Avocado and coffee leaves through waves: evaluation of tropical plant tissues by acoustic and opto-acoustic techniques

José Luis Castaño Bernal, researcher and PhD student in Applied Mechanics. Credit: Communications Office, Faculty of Engineering.   

Nowadays, when natural phenomena such as "La Niña" generate climatic changes in many regions dedicated to the cultivation of agricultural products, phytosanitary variables are required to ensure the care and monitoring of crops and harvested food. Among these variables are moisture content in plant leaves and fruit firmness. However, the techniques traditionally used subject these plant organisms to invasive or destructive practices. A research project seeks to develop, using ultrasonic waves, non-invasive techniques useful for measuring these parameters.  

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A futuristic bet for the field: ultrasound in the field of crops and post-harvest conservation 

Changes in climate and seasonal cycles are becoming increasingly difficult to predict around the world due to the effects of global warming. This scenario impacts, among other sectors, communities that depend on the regularity of the weather to ensure the success of food production.  

To face the challenges in terms of analysis and preservation of products that these climatic conditions entail, the physical engineer and researcher José Luis Castaño Bernal, as part of his studies in the PhD in Applied Mechanics of the School of Civil Engineering and Geomatics of the Universidad del Valle, proposed the study of techniques derived from ultrasound, such as opto-acoustic techniques, with the aim of providing new ways to evaluate the condition of fruits, especially their plant tissues.  

The research, led by the researcher and professor of the School of Mechanical Engineering, Joao Luis Ealo Cuello, and funded by the National Royalties System through the Bicentennial Doctoral Excellence Scholarship offered by the Ministry of Science, Technology and Innovation, conducted the examination of physiological processes in coffee and avocado leaves using two methods, air-coupled ultrasonic spectroscopy and laser-induced elastic waves, which aspires to become the starting point for a radically innovative area of research that could have uses and impact in areas such as food engineering and plant physiology. 

This work builds upon the previous doctoral research conducted by David Alejandro Collazos-Burbano. Furthermore, its initial progress was made possible through collaboration with Tomás Gómez from the Spanish National Research Council (CSIC), who introduced us to the technique of contactless ultrasonic spectroscopy and is regarded as a global authority in this field.

Research: air-coupled ultrasonic spectroscopy and laser-induced elastic waves 

The work carried out by researcher José Luis Castaño Bernal started by finding fruits typical of the area of Caldas, department of which he is a native, and after considering different options, he opted for the ripening of avocados and events of dehydration and rehydration of coffee leaves. 

His objective was to test two techniques based on acoustic principles (air-coupled ultrasonic spectroscopy and laser-induced elastic waves), to observe in-vivo physiological phenomena in plant tissues and obtain information about their mechanical properties without compromising the integrity of the tissues.  

"The idea is that we can have a measurement system not only for research, but also so that a producer can determine the drought resistance of coffee plants (extendable to other plants), a quantification of when irrigation could be done, and also determine how many days are left for a fruit to ripen, without the need to destroy it, open it or touch it", explains the researcher Castaño Bernal.  

He also explains the characteristics of both techniques: 

The first, called air-coupled ultrasonic spectroscopy, was applied to coffee leaves, and consists of two transducers located one in front of the other, which emit and detect ultrasonic frequencies. The coffee leaf sample is placed between them, so that the generated wave is transmitted through the different structures of the leaf where information on the propagation of the wave is collected. The result makes it possible to determine the changes and structures inside the leaf case, for example, it has been subjected to a dehydration process. "There are a series of phenomena associated with the structures, morphology and changes that are occurring inside the leaf that we can detect by means of the transmitted wave," adds the researcher. 

The second technique uses a phenomenon called laser-induced ultrasound which, through the exchange of thermal energy generated by a pulsed laser, generates elastic waves in a material. This wave is detected by another laser, so that it is possible to track the wave in order to determine its evolution and propagation through the medium.  

Once both methodologies were available for the evaluation of plant tissues, it was proceeded to analyze, in the case of coffee leaves during hydration and rehydration cycles, the degree of water loss related to their elasticity and turgor. In the case of avocados, elastic waves were introduced to analyze dispersion and attenuation curves, and a model of wave propagation in cylindrical geometries was applied to determine the longitudinal and transverse velocities during the ripening process; the latter in order to identify the degree of ripeness of the fruit. 

Promising results in an underexplored area of research 

When working with biological elements, the research faced a number of challenges, given the sensitivity of plant tissues to changes in the environment in which they were treated at. Faced with this, the researcher José Luis Castaño Bernal identified several parameters that allowed him to identify ideal conditions to carry out his analysis successfully. Once this process was carried out, the results obtained allowed the researcher to demonstrate significant differences in the mechanical response of the avocados throughout the ripening period: the longitudinal and transverse velocities decreased as the fruit ripened and lost mass. As for coffee leaves, changes in the elastic conditions of leaf tissues were evidenced, where the diffusivity constant (the speed at which a substance is transported through a medium) was a differentiating parameter between dehydration and rehydration. In his opinion, the fact that these results were obtained by means of a non-contact technique represents a great advance:  

"Techniques that usually evaluate physiological parameters in plants are destructive. You have to cut the leaf, subject it to the pressure chamber and then extract water from it. The leaf is destroyed. In the case of fruit, you have to penetrate with a probe into different sectors of the fruit and at different stages of ripening to get a mechanical response and the fruit is bruised and shattered.  In addition, you have to do multiple repetitions to get verifiable results. We know that with the techniques we are implementing we are able to develop non-contact techniques that can measure plant leaves and avocados without destroying them, and we could even measure plant organisms directly on the plant. That implies a greater difficulty, because you are no longer studying a leaf, but a soil-plant-atmosphere network that interacts in a more complex system, but, at the same time, more interesting." 

Innovative nature of the doctoral program 

Researcher José Luis Castaño Bernal believes that, after conducting tests with air-coupled ultrasonic spectroscopy and laser-induced elastic waves, both techniques have high potential to be miniaturized, which would make them susceptible to be industrialized and marketed. According to him, as progress is made in the development of these techniques, it could be thought in the future that growers will have devices that allow the measurement of fruit and leaf indicators, avoiding invasive practices currently used.  

On the other hand, the little use that is being made of ultrasound opens the door for this research to become a reference both regionally and nationally, as well as for those who wish to use acoustics in little explored fields such as plant physiology. "Like all research, I believe that we are in an exploratory stage with promising results [...] As new developments, implementations and more research advance, the techniques will acquire sufficient maturity to use them in different and broad areas", suggests the researcher about the current state of his project. 

If interested in being in touch with the student or any further information about the investigation, please write the Faculty of Engineering Communications Office: comunicaingenieria@correounivalle.edu.co.