Sieving is the separation stage that converts the mixed output of each milling passage into distinct flour fractions. In a modern roller mill plant, every grinding stage is immediately followed by a sieving stage — the two functions are inseparable. The quality parameters that define the finished flour type — primarily ash content, but also granularity, moisture, and protein — are the direct result of how efficiently this separation is achieved at each passage.
Plansifter Operation
The plansifter is the workhorse of flour milling separation. It consists of a stack of sieve frames, each fitted with nylon or metal wire cloth of a defined aperture, enclosed in a box that moves in a circular horizontal motion driven by an eccentric shaft. The circular motion causes ground material to spread across the sieve surface and stratify by particle size — fine particles migrate downward through successive sieves while coarser particles are retained and directed to further grinding.
A single plansifter box may contain 24–32 sieve frames in multiple compartments, with the sieve arrangement configured to produce several distinct outputs simultaneously: patent flour (finest fraction), clear flour (coarser endosperm), semolina (for further reduction), and bran or overtails (directed back to break rolls or removed from the flour stream). The sieve cloth apertures used in Polish mills for white flour production range from approximately 100–150 micrometres for the finest flour fraction to 500–800 micrometres for coarse break overtails.
Sieve Cloth Specifications
Nylon bolting cloth is specified by the number of threads per centimetre in each direction and the resulting open area percentage. For the finest flour sieve in a white flour plant, woven nylon with 36–42 threads/cm and an open area of around 27–32% is typical. Wire cloth is used in break sieving where abrasion resistance is more important than fine separation.
Sieve cloth condition is a significant operational variable. Blinded or torn sieves allow particles to pass without separation, causing elevated ash in the flour stream. Polish mill quality procedures include weekly sieve inspection, with immediate replacement of any cloth showing tears, holes, or blockage exceeding 10% of the surface area.
Centrifugal Sifters and Control Sifters
Before flour leaves the mill and enters the finished product bin, it passes through a final control sifter — typically a centrifugal sifter (entoleter or rotary sifter). These machines use centrifugal force rather than gravity to move flour through a fine sieve at high throughput rates. Their primary purpose is to remove any insect eggs or large particles that have bypassed the plansifter system, and to ensure that no oversized particles enter the finished flour.
In Poland, customer specifications for retail flour typically require that 99.5% of the product passes through a 212 µm sieve (for Type 450 and 500) and that no particles above 315 µm are present. Industrial bakery customers may specify tighter or looser tolerances depending on their mixing and water absorption requirements.
The Polish Flour Type Designation System
Polish flour types are defined under PN-A-74022 (wheat flour) and PN-A-74032 (rye flour). The type number represents the ash content expressed as milligrams per 100 grams of flour on a dry matter basis. A Type 550 wheat flour must have ash content between 0.50% and 0.60% (dry matter basis), and a Type 2000 wholemeal rye flour can have ash content up to 2.0%.
| Flour Type | Ash Content (% d.m.) | Protein min. (%) | Primary Application |
|---|---|---|---|
| Type 450 (wheat) | ≤ 0.45 | 9.5 | Pastry, cakes, biscuits |
| Type 500 (wheat) | 0.45–0.55 | 9.5 | All-purpose cooking flour |
| Type 550 (wheat) | 0.50–0.65 | 10.0 | Standard bread, rolls |
| Type 650 (wheat) | 0.60–0.75 | 10.5 | Bread, pizza dough |
| Type 750 (wheat) | 0.70–0.85 | 11.0 | Artisan bread, rye-wheat mix |
| Type 1850 (wheat) | 1.70–2.00 | 12.0 | Graham-type bread |
| Type 720 (rye) | 0.68–0.78 | 8.0 | Light rye bread |
| Type 1400 (rye) | 1.30–1.60 | 8.5 | Medium rye bread |
| Type 2000 (rye) | 1.80–2.10 | 9.0 | Dark rye, pumpernickel |
Laboratory Quality Checks on Finished Flour
At the point where flour enters the finished product bin, Polish mills are required to perform batch testing under their internal quality management systems (typically ISO 9001 or BRC Food certified). The standard test battery for each flour batch includes:
- Ash content — muffle furnace incineration at 550 °C for 4 hours (PN-EN ISO 2171)
- Moisture content — oven drying at 130 °C (PN-EN ISO 712) or NIR inline measurement
- Protein content — Dumas or Kjeldahl method (PN-EN ISO 16634-1)
- Falling number — Hagberg method (PN-EN ISO 3093)
- Granularity (particle size distribution) — sieve analysis using nested sieves on a laboratory shaker
- Wet gluten content — for bread flours, by gluten washing and centrifugation (ICC Method 137/1)
- Farinograph water absorption — Brabender farinograph for industrial customers
Wet gluten content is distinct from protein content. Protein includes all nitrogen-containing compounds in the flour, while wet gluten measures specifically the viscoelastic gluten complex — the functional protein network critical for bread structure. A flour may have 12% protein but relatively weak gluten if the gluten-forming proteins (glutenins and gliadins) are of poor quality.
Dust and Particle Emission Control
Sieving operations generate fine flour dust, which is both a product loss issue and an explosion hazard. Polish mills operating under ATEX Directive compliance (Directive 2014/34/EU for equipment in explosive atmospheres) are required to fit sieving and conveying equipment with dust-tight connections and to maintain flour dust concentrations in mill air below the minimum explosive concentration of approximately 50 g/m³.
Dust collection systems use bag filters with pulse-jet cleaning, and the collected fine flour is either reintroduced into the product stream or disposed of as feed-grade material depending on particle size and contamination assessment.