Color sorting in flour and durum mills

by Teresa Acklin
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Industy consultant David Sugden discusses the application of color sorting techniques in flour and durum mills.

   Color sorters in flour and durum mills are being used more frequently because of increased emphasis on product purity, safety, yield and return on investment.

   Color sorters have been around for a long time, especially in the rice milling industry. Their job was, and still is, to separate discolored grains and specks. The technology now encompasses robustness, reliability and even more searching separations.

   Traditionally, durum and common wheat cleaning is carried out using the time honored principles of separation by width (sieving), length (trieur indented or disc separators), density (vibration and air flotation), magnets, non-magnetic metal detectors and air resistance (aspiration). Color sorting is an extra principle gained by means of specially selected fluorescent lamps, photocells, amplifiers and CCD (Charge Coupled Device) cameras.

   The CCD camera is similar to a video camera. Comparisons are made between a chosen contrasting background and the material to be sorted. Defects the size of a pin head as well as discoloration can be detected.

   Wheat is fed by gravity and spread across a series of channels so that each kernel is presented individually to an optical head. Once a defect has been detected, a compressed air ejector removes the unwanted particle and sends it to the “reject” outlet. Ejectors work at very high speeds, usually around 0.5 milliseconds.

   Color sorting machines can be configured to have both a coarse and a retreatment separation. Features of such equipment include remote and local control, sensitivity adjustment, alarm and shut down capability, test facilities, a memory bank for different materials to be sorted and remote holding of information.

   Other color sorting machines can be fitted with infrared detectors in tandem with, or instead of, CCD cameras. One particular use of the invisible spectrum infrared sensor is to differentiate glass particles from white rice.

   Optics can be placed opposite each other in order to measure both sides of a particle. To avoid dust build up, a wiper blade similar to a windshield wiper is incorporated to operate periodically. Lamps tend to deteriorate over time and are automatically compensated for in up-to-date versions.

   In the milling industry, color sorting has a number of effective uses. These include removal of ergot from rye or wheat; black points from semolina or couscous; dark or stained pieces from pasta; discolored kernels from maize, oats and barley; stones; mold; and plastic debris.

   Product liability is becoming not only more and more expensive to insure; in some cases, adverse publicity related to liability can be highly damaging to the reputation of a mill or business. Accordingly, protection that can be achieved by using the extra step of color sorting must be an additional defense.

   Moreover, with customers demanding ever-greater consistency and quality, it is a further comfort to them to know that the supplying miller has taken another new shielding step. The competitive benefit, when others are not using such a technique, will not be lost to the technically inclined.

   Diagram I, which is a simplified example, depicts the position of a color sorter in a typical common wheat flow. The purpose in this case is to recover broken and small wheat that would otherwise be lost to screenings (dockage).

   The bulk density separator (2) is set to deliver between 25% and 40% of full feed to the round seed separator (4). This in turn sends about 1% to 1.5% of full feed to the color sorter. Recovered broken and small wheat of 0.5% to 0.75% of full feed is returned to the main stream with rejected material, consisting of round seeds and damaged small grain, directed to screenings.

   The broken and small wheat reclaimed by the color sorting process is valuable when accumulated over a year compared with the value of millfeed that it would otherwise become. This return on investment is clearly greater in larger capacity plants.

   Turning to the example in Diagram II (also simplified), the application in this case places the color sorter in the main stream at full capacity between the indented cylinders/disc separators and the following scourer-aspirator before tempering. The concept here is not only to replace the gravity table and spiral seed separator, but also to eliminate by color.

   The gravity table is relatively low in capacity and problematical to keep set at maximum efficiency. The spiral seed separator is not particularly accurate or flexible. The gain by using the color sorter is one of product purity and accuracy.

   Pay-back on many applications is about four years (25%) or better. Machine capacities on wheat (and any other commodity for that matter) vary according to the degree of contamination, but capacities for wheat start from 15 tonnes per hour downwards. Smaller size facilities range down to around 200 kilograms per hour with laboratory scale equipment handling individual samples.

   Other variations on the sorting principle are possible. One unusual option is to separate healthy dark kernels of common wheat from light kernels. Darker kernels tend to contain greater protein, therefore are of higher value. By color sorting it is possible to add value.

   A good step for millers who are considering using color sorting equipment is to arrange for samples to be tested by the manufacturer. A visit to witness the testing is even better.

   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.

   Durum wheat cleaning

   Diagram II

   Color sorter location to dispose of

   discolored seeds & grains


   1   Milling separator/aspirator

   2   Bulk density separator

   3   Round seed separator*

   4   Oats & barley separator*

   5   Scourer/aspirator


      *Trieur cylinders (or discs)