Storage and Cleaning Wheat

By : Novozymes and Buhler

Storage Wheat

Following are the options for the storage of cereals :

• Storage in sheds (bagged or in bulk)

• Horizontal silos

• Big space bins (flat bottom)

• High silos with outlet cone (self-emptying)

The storage shed is a low investment solution but it needs a large surface area. It is generally not a common method to store wheat. The big space bin is a good investment solution. 

This storage type is suitable for plants processing large quantities of the same variety of wheat for long storage times. The disadvantages include the manual emptying of its bottom part and infestation, which is difficult to control. 

The high silo with bins of different sizes is the ideal solution for the storage of cereals but it requires the biggest investment. Different shapes are built (rectangular, square and round bins) from concrete or steel sheet.

Cereals during storage respire. With respiration, oxygen is consumed and the inherent carbohydrates are de-composed. As a result, carbon dioxide, water and heat are produced. This decomposition causes a loss of weight. So how intense is this respiration as a factor for moisture and temperature of the grain?

At an increased respiration activity, much water and heat (energy) are released. The water is again absorbed by the grain, leading to a more intensive respiration. 

If such a development is not controlled in time, this self-heating can lead to a rapid rotting of the grain. Mold and other microorganisms help accelerate this process, as they develop fast in humid an warm air.

There are certain measures that can be taken to control the respiration activity of wheat. First is aeration, which involves running the wheat again from time to time through the pre-cleaning separator with an attached aspiration channel. 

A second method is cooling the grain, using which wheat with moisture of even up to 20% can be stored for a few months. In this approach, fresh air is cooled via a cooling unit and afterwards blown into the respective silo bin.

This process is continued until the complete batch of wheat has reached the desired low temperature. Usually grain temperatures of approximately 8 – 10 ⁰C are targeted. At these low temperatures, insect growth can be stopped, which is an additional advantage.

Monitoring the temperature of stored grain is a very good indicator of safe storage conditions. Today, modern temperature control installations are connected to a computer system and measuring points are distributed regularly over the complete height of the bin. 

The following criteria should be taken into account before raising an alarm :

• Maximum permissible temperature 

• Maximum permissible temperature increase to the value measured after the reception of the lot

• Maximum permissible temperature increase since the last measurement was taken

Cleaning Wheat

Wheat cleaning is done in two stages. First stage is receiving bins up to the tempering bins while the second stage is the tempering bins until the first break. The amount of impurities might vary between 1-3%. It is, of course, possible that this percentage is much higher. 

After pre-cleaning, the quantity of impurities is still high. If impurities are not removed from the wheat, the following consequences can be expected: increase in flour ash content, deterioration of baking performance, reduction of flour yield, deterioration of flour color, smell and overall hygiene.

Separating machines used in the cleaning section are based on the following differences to the wheat : size, (width, thickness), air resistance, density, length, shape, magnetic reaction, impact reaction and color. 

To achieve the highest efficiency of the cleaning machines, position of the different machines in the cleaning process is of great importance. A typical sequence would be :

• Removal of iron particles

• Separation of all impurities which are bigger or smaller than the wheat in conjunction with a good aspiration system to remove all the light impurities

• Separation of stones, glass, and non-ferrous metal particles

• Separation of foreign seeds and foreign cereals

• Cleaning of the grain surface by scouring

• Conditioning by dampening and tempering

• Scouring of the grain surface

The task of the de-stoner is to separate the stones from wheat. For an optimal functioning of the dry de-stoner, the grain must be free of coarse and fine impurities. Coarse impurities cause disturbance at the inlet, in the separating zone and at the stone outlet section.

Smaller impurities and small stones with a diameter of less than 2 mm in particular obstruct the wire mesh and disturb the distribution of the air.  

Furthermore, the material should be well aspirated before de-stoning so that the machine generates as little dust as possible. Observation of the de-stoning process is inhibited when the sight glass is covered with dust.

The method of separation is based upon the principle that stones, glass and metal particles have a higher specific density than grain.

The perforated separating table is slightly inclined. The vibro motor, which is directly attached to the table, sets it into a vibrating motion at a certain angle. 

This causes an upward movement of all particles that are in contact  with the wire mesh. The air, which passes through the openings of the wire mesh, carries the lighter particles (in this case the wheat) to the lower end of the inclined table.

