Wheat blending and mixing
January 01, 1996
by Teresa Acklin
Industry consultant David Sugden describes the secret to consistency in finished flour products.
Often it has been said that the most important art in flour milling is wheat purchasing and use. Rather, wheat purchasing and use is a science, a percentage risk game that can be calculated for the most favorable outcome. If the process starts right, it stays right, creating the best chance for optimum quality and financial return. Wheat blending and mixing is at the very center of this process.
Blending in flour milling is defined as proportioning together products in this case wheat of different characteristics with a particular final flour specification in mind. Mixing is the activity of homogenizing blends to ensure consistency without variation. Both processes are important for the greatest effect.
Why is it necessary to blend and mix wheat? The main benefit of a sound blending and mixing process is attaining near-perfect and uniform quality. In today's trading climate, this result is critical, especially in medium sized or large plants. The wheat blending and mixing process subsequently allows for end-product control and improved financial return.
Wheat blending and mixing processes typically consist of either a continuous or batch system, as illustrated in the accompanying diagrams.
Diagram 1 demonstrates a classic system of continuous grain flow. Starting from the top left (1), wheat moves from the elevator or silo to the intermediate wheat bins (2). The bins are equipped with multi-hopper outlets (3) for laminar, or first in first out, flow.
Laminar flow is particularly important to eliminate test weight variation and ensure consistency. Without laminar flow, heavier wheat tends to discharge first, followed by lighter weights, defeating the objective of consistency.
The first blending section (4) can be either volumetric or by weight. The first mixing conveyor (5) typically is more than 2.75 meters in length and collects wheat from any or all of the five bins. Mixing conveyors should be screw-type to ensure optimum results.
The flow continues through the wheat cleaning plant (6) and through a conveyor (7) into the conditioned wheat bins (8). Another set of multi-hopper outlets (9) feeds the second blending system (10), from which wheat is carried in the second mixing conveyor (11) to the mill (12).
Diagram 2 illustrates a batch system, with wheat feeding from conveyors (13) to either intermediate or conditioned wheat bins (14). Multihoppers (15) discharge wheat via a fixed-speed auger screw conveyor (16) into a ribbon blender mixer (17), which is typically used in animal feed milling. This mixer, set on load cells, discharges into the hopper (18), from which wheat enters a conveyor (19) to either the wheat cleaning plant or the mill.
Blending by volume is a valid method, although it is not as accurate as weighing. Two principal types of volumetric blenders are available.
The first is a rotary type with a slow, constant speed, whereby grain is choke-fed by gravity into pockets pre-set by the operator. This type normally can discharge proportionately between 100% to not less than 10%.
The second type feeds in fixed volumes, but speeds can be varied with inverters or gear boxes. The second type of blender is more expensive, but allows a discharge range from 100% to as little as 2%.
Both methods of volumetric blending are cheaper and simpler than weighing and require minimum space and headroom. One disadvantage is inventory reconciliation; because no weight control exists, it is difficult to account for variances in weight per volume unit.
Weight-based blending systems range from those providing absolute accuracy, which are the most expensive, to less accurate systems, which are not true weighers. The most accurate and expensive are small batch-type weighers.
Another system is the constant weight feeder, which uses a moving band conveyor of wheat mounted on a weighing linkage. A third method is the tubular weigher, which is virtually continuous but weighs only around 15% of total feed.
Yet another type, known by various names, is an impact or so-called gravimetric feeder. This type is popular and is less expensive than outright classical weighing, although more costly than volumetric methods. It has the advantage of low space requirements and is capable of fairly accurate inventory control.
The consistency of final finished products depends on blending together as many bins as possible by volume or weight. Imagine at one extreme running a single bin of wheat through the system described. The wheat from this bin may well have a natural variation of plus or minus 0.5% for protein; the finished flours will have a similar variation, resulting in a protein spread of 1%. Other quality parameters would vary similarly.
But if 10 bins are blended together, the variation typically will be reduced to 0.1%, and flour quality will be much more consistent because peaks and troughs were smoothed out. The results will pay off financially because the flour user also will be able to make more consistent products.
Reducing variation in finished products by ironing out ups and downs enables the miller to lower his cost of wheat. The miller can calculate to balance the quality and specification demanded by customers against the sundry wheat grades available with their different costs. In other words, get the target right and blend accordingly.
Another way to view the benefits of blending and mixing is that the process can reduce the cushion or insurance needed to assure meeting minimum requirements. But reducing the cushion also means keeping a careful watch on results by using the laboratory to monitor outgoing flours and incoming wheats and to help diagnose faults within the plant.