## 3 Experimental Study on the Coefficient of Restitution of Wheat Grains
### Characteristics of Research Samples  
The complex structure of cereal grains--irregular shape, variable moisture content and multi-layered anatomy--makes them challenging subjects for collision analysis and for determining the coefficient of restitution (\(CoR\), \(e\)). Such intrinsic variability can introduce inconsistency and bias if the grain population is not tightly controlled. Consequently, many studies use nearly spherical seeds--pea grains, lentils, soybeans, rapeseed and beans [10; 42; 43; 44]--to simplify the geometry.  
In the present work, we selected _Tritcium aestivum_ 'Memory', a winter wheat widely grown in Greater Poland and rated in the top-quality groups (A/B). Certified seed was obtained from a registered seed centre, guaranteeing provenance from laboratory fields and prior mechanical cleaning. Multi-stage sieving and vibratory-sorting with triers and winnowers had removed undersized, oversized and damaged kernels, yielding a homogeneous batch.  
This careful sourcing and characterisation minimise sample variability and provide a robust basis for analysing grain impacts at different moisture levels. Prior to the commencement of the study, the grains were characterised for physical properties that could affect the measurements, using the instruments listed below:  
* Grain mass: determined with an analytical balance (RADWAG PS 1000/Y; accuracy \(\pm\)0.001 g); 200 randomly selected grains were weighed individually on a controlled surface.
* Moisture content: measured with a moisture analyser (RADWAG MA 200.3Y WH.B) and the routine reference method for cereals (PN-EN ISO 712:2009), which dries a prepared analytical sample at 130 \({}^{\circ}\)C.
* Principal dimensions: obtained by a photogrammetric method. High-contrast images were captured on a custom light table that provided back-lighting of the samples, then processed with GABAR software 1.0 developed at the Department of Working Machines, Poznan University of Technology (Figure 8a). Mean values and standard deviations of grain length, width and thickness were recorded.  
As a result of the measurements, the following material characteristics were established:  
* Average grain mass: 0.045 \(\pm\) 0.008 g;
* Baseline moisture content: 7%;
* Grain length: 6.41 \(\pm\) 0.50 mm;
* Grain width: 3.04 \(\pm\) 0.38 mm.  
Moisture-content adjustment was carried out by conditioning 100 g samples: the required volume of distilled water was added, and the samples were sealed in airtight containers, stored at 3 \({}^{\circ}\)C and shaken periodically to ensure uniform distribution. The mass of water to be added was calculated with  
Figure 8: Identification of geometric features: (**a**) measurement plate; (**b**) software interface for grain-geometry measurement [7].  
\[\Delta_{mw}=\frac{w_{1}-w_{0}}{100-w_{1}}m \tag{16}\]  
where  
\(\Delta_{mw}\)--mass of the missing water,  
\(M\)--mass of the grain,  
\(w_{0}\)--initial moisture level,  
\(w_{1}\)--target moisture level.  
This range of 7-16% was selected because drying below 7% requires disproportionately high energy, whereas moisture contents above 16% accelerate microbial degradation of the grain.