Influence of ultrasound on the microstructure of plant tissue

- The influence of ultrasound on material structure and cell components was examined.
- Thermal and mechanical effects of ultrasound were confirmed.
- Irreversible changes in structure were found.
- Aberrations in cell walls or membranes due to ultrasonic passage were not observed.

The aim of this work was to assess the influence of ultrasound on the microstructure of plant tissue during direct sonication via air (without liquid medium). Two different materials (apple and onion) were tested under two experimental conditions: direct sonication in an acoustic chamber and convective drying enhanced with ultrasound in a laboratory hybrid dryer. The impact of ultrasound on the material being processed was analyzed through direct observation with the naked eye, analysis of thermograms, and observation of dyed in Lugol's solution specimens, using an inverted optical microscope and SEM images. The results obtained revealed both the thermal and the mechanical effects of ultrasonic action. The temperature of the materials processed with the use of ultrasound attained values higher than the temperature of ambient air, even during convective drying at 313 K. Moreover, application of ultrasound