Cross section of de-stoner (Picture is a courtesy of Buhler AG, Uzwil, Switzerland)

Paricles with a higher specific weight sink onto the screen of the separating table and are carried to the upper end of the inclined table, where the final separation of the stones takes place. 

At the final separating zone, a plate which is placed across the separating table, causes a counter air flow to the stones leaving the machine. The speed of this air can be adjusted by changing the position of this plate, allowing only the heavy particles to pass.

There are two basic types of indented surface separators in the market : the indented cylinder or Trieur and the indented disk separator or Carter.

Both work according to the same separating principle but are different in design. The separation method depends on the difference in length between the grain and the impurities. Thus, it can be used for all products which have more or less the same thickness as grain but are different in length.

Depending on the material to be separated, separators can be distinguished as round-seed and long grain separators. 

As indicated by the name, the round-seed separator picks out products that are shorter than wheat and have a more round shape such as cockle, vetches, goose grass, broken kernels, and mustard seed. 

The long-grain separator lifts up wheat which is shorter than other particles such as barley, oats, ergot and straw.

The main component of the indented cylinder is a steel sheet drum, having an indented surface on the inside. The diameter of the Trieur cylinder varies between 400-900 mm, depending on the capacity. 

The diameter of the indents has to suit the product to be lifted out. Rotation of the drum helps lift the shorter particles and drop into the collection through, from where they are conveyed to the discharge by a small screw.

Although the Carter disc separator works according to the same principle of separation, its design is completely different from the Trieur cylinder. 

Steel plate housing connects the two cast-iron end walls, which act at the same time as supports. The horizontal main shaft is supported by bearings that are installed at both end walls. The shaft rotates with a speed of 55-60 rpm and can fit up to 33 discs.

The number and diameter of these discs vary according to the required capacity. On the surface of the discs are pocket-like indents on both sides. Depending on the type (size) of pockets, either the black seeds and broken kernels or the wheat itself are lifted out. 

As the discs continue their rotating movement, the picked up material is tipped out of the pockets into collecting channels, which are arranged between the discs. In this way, the lifted product is lead either into the discharge hopper or to the return screw, depending on the setting of the flaps above this screw. 

This collecting screw is located at the front of the housing and leads the selected material back to the inlet of the machine. By returning this material back to the inlet, a re-separation of the product is conducted. 

The product, which is not lifted by the discs, is advanced by the flights along the entire length of the machine, and it finally over-tails at the front end.

To regain the broken kernels, the spiral separator can be applied. Particles that differ in shape and surface characteristic initiated by the centrifugal force develop different speeds while running down a spiral column. 

In this way, it is possible to separate broken kernels from round seeds (e.g cockles and vetches). The spiral separator receives the product from the round grain re-treating cylinder.

The spiral is fiited around a vertical column. Unlike a conveying screw, the diameter increases from top to bottom. It can consist of a single or a double screw. At the circumference of the spiral, a small vertical rim prevents the grains from falling off the edge.

Depending on the shape, surface and gravity of the kernels, each particle can descend at a different speed. Round seeds with a smooth surface roll down very fast and are carried by centrifugal force over the edge of the small inner spiral towards the circumference. 

Broken kernels, due to their edgy shape, slide down at a slower speed and therefore, the effect of the centrifugal force is little and they remain close to the inner part of the column. 

At the bottom end of the spiral, the graded products can be split into three fractions (broken kernels; small wheat kernels & oval seeds; round seeds) by adjustable flaps.

The next stage is scouring. Following are the objectives of scouring the grain in the cleaning section :

• Removal of dirt from the surface of grains

• Dataching any mud balls or weed seeds

• Rubbing off of the outer bran layers

• Reduction of the bacteria count

The product enters the machine and is taken into the scouring chamber by the screw type surface of the rotor plates. In the working chamber, it gets an intensive action of friction, due to the narrow space between the rotor and the sieve jacket.

The scouring action is based upon friction of the grains among each other, grains between the corrugated rotor and stator surface, grains between the spikes of the rotor and the stator, and grains between the spikes of the rotor and the sieve jacket.

A recent advancement in wheat cleaning is color sorting. High resolution monochromatic visible cameras and custom designed camera lenses detect even subtle color defects and foreign materials, enabling a highly efficient outcome in the removal of unwanted material from the product stream.

Difference between optical sorting and mechanical cleaning wheat (Picture is a courtesy of Buhler AG, Uzwil, Switzerland)

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