## 2 Materials and Methods
### Experimental Procedure to Evaluate the Thermal Uniformity  
Following the results obtained with the numerical simulation, the MW prototype was built and tested to check its correct operation.  
Almonds (_Prunus dulcis_) of the Tuono cultivar were used for the experimental tests. The harvested almonds were hulled, sun-dried, shelled and then stored at a controlled temperature of 18 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{l c c} \hline \hline \multicolumn{1}{c}{**Symbol**} & **Value** & **Description** \\ \hline f\({}_{0}\) & 2.45 GHz & Microwave frequency \\ \(\varepsilon^{\prime}_{r,al}\) & 1.42 & Dielectric constant of almonds \\ tan \(\delta\) & 0.14 & Dielectric loss factor of almonds \\ \(\varepsilon^{\prime}_{r,pl}\) & 2.30 & Dielectric constant of polypropylene \\ tan \(\delta_{pl}\) & 0.002 & Dielectric loss factor of polypropylene \\ \(\sigma\) & \(7\times 10^{6}\) Sm\({}^{-1}\) & Electric conductivity of stainless steel \\ k\({}_{\rm a}\) & 0.1 Wm\({}^{-1}\)K\({}^{-1}\) & Thermal conductivity of almonds \\ k\({}_{\rm p}\) & 0.24 Wm\({}^{-1}\)K\({}^{-1}\) & Thermal conductivity of polypropylene tube \\ k\({}_{\rm s}\) & 65.2 Wm\({}^{-1}\)K\({}^{-1}\) & Thermal conductivity of stainless steel \\ C\({}_{\rm p}\)a & 1400 Jkg\({}^{-1}\)K\({}^{-1}\) & Specific heat of almonds \\ C\({}_{\rm pp}\) & 1000 Jkg\({}^{-1}\)K\({}^{-1}\) & Specific heat of polypropylene tube \\ C\({}_{\rm ps}\) & 450 Jkg\({}^{-1}\)K\({}^{-1}\) & Specific heat of stainless steel \\ \(\rho_{\rm a}\) & 800 kgm\({}^{-3}\) & Mass density of almonds \\ \(\rho_{\rm p}\) & 2200 kgm\({}^{-3}\) & Mass density of polypropylene tube \\ \(\rho_{\rm s}\) & 7870 kgm\({}^{-3}\) & Mass density of stainless steel \\ d\({}_{0}\) & 125 mm & Outer diameter of the propylene tube \\ d\({}_{\rm i}\) & 90.8 mm & Inner diameter of the propylene tube \\ L\({}_{\rm f}\) & 395 mm & Length of the honeycomb filter \\ W\({}_{\rm f}\) & 180 mm & Width of the honeycomb filter \\ H\({}_{\rm f}\) & 552 mm & Height of the honeycomb filter \\ W\({}_{\rm g}\) & 60 mm & Width of the grid honeycomb filter \\ H\({}_{\rm g}\) & 60 mm & Height of the grid honeycomb filter \\ L & 3000 mm & Length of the treatment chamber \\ W & 564 mm & Width of the treatment chamber \\ H & 552 mm & Height of the treatment chamber \\ \hline \hline \end{tabular}
\end{table}
Table 1: Electrical, thermal and geometrical parameters.  
Experimental test plans were carried out to measure the surface temperature of the outgoing almonds at residence times of 80 s, 100 s, 120 s, 140 s, 160 s, 180 s and 200 s, obtained by adjusting the electric motor frequency drive of the helix in the range between 30 Hz and 80 Hz. The measurement of the temperature of the almonds in the different test conditions was carried out using an infrared thermometer (Omega Engineering, mod. OM051-LT, Norwalk, CT, USA) installed at the exit of the treatment chamber. The data detected by the thermometer were reported on the control PLC of the MW plant.  
Total MW power was 7.5 kW. The almonds used for the experiments had been previously stored and kept at a constant temperature of 16 \({}^{\circ}\)C (the same initial temperature assumed for the numerical simulation) in a temperature-controlled chamber. For each trial, a homogeneous batch of almonds of 150 kg was used. Each trial was repeated 10 times.  
During trials, the flow rate was determined by measuring the time needed to treat the 150 kg batch of almonds; in addition, the surface temperature of the almonds at the end of the treatment was determined using a thermal imager (Testo 876, Testo SE & Co. KGaA, Lenzkirch, Germany) with uncertainty of \(\pm\)2%, an FPA 160 \(\times\) 120 pixel, a range measurement spectrum from 8 up to 14 \(\upmu\)m and an update frequency of 9 Hz. The thermal pictures were captured at 0.5 m. Thirty thermal images were processed for each test condition.