Purification in Flour Mills

by Teresa Acklin
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This is the second in a series of articles by industry consultant David Sugden explaining the gradual reduction system in a flour mill. The first article on break systems appeared in the September issue of World Grain. The final article on reduction systems will be published in November.

   The purpose of purification is to separate the outer branny material from the inner white endosperm by the combined means of air resistance, stratification, terminal velocity, sifting and gravity. This all takes place within the same machine — the purifier.

   The advantage of purification is to gain a longer extraction of flour from wheat for a given mineral content or degree of whiteness.

   This article discusses a typical purification system for semi-hard and hard wheat bread flours, simply because this is the largest production tonnage using purifiers worldwide. Reference also will be made to durum milling for pasta.

   Purifiers will only function on semolina and middlings, which are essentially larger particles of endosperm contaminated by degree with bran before milling into flour.

   The words used can be confusing. The term “middlings” is widely used in North America, such as coarse, medium or fine middlings and midds. In the United Kingdom and some other parts of the British Commonwealth, the equivalents are coarse, medium or fine semolina and middlings. While European countries are all slightly different, the logic of any mill diagram is nevertheless similar.

   Some mills do not have purifiers mainly because 100% soft wheat milling does not lend itself to efficient purification, though purifiers can be and are used in mixed grist mills. Soft wheat is defined here as several European varieties, Pacific soft white, some red winter wheats, certain Ontario winter and lower protein Australian.

   There is no point in spending money on purifiers in all soft wheat mills because they rarely make top patent flours and the endosperm, being soft, breaks up too easily, creating attrition flour. The latter tends to be lost in purifier exhaust systems and blocks purifier sieve covers, rendering the machine inefficient with the tendency to lose extraction.

   One classic flow for a mill without a purifier (not shown) will have the usual break steps, which then feed their own sizing (corrugated) rolls with following plansifters, thereafter grading to feed the requisite number of reduction roll passages. Sizing, in this instance, takes the place of purifiers.


   The feed to any purifier should be of a narrow range of particle size. In the first purifier shown in the diagram of a typical purification system (see Page 32), the feed, or left-hand side, has a particle size between 1000 and 570 microns (one thousandth of a millimeter). The particle size for the sixth purifier ranges between 470 and 250 microns.

   Purifiers, because of their physical limitations, will not work on coarser bran, finer dunst or flour. These stocks are either too large and light or too small and light in bulk density, and can be easily lifted out by air exhaust.

   Break sifters, which provide the feed to each machine, have the action of stratification. This is beneficial to the purifier because it naturally tends to provide heavier bulk density of material. Stratification means that the lighter particles stay on the top and the heavier particles stay on the lower part of any sieve. The break sifter therefore “grades” material for purification.

   It is important that purifier feeds be dust-free. If not, extraction will be lost to air exhaust.

   The purifiers illustrated have three decks, one above the other, and each deck has four sieves. (Purifiers also are available with one or two sieves.) It is important that each sieve is tight. If the sieve sags, air currents drawn through from the top will take the line of least resistance where there is no material.

   The feed needs to be spread evenly. Each sieve has either a brush or a set of ball cleaners underneath and must be operating properly all the time. If not, blind sieves will cause mill imbalance, meaning lost extraction.

   The slope of the sieve deck is adjustable. When the purifier is set up with the best combination of sieve aperture openings — a matter of trial and error — the top tail sheet should be only lightly covered, with a gently observed aspiration of material coming away. Of course, the sieve deck must be level or material will work to one side, nullifying the effect.

   Adjustment of the “throw” is available on most purifiers. Large and heavy particle stocks need a coarse throw, not just to convey material along the length of all sieves but also to allow the negative pressure (suction) air exhaust to more easily lift off branny particles (as illustrated in the first and second purifiers).

   Fine throw is used for small and lighter particles, otherwise too much will be lifted by air and the stock will travel too quickly from the head to tail of the machine. Throw is a mixture by degree of vertical to horizontal movement.

   Eddies of air currents, bare patches or uneven feed can be corrected by adjusting the various elements listed above.

   Pay careful attention to air adjustment. Most purifiers have up to four compartments over each of the four top deck sieves. Every air distribution compartment has its own individual variable air valve. Illumination is often provided on the modern purifier; if not, a torch light will do the trick. Make any observations and adjustments while sampling the throughs.

   Air trunking within and outside the purifier is designed to be self cleaning. Air carrying lifted stock is collected at the dust filter and thereafter fed to the IV break system. This stock is very high in mineral content and high in protein, but not in baking quality.


   Ultimately, trial and error is necessary in selecting sieve apertures. The source, variety and conditioning or tempering of wheat has a significant influence, as do break releases.

   In the first purifier illustrated, I and II break coarse middlings (coarse semolina) with a feed particle size between 1000 and 570 microns, the top deck head sheet or sieve has a 1050 micron aperture. The fourth, or tail, top deck sheet has a 1350 micron opening.

   As the illustration suggests, the aperature increases from head to tail. This also is the case with the middle and lower decks (850 to 1170).

   The throughs of the lower deck are virtually infinitely adjustable and destined to either 1 Sizings (A) or 2 Quality (B2). The former is reserved for the lowest mineral or ash content (purest). The lowest mineral content, much lower than the parent feed, always comes from the head end of the machine.

   The highest ash content, much higher than the machine feed, comes from the overtails of the top deck, in this case destined for the IV break. Overtails of the middle deck are shown with the alternate destinations of III or IV break, with the bottom deck optionally to 2 Quality (B2) reduction or III break passage.

   While these destinations are a matter of judgment by the miller, it is usually fairly obvious where the finished stocks should be sent.

   The remaining purifiers should be set in a similar manner. The III break purifiers, numbers five and six, carry higher mineral content origin of feed material. This means that the ultimate throughs and overtails also are higher.

   These stocks contain a relatively high proportion of fine contaminating, oily wheatgerm, which has the tendency to gradually blind the sieve covers with germ oil. The only solution, everything else being equal, is to renew the sieve covers to maintain mill performance (extraction). The effect otherwise is to tail relatively good stock farther down the mill.

   However, one of the most productive points of near whole germ for those requiring maximum germ yield is from the throughs of the tail sheet or sieve of the I and II break coarse middlings (coarse semolina) purifier and overtails of the bottom deck. To a lessening degree, the same applies to the remaining purifiers, two through six.

   To make top patent flours, stock is taken normally from the head sheets of purifiers one through four or, in other words, from I and II breaks. Top patents can be defined as being around 0.4% to 0.45% dry basis mineral content.

   The illustration is but an example. Variations abound depending on customer demand, wheat availability and local practice.


   All the major suppliers furnish suitable machinery. In most cases, they look similar. There are also extra-large machines on the market. Commonly, to save floor space, most purifiers can be erected one on top of the other.

   Individual sieve sizes range from 300 millimeters to 750mm (12 inches to 30 inches). Capacities range from around 400 kilograms to 5000-plus kgs per hour (900 lbs to 11,000 lbs), depending on material and particle size.

   Air requirements for a purifier vary from 40 cubic meters to 70 cu meters per minute per double machine (1,400 cubic feet to 2,500 cu feet). Motive power is typically provided by two vibratory motors for a double machine, each around 0.4 kw to 0.5 kw, running at about 700 rpm. These motors drive the two sets of decks synchronized, in harmony, and vibration-free to the main frame.

   The number of machines required for a typical 250-tonne to 300-tonne wheat per day bread flour mill (4,000 cwts to 5,000 cwts flour per day) is around six double purifiers of 450mm (18-inch) sieve size.

   To give an idea of physical dimensions, each 450 mm double purifier measures about 270 centimeters by 135 centimeters (9 feet by 4.5 feet) as a footprint, and is135 centimeters high. Typical shipping weights are around 1 tonne to 1.5 tonnes. These vary according to the supplier.

   By contrast, the durum mill has many more and extensive purifier passages — 16 are common. Many are retreatment and are necessary to take brown specks out of semolina. (Durum wheat cleaning is responsible for coping with black specks.) The aim is to provide a long extraction of speck-free semolina for pasta manufacture.

   This type of plant relies heavily on purification. Durum wheat is extremely hard. The idea is to make as little flour as possible — the reverse of flour milling.

   Purification is not simple and requires daily monitoring to pick up faults that have occurred overnight or that are insidiously creeping up on the unwary. Certainly, it pays to review major settings and sieve sizes after a new harvest.

   A bread flour mill equipped with purifiers will outperform one that does not by some 2% or 3% extraction, provided they are properly maintained.

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

      Diagram of a typical purification systems

From I & II breaks
I & II break
Coarse middlings (coarse semolina)
1050105011701350IV break
85094010501170III break/IV break
850940105011702 quality (B2)/III break
1 sizings (A)2 quality (B2)
From I & II breaks
I & II break
Medium middlings(medium semolina)
660660710780IV break
610660710780III break/IV break
5606106607102 quality (B2)/III break
1 sizings (A)2 quality (B2)
From I & II breaks
I & II break
Fine middlings (fine semolina)
4404704905202 tails (J)
4104404704902 tails (F)/2 tails (J)
4104404704903 middlings (D)/1 tails (F)
1 middlings (B)3 middlings (D)
From I & II breaks
I & II break
Middlings re (middlings)
3503503703902 tails (J)
3203403503705 middlings (G)/2 tails (J)
3103203403503 middlings (D)/5 middlings (G)
1 middlings (B) 3 middlings (D)
From III break
III break
Coarse middlings (coarse semolina)
710780850940IV break
6607107808501 tails (F)/IV break
6106607107801 tails (F)
2 quality (B2)1 tails (F)
From III break
III break
Fine middlings (fine semolina)
520560610610IV break
5205605606101 tails (F)/IV break
4104404704901 tails (F)
1 middlings (B)3 middlings (D)
Note: Numerals are in microns aperture width.
U.S. terminology- no brackets;
U.K. terminology- in brackets.
Same terminology where no brackets follow.