SOP: Particle Size Analysis
Particle size decides whether chocolate feels smooth or gritty, whether flour hydrates evenly, and whether a powder flows or cakes. Sieve analysis remains the practical shop-floor method, while laser diffraction serves the lab.
Physical measurements translate what consumers see and feel into instrument readings that production can control. These tests are sensitive to sample handling in ways chemical tests are not: a few degrees of temperature difference or a slightly different sample shape changes the number. Standardized preparation is therefore part of the measurement.
Method Principle
Sieve analysis separates particles by size through a stack of woven-wire meshes with certified openings. Mechanical agitation moves particles across each mesh; those smaller than the opening pass through while larger ones are retained. The mass retained on each sieve, expressed as a percentage of the total sample, builds the particle size distribution. The method assumes particles are roughly equidimensional; elongated particles may pass diagonally and bias results toward finer fractions.
Purpose
To determine particle size distribution of powders and granules by sieve analysis for quality control.
Scope
Applies to flours, spices, powdered ingredients, and granulated products.
Equipment and Materials
- Test sieve stack (certified mesh sizes per the product method)
- Sieve shaker
- Balance, 0.1 g
- Soft brush for cleaning sieves
- Sample divider
Procedure
- Check sieves for damage and verify certification is current.
- Weigh 100 g of representative sample obtained with the sample divider.
- Assemble the stack coarsest on top, pan at the bottom. Pour in the sample and cover.
- Shake for the specified time (commonly 10 minutes) on the shaker.
- Weigh the residue on each sieve and in the pan.
- Calculate the percent retained on each sieve. Total recovery must be 98 to 102 percent of the starting weight.
- Brush sieves clean from the underside; never poke the mesh.
Recording Results
Report percent retained per sieve and cumulative percent passing. Compare against the product grading specification.
Quality Control
Calibrate instruments per the manufacturer schedule and verify with certified standards before each use session. Run a stable in-house reference sample with every batch and plot results on a control chart; investigate drift beyond two standard deviations. Standardize sample preparation rigorously, since physical measurements are highly sensitive to sample geometry, temperature, and handling. Train all operators on the method and verify competence annually through supervised replicate testing. Record instrument ID, calibration due dates, and environmental conditions (temperature, humidity) with every batch of results.
Troubleshooting
- Poor recovery: sample lost in transfer or stuck in mesh. Work carefully and include brushings.
- Blinded sieves: near-size particles wedged in mesh. Clean ultrasonically per the sieve maker guidance.
- Inconsistent results: the sample was not representative. Use the divider, not scooping.
- Results differ between instruments: cross-calibrate with the same reference sample on both units before comparing data sets.
- Seasonal drift in results: ambient temperature and humidity affect many physical tests. Log conditions and consider climate control.
Interferences and Limitations
Physical measurements respond to sample history: temperature, age, and handling all shift readings, so strict adherence to the conditioning steps is essential for comparability. Heterogeneous products give variable results; increase replicate counts rather than averaging away real variation. Instrument readings are comparative, not absolute: values from different instrument models or probe geometries are not interchangeable without cross-calibration. Operator technique influences results in manual tests; verify operator competence before assigning compliance testing.
Safety Notes
- Dust hazard with fine powders; wear a dust mask for large batches.
- Secure the shaker stack before starting.