaporation) in the mill to avoid problems in the sifting if the stocks as well as to ensure a safe storage of the bran.

The following guidelines show the ideal moisture levels at the 1st break from a technical point of view :
  • Very hard : 16.5-17.5%
  • Hard : 16.0 – 16.5%
  • Semi Hard : 15.5-16.0%
  • Soft : 14.5-15.0%

These levels have to be adjusted from plant to plant to match commercial aspects. The formula for the calculation of the water to be added is :

To make the endosperm softer, and right up to the center, the water must have enough time to penetrate all the way into it. This called “tempering time”. The tempering time depends on various factors such as structure of the wheat, hardness degree, temperature of the wheat, initial moisture content, and condition of the outer bran layers, especially the epidermis and seed coat, distribution of the water onto the kernel surface, quantity of protein. The tempering time indicated in Table :
Type of Wheat
Tempering time    (h)
Very hard wheat

6. The Second Cleaning Section

The second cleaning section starts below the tempering bins and ends at the first break bin. It is the task of the second clening section to remove all kind of surface dirt and outer (loose) bran layers to eventually provide impact treatment to the wheat and to condition the bran to achieve an optimum grinding condition.

While mixing water with the wheat in the intensive dampeners ahead of the tempering bins and due to the friction of the grains among themselves, the outer bran layers as well as dirt tend to become loose. The scouring work already started by these intensive dampeners should be completed mechanically in the scouring machine.

The wheat now reaches the scouring machine with a capacity slightly higher than the capacity of the mill. This machine is now required to scour, as intensively as possible, the surface of the wheat. However it should avoid tearing up the bran, or damaging it, in any way as that would negatively influence the break system. It is also essential to avoid knocking off the grain tips and germ to prevent yield losses.

The overs, wheat and larger loosened bran particles, go on to an aspiration channel. This channel has to aspirate the wheat and take away the loosened light particles. The wheat cleaned this way now reaches the dampening section ahead of the 1st break. If the relative humidity of the air is low, hard wheat has to be mainly milled or a high extraction of low ash flour needs to be produced and the bran has to be dampened once again before reaching the 1st break rolls.

The quantity of water addition here is between 0.3-0.5%. the bin after the 1st break dampener should not have more than 30 minutes holding capacity. Test have shown that it takes about one hour for the water to pass the bran layers. The wheat therefore, should not remain too long in the 1st break bin, if water is added for bran conditioning. It is also essential to the water arriving at the aleuronic layer of the wheat kernel, as this would make it increasingly difficult to separate bran and endosperm in the break system.

The moisture content of the flour does not increase much by this water addition. And mistakes made during main conditioning, cannot be corrected anymore at this stage.


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