Post-milling Flour Processing

by Teresa Acklin
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Industry consultant David Sugden describes physical treatment processes to assure high-quality flour after milling.

   After wheat is milled, the resulting flour requires additional “processing” or treatment before storage. The reason for this processing is to ensure that the customer receives as pure a product as possible, consistently.

   The accompanying diagrams 1 and 2 depict the steps in post-milling flour processing. While many variations are not only possible but also practiced, these steps together show a typical scheme.

   Each stage should include exhaust systems to enable proper dust suppression. The systems can be controlled by programmable logic controllers, which also are not shown in these illustrations.

Process and Equipment

   At the top of Diagram 1, flours 1 and 2 at points (1) and (2) are top and low grade qualities that are being milled and collected separately and simultaneously. Additive feeders (3) sit on the flour collecting screws (4). These feeders may be volumetric or loss-in-weight, with the latter a very accurate and reliable, if expensive, option.

   The feeders deliver various ingredients as dictated by final flour specifications. Additives may include fungal enzymes, ascorbic acid, gluten, vitamin enrichment — the list is extensive, limited only by law and customer demand.

   Agitators (5) are shown below the flour collecting screws. The agitators allow for gas addition such as chlorine, chlorine dioxide or sulfur dioxide.

   Flour redressers or rebolt sifters (6), matched to the capacity of the mill, are plansifter type and are clothed with relatively fine covers, around 230 microns. This level is fine enough to ensure that no oversize particles pass through, and it detects burst covers farther back in the process.

   These redresser plansifter covers cope with considerably more work than those in the main mill itself and so need more maintenance. Foreign oversize matter is also dealt with at this point.

   Divide valves (7) are set under the sifters. Their purpose is to make a variety of low ash (patent) or high ash content (low grade) flour (see November-December 1996 World Grain, page 26).

   The divide valves may be remotely controlled by the on-line near-infrared measuring instrument (10) according to a closed loop feedback system. This instrument measures and can control moisture and protein content, for example.

   Magnets (8) are placed to capture any metals attracted to the product. The magnets are necessary in any set-up to protect not only the client but also the subsequent machinery, especially infestation destroyers (12).

   Metal detectors (9) are an excellent further insurance policy. These instruments sense non-magnetic as well as magnetic metals. In some large sophisticated plants, metal detection systems not only feature automatic rejection but also recover reprocessed flour that was previously contaminated.

   Metal detection is not particularly accurate, but it certainly acts as a purity safeguard as well as a good legal defense as evidence of due diligence.

   An on-line near-infrared system (10) has been interposed to monitor and control the performance of the mill (see July/August 1996 World Grain, page 8). Autosamplers (11) are shown to provide a regular and timed supply of flour samples to the laboratory for confirmation of results.

   Another important feature is the infestation destroyer (12) or Entoleter. This 3,000 r.p.m. machine kills all insects and eggs by centrifugal force.

   The two flours then pass through weighers (13) via varispeed screws (14) and on to blowlines, or air conveyors, (15). These positive pressure units discharge to the flour bins (16).

Further Steps

   By now, flours are fairly pure, though not necessarily finished — which is the purpose of the system shown in Diagram 2. It pictures the two flours 1 and 2 entering the flour bins (3). Together with any number of additional additive feeders (4), the bins supply the weigher mixer (5).

   The additive feeders at this point should be weigher type to ensure pinpoint accuracy because flours are at the final stages before delivery. Indeed, the purpose and capability of these three components — bins, feeders and the weigher mixer — are to blend for consistency and precision to maintain the client's goodwill.

   The on-line NIR (6) measures and controls the blending apparatus. Analyses of a number of parameters, especially protein and moisture content, take place at this point. Subsequent equipment consists of a magnet (7), metal detector (8), autosampler (9) and infestation destroyer (10), all serving the same purposes as in Diagram 1.

   The redresser (11) here is the rotary type using brushes or impellers. This type generally is recommended because the capacity requirement is at least three times greater than for the redresser shown in Diagram 1. Consequently, plansifter types are rarely used, and the aperture for white flour sieving here is unlikely to be smaller than 1,000 microns.

   This final redresser ideally should be capable of sieving as finely as its predecessors, but practicality typically precludes this; because of sieve surface allocation, aperture and capacity, the capital cost and space requirements are much greater. However, customer demand in some countries and locations dictate the use of plansifters at this processing point.

   The facilities described here are fairly comprehensive. Client requirements and the competitive and legislative atmosphere dictate the degree of sophistication of post-milling flour processing.

Maintenance Tips

   Maintenance on a regular basis according to suppliers' advice is critical if the benefits of flour processing are to be optimized. Additive feeders of any type at both pre- and post-bin stages require daily inspection for calibration or entrapment of foreign material. Such inspection normally does not take long.

   Agitators seldom need much attention because the bearings are sealed-for-life. Nonetheless, the agitator impellers may show signs of distortion; excessive vibration is a good telltale sign.

   Magnets should be cleaned off daily for two reasons. The first is to hold top performance and protection; the second is to determine, from looking carefully at the clean-out material, the source of contamination. Intelligent deductions can be made from such scrutiny, and tracing such matter early on can save serious trouble and cost later.

   Metal detectors have a test sample for determining sensitivity and therefore performance. These also must be examined daily and adjusted if necessary.

   The on-line NIRs, as with laboratory bench instruments, need checking daily. These instruments are prone to drifting, thereby churning out misleading results — and leading to off-specification flours if the system is fully automatic. By use of local modems, most suppliers can assist with fault diagnosis and correction from their base.

   Automatic sampling machines are usually of the carousel or magazine type, simple in concept and construction; daily checks of their function will ensure the laboratory is sustained as scheduled. Infestation destroyers are unlikely to develop problems; the most common trouble is excessive vibration, which can be corrected by stripping and reassembling.

   Weighers need daily calibration and checking for inventory control. Although varispeed screws and the flour divide valves seldom give cause for concern, checks should be carried out weekly or more frequently. Blowers, well maintained, are very reliable provided the manufacturers' recommendations are followed.

   The weigher mixer in Diagram 2 demands daily inspection and calibration. In this case, it is not just for inventory reasons, but to optimize the integrity of each finished flour.

   The sequence of operations on both diagrams has a number of optional positions in the flow, yet some operations must be fixed.

   In Diagram 1, positions (1) to (6) in the flow are fixed (flour ex mill to the redressers). The divide valves (7) must be after the redressers (6) to maintain a constant load. The on-line NIR (10) and autosamplers (11) need to come after the divide valves (7) so that the on-line NIR can at least check that the divides are correct; some configurations feed back and control the divide valves automatically.

   The weighers (13) with varispeed screws (14) come last for inventory control. The remaining equipment may be placed within reason in a number of optional positions, although the flow shown is preferred.

   In Diagram 2, positions (1) to (6) are fixed because this unit, as it were, is interdependent. The redresser (11) should be the final piece in the flow, with the positions of the remaining equipment optional.

   These diagrams show commonly installed flows, but they have little to do with ISO 9000 certification in the sense that any configuration of any flow will be acceptable under ISO, provided it is properly documented, logged and checked.

   Systems as shown in the diagrams are capable of a relatively high degree of automation. The risk, particularly with instrumentation, is one of reliability; the frequent checking of equipment for optimum function by suitably trained personnel is still vital.

   Further developments in new instrumentation, accuracy and reliability are being seen constantly, but the need to support all systems with good staff will not disappear. After all, customer demand for pure and consistent flour products increases all the time.

   David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingateston, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.