Colombian coffee production is well-known, and selective manual harvesting plays a vital task in guaranteeing high ripe coffee fruit rates in this process, leading to its known worldwide aroma and flavor. To maintain this quality approach, selective harvesting methods based on mechanical vibrations are a promising alternative for developing technologies that could accomplish the challenging Colombian coffee production context. In this study, a vibrations analysis in coffee fruits at three ripening stages was carried out to evaluate the dynamic behavior at two frequency windows: 10 to 100 Hz and 100 to 1000 Hz. Two groups of fruits previously classified in the CIELab color space were chosen for the vibration test study samples. Time and frequency signals were characterized via FFT (fast Fourier transform), and bump wavelets were determined to obtain the frequency–time magnitude scalograms. The measurements were obtained in three degrees of freedom over the fruits: one for measuring the input force (computed in voltage way) and the other two measured by the velocity. The results revealed frequency ranges with specific resonant peaks between 24 and 45 Hz, and close to 700 Hz, where the ripe fruits presented higher magnitudes in the calculated parameters. FFT of the velocity and scaled mechanical impedance were used to estimate these frequency ranges. This work is an important step to identify a “vibrational fingerprint” of each Coffea arabica var. Castillo fruit-ripening stage. However, we consider that more experiments should be performed to reconstruct the modal shape in each resonance. In future studies, fatigue analysis could show which are the most effective frequency ranges to detach the ripe fruits from the perspective of a real selective coffee-harvesting scenario.

In the last few years, Colombian coffee production has decreased by different economic factors related to the increment of costs in manual selective harvest operations. Consequently, an interest has arisen for studying selective harvesting methods based on mechanical vibrations in seeking for an alternative technology. In this sense, it is mainly essential to understand the dynamic behavior of the coffee fruits at different ripening stages. In this study, an experiment was proposed to observe the vibration shapes of coffee fruits (Coffea arabica var. Castillo) in defined frequency intervals. The ripening stages were categorized by a strategy based on color measurements in the CIELab space. Four ripening stages were determined from color data taken of 84 fruits, which allowed the development of the dynamic experiments over classified fruits. The dynamic excitation was applied to the coffee fruits between 0–1000 Hz in which ripe fruits evidenced higher vibration amplitudes in specific frequency intervals. These motions cannot be visualized from a normal video-camera (60 Fps). Therefore, a high-speed camera (Chronos 1.4) was used to record these during harmonic excitation. Eulerian video magnification technique was applied to the video-based measurements to visualize the amplified vibration shapes. The magnified motions were contrasted with vibrometry measurements and a piezo sensor attached to the fruit pedicel. Results show that the ripe fruit can vibrate with higher amplitudes in comparison with the other ripening stages, this information could be used to harvest selectively only ripe fruits.

The development of selective coffee harvesting technologies requires detailed knowledge of the geometrical, physical, and mechanical properties of the subsystems of the coffee bush, including the elastic properties of the substructures of fruit and the coffee bean, which are directly related to the selectivity problem. The elastic properties of the mesocarp–exocarp and bean are not described in the literature due to the difficulty of characterizing these locally, since measuring each component of the coffee fruit structure (mesocarp–exocarp and bean) is not an easy task. However, determining the elastic properties (of the mesocarp–exocarp and bean) could help create realistic simulations as an initial estimation for selective coffee harvesting studies. The present work aims to bridge the gap in the mechanical characterization of the sub-structures of the coffee fruits by assessing the elastic properties of the mesocarp–exocarp and bean. Indentation tests were performed on eighty Coffee arabica L. var. Castillo fruits and beans, which were previously classified into four ripening stages using fruit color data in the CIELab color space. Young’s modulus and indentation hardness of the mesocarp–exocarp structure and beans were calculated, applying the Oliver and Pharr indentation model and Hertz contact theory.