Particle Size, flour, milling, baking

Particle Size Index

Also known as endosperm mechanical resistance to particle size reduction

What is the Particle Size Index?

The particle size index (PSI) is an indicator of wheat milling and baking performance associated with wheat kernel hardness and flour granularity.

The hardness of the wheat kernel has tremendous implications in its milling and baking qualities. Parameters such as damaged starch, water absorption, gas production and gas retention are all affected by how hard the wheat kernel is.1 The PSI test helps millers and bakers to:

  • Classify wheat according to degree of hardness
  • Adjust tempering settings at the mill
  • Get an indication of the milling/baking performance and end-product functionality of wheat

How does it work?

The particle size index (PSI) is a physical test that determines the relative hardness of wheat classes by grinding and sieving. Data obtained from this test can be converted to relative hardness via a standard calibration chart.

According to AACC International Official Method 55-30.01 (Cereals & Grains Association), the PSI test requirements include:2

  • Equipment
    • Lab mill fitted with No. 2 (fine) burr, revolving at 3,500 ± 100 rpm.
    • Screen or sieve, U.S. Standard No. 200 with 75 µm opening, fitted with lid and bottom pan.
    • Sieve shaker
    • Balance (electronic)
  • Sample preparation
    • Only cleaned wheat kernels should be used
    • Thoroughly pre-blend wheat kernels
    • Moisture content must be 11–13%
    • Drying of sample should be carried out if wheat moisture is greater than 14%
  • Method
    • Adjust mill to its finest setting (burrs should be barely touching each other)
    • Mill 23 g of wheat into meal.
    • Tare bottom receiving pan.
    • Weigh approximately 10 g of meal.
    • Transfer to U.S. No. 75 sieve (pan previously fitted to sieve)
    • For proper sieving, add 50 g whole wheat kernels or six plastic sieve cleaners; cover and sieve for exactly 10 min on sieve shaker.
    • Brush any fines adhering to bottom of sieve into bottom receiving pan and reweigh to obtain weight of throughs (W).
    • PSI (%) = (W/Sample weight) × 100
  • Relative hardness scale
Category PSI (%)
Extra hard Up to 7.0
Very hard 8.0–12.0
Hard 13.0–16.0
Medium hard 17.0–20.0
Medium soft 21.0–25.0
Soft 26.0–30.0
Very soft 31.0–35.0
Extra soft Over 35.0

As can be seen from the above table, hard wheats are characterized by lower PSI values while soft wheats have higher PSIs. As PSI increases, the particle size of the flour (or granulation) decreases. Hard wheats are more difficult to grind than soft wheat kernels which readily break into smaller pieces.


Wheat hardness is key for bakers and millers. The following table shows the relationship between class, protein content, kernel hardness, and application of wheat:

Kernel Hardness % Protein Wheat Class End-Product Utilization
Soft 7.0–8.5 Soft wheat Cake
Soft 7.2–9.5 Soft wheat Cookies
Medium soft / medium hard 8.0–10.5 Soft wheat and hard winter Doughnuts
Medium soft / medium hard 8.5–12.5 Soft wheat and hard winter All purpose flour
Medium soft / medium hard 7.5–11.0 Soft wheat and hard winter Crackers
Medium soft / medium hard 9.5–10.5 Hard winter Noodle
Medium hard 10.5–12.5 Hard winter Flat bread
Hard 11.5–14.0 Hard winter

Hard spring

White pan bread
Extra hard 12.5–15.0 Hard spring

Durum wheat

Very hard 13.5–15.5 Hard spring

Durum wheat

Hearth bread
Very hard 13.5–16.0 Hard spring

Durum wheat

Whole wheat bread

Factors that affect PSI

  • Water content of kernel
  • Milling device type and configuration / milling intensity
  • Vitreousness and starchiness of kernel
  • Starch-protein interaction
  • Wheat class

Hardness, PSI and baking quality

The hardness of wheat kernels, a measure of their physical resistance to crushing or shear forces, is directly correlated with:

  • Gluten-starch interaction
  • Extraction rate (ER)
  • Mean particle size after milling (further grinding is needed to attain final granularity)
  • Force required and  energy consumption during  wheat milling into flour
  • The amount of damaged starch and  flour protein content
  • Flour water absorption capacity
  • Susceptibility of  flour to amylase action
  • Breadmaking performance of flour
  • Mixing time


  1. Carson, G.R., and Edwards, N.M. “Criteria of Wheat and Flour quality.” Wheat Chemistry and Technology, 4th edition, AACC International, Inc., 2009, pp. 97–118.
  2. AACC International. Approved Methods of Analysis, 11th Ed. Method 54–40.02. Mixograph Method. Final approval November 8, 1995; Reapproval November 3, 1999. Cereals & Grains Association, St. Paul, MN, U.S.A.

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