## Materials and Methods
### Experimental Setup for Determining Frequency Response Functions  
In this section, an experimental setup was applied to evaluate the dynamic behavior of coffee fruits analyzing their response functions in time and frequency. Therefore, the primary sense of this experiment is to compute the dynamic response of the six coffee fruits classified previously and listed in Table 2. For that purpose, experiments were executed for pairs of fruits of the same ripening stage, which means that two groups of samples were evaluated. Each fruit was dynamically tested using an electrodynamic shaker (Mini shaker, Sentek Dynamics, Santa Clara, CA, USA) that applies the vibrations through a 3D-printed support fastened to it. Input motions were applied to the fruits in two different degrees of freedom according to the fruit orientation system \(e_{1}\)-\(e_{2}\)-\(e_{3}\). For our case, points P1 and P2 were assigned for the orientations \(e_{1}\) and \(e_{2}\), as illustrated in Figure 3a. The support was prototyped in a PLA (polylactic acid) frame with a piezoelectric transducer (brass platform) at the core to hold from the pedicel the fruit joined using a layer of epoxy adhesive, as shown in Figure 3b. A laser vibrometer POLYTEC CLV-2534 (Polytec Inc., Auburn, MA, USA) was utilized to measure the fruit's vibrational velocity at each point. A data acquisition card (NI DAQ-6211, Austin, TX, USA) was used for programming and measuring the excitation and response signal. In addition, a laser vibrometer was connected to the analog input, and the electrodynamic shaker was connected to the analog output of the card. To assess the frequency response of the fruits, vibration tests were applied by two windows of chirp excitation signals for the two groups of fruits. The signal one was set up from 10 to 100 Hz, with 0.12 V of shaker amplitude, 100 s of excitation time, and 2 kHz of sampling rate. Signal two was defined from 100 to 1000 Hz, keeping the same amplitude and time in the shaker but applying 10 kHz of the sampling rate. Each sample was tested by applying the same signals. The main idea of this experiment is to understand the dynamic behavior of the fruits identifying their resonance frequencies and the velocity amplitude at both frequency windows. The identification will allow us to choose monitoring peaks that could be tracked from the fast Fourier transform and a wavelet analysis. On the other hand, for carrying out a modal analysis, the input vibrational force applied to the samples was read as voltage signals by a piezoelectric transducer placed inside the excitation support, as detailed in Figure 4. A lateral view, frontal view, and upper view of the experiment can be observed in Figure 4a-c, respectively. As detailed in Figure 4d, the time domain of the voltage signal of the tested fruit is acquired in the analog input of the DAQ with which the FFT of the input voltage is computed for all the samples in the \(e_{1}\) and \(e_{2}\) directions, respectively. This experimental approach was applied for computing the mechanical impedance of the fruits and determining the rate (input force--output velocity) of the tested fruits. Piezoelectric transducers have been used at different applications to sense vibrations signals over structures such as the case when residual forces are monitored in concrete structures, as was implemented by [PERSON] et al. [41]. To understand the relations between force/velocity modal analysis theory is reviewed [42]. In the experimental configuration the velocity is measured from the vibrometer and the force is determined with the piezoelectric sensor added to the fruit pedicel. Let us consider the motion equation in the frequency; this can be expressed as  
\[(-M\omega^{2}+Cj\omega+K)\{v_{out}\}e^{i\omega t}=\{F_{in}\}e^{i\omega t}. \tag{15}\]  
where \(M\), \(C\), and \(K\), constitute the matrix of mass, damping, and stiffness. \(F_{in}\) and \(v_{out}\) are the input force and the output velocity, respectively.