Wheat Milling Part 5 (Blending and Packing)

By : Novozymes and Buhler

9. Blending

the flour after milling is stored in storage bins usually made of reinforced concrete or steel. from there, it is either transferred directly to the packing line or mixed with other flours and ingredients to achieve the desired quality characteristics. in the past, grain gristing (mixing) was used based mainly on equal wheat hardness. however, if the capacity of flour mixing equipment is adequate it is no longger necessary. Flour blending can be done by the following means :
  • continuous, volumetric dosing and mixing
  • continuous, gravimetrical dosing and mixing
  • batch mixing system

the continuous volumetric system can be applied, if no high demand in regards to mixing accuracy is necessary. Frequency converters control volumetric bin dischargers. the desired mixing ratio can be adjusted by selecting the speed of the bin dischargers. Screw conveyors collect the continuously discharged flours and lead them to a pneumatic lift. The volumetric mixing system should only be applied, if larger batches of mixes have to be produced. During every start up or stopping period, an increased inaccuracy of the mixture may be noticed, because not all the discharges might reach full operation at the same time.

An additional problem in terms of accuracy could arise, if the bulk density of different types of flour keeps changing. This simple installation however, can be an economical solution for mixing flour at a high capacity.

When large batches of flour mix with a relatively low number of components have to be produced, the continuous, gravimetrical dosing and mixing system should be applied. The basis of this system is the continuous weight recording and regulation of the through-flow capacities with a following homogeny mixing of the continuously arriving product streams.

Continuously working scales, which are installed below the flour bins, join together the different types of flour at a selected flow rate. Chemical feeders can be additionally installed for the continuous addition of the micro ingredients. A homogenizer mixes the arriving products into a uniform mix. Every component of the mix requires an independent scale. The scales are controlled via a computer program, which contains different recipes for the mixes.

Such a system is favored because it facilitates accurate blending of the main and special flours, small ingredients according to recipe, especially suitable for lot sizes of 10 to 100 t and higher and easy alteration of capacity.

If a large number of different flour types have to be produced, a batch mixing plant can offer several advantages :
  • increased flour yield, due to the possibility of separate milling of differing wheat varieties in regards to the degree of hardness
  • Milling of larger wheat quantities of the same grist, resulting in less grist changes and therefore, less labor work and less off-grade flour, which is produced during grist changes
  • Better flour storage bins utilization
  • High accuracy of the flour blending due to dosage by weight
  • More flexibility concerning flour varieties or just in time deliveries
  • Possibility of correcting mistakes made in the cleaning or grinding section with little effort
  • Mixxing a small lots of even 500kg
  • Easy extension of the flour assortment, therefore, very flexible to meet special customer requirements
  • Lower investment costs if a large number of different flours and ingredients have to be mixed
  • Reduction of mixing time, due to the application of high speed mixers

With the batch mixing system, the different compounds of a mixture are weighed and accumulated in the batch scale. After completion, the batch is filled into the mixer. When mixing is over, the content of the mixer is dropped into a hopper and transported to the packing or loading bins. All this is a discontinuing process. The following four steps are included in one mixing cycle of a batch mixer :
  • Preparing the different types of flour components and additives by weight according to the recipe
  • Emptying the scales into the mixer and mixing
  • Emptying of the mixer
  • Transporting the batch into storage, packing or loading bin

If the amount of flour mixes to be produced is rather small and the addition of micro ingredients rather seldom, micro components could be added manually. Under normal conditions however, it is worthwhile to install a system to add the micro ingredients. Such an installation serves the purpose of storing the micro ingredients, feeding them to a small scale, registering the weight as well as feeding ingredients to the batch. Given that we have to deal with rather small quantities of micro ingredients, they can be filled manually into the small storage hoppers. Most of the micro ingredients have very bad flowing characteristics. Special discharging and dosing units are, therefore, required.

10. Packing

To be sure that no foreign objects remain in the flour, a rotary sifter should be installed ahead of the packing unit or bulk out-loading system. Depending on the capacity, the granulation and the flowing characteristic of the flour, sieve openings between 1.0 to 3.0 mm have to be selected. Metal detectors further boost product safety. For destroying insect egss, sterilities can be installed prior to the packing system. Finally, flour is packaged either in bags or silo trucks.
Wheat Flour

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