Last reviewed 1 Oct 2026 · 8 min read
Why study engineering properties?
Machines for cleaning, grading, conveying, storing, drying, milling and packaging must be designed around the properties of the material they handle. Engineering properties tell us how the produce flows, falls, slides, resists crushing, heats and cools, and they vary with variety, moisture content, temperature and maturity.
| Group | Properties | Used in design of |
|---|---|---|
| Physical (geometric) | size, shape, volume, surface area, 1000-grain weight | sieves, graders, hoppers |
| Gravimetric | bulk density, true density, porosity | storage bins, silos, transport |
| Frictional | angle of repose, coefficient of friction, angle of internal friction | hoppers, chutes, conveyors, bin loads |
| Aerodynamic | terminal velocity, drag coefficient | winnowers, pneumatic conveyors, aspirators |
| Thermal | specific heat, thermal conductivity, thermal diffusivity | dryers, coolers, cold storage |
| Mechanical (rheological) | hardness, compressive and shear strength, modulus, rupture force | milling, shelling, threshing, handling damage |
| Optical and electrical | colour, reflectance, dielectric constant | sorters, moisture meters |
| Hygroscopic | equilibrium moisture content, water activity | drying and storage |
Size and shape
Size
For a grain with three perpendicular dimensions — length , width , thickness — measured by a vernier calliper or micrometer on a sample (usually 100 seeds):
- The geometric mean diameter is the usual size for the equivalent sphere.
- Size distribution is found by sieve analysis; the fineness modulus and mean diameter are used for flour and ground material.
Shape
| Term | Definition | Formula |
|---|---|---|
| Sphericity () | how closely the grain resembles a sphere | |
| Roundness | sharpness of the corners | area of the largest inscribed circle ÷ area of the smallest circumscribing circle (or ratio of mean radius of corners to the radius of the maximum inscribed circle) |
| Aspect ratio | slenderness | |
| Surface area | for a sphere-like body: | |
| Volume | by the liquid displacement method (toluene), or for a sphere-like body |
- A sphericity close to 1 means the grain is nearly spherical (rolls easily); a low sphericity means it is flat or long (rice, oats). Shape influences flow, sieving, orientation in cleaning, and drying rate.
A wheat grain has mm, mm and mm.
; ; mm.
Density and porosity
| Property | Definition | Measurement |
|---|---|---|
| Bulk density () | mass of the grain per unit bulk volume (including the void spaces) | fill a container of known volume with grain under standard conditions (drop height) and weigh it |
| True (particle) density () | mass per unit volume of the solid grain (excluding pores) | liquid displacement (toluene/water), gas pycnometer |
| Porosity () | fraction of the bulk volume occupied by voids | |
| 1000-grain weight | mass of 1000 grains | counting and weighing |
Typical values (indicative): wheat kg/m³, kg/m³; paddy kg/m³; rice kg/m³; maize kg/m³; soybean kg/m³. Density decreases with an increase in moisture for most grains (because the swelling increases the volume more than the mass).
- Bulk density governs storage capacity and bin loads; true density governs separation by specific gravity (gravity tables, floatation); porosity governs airflow resistance in dryers and aerated storage, and heat transfer.
Wheat: kg/m³, kg/m³. Porosity . A silo of 500 m³ holds about t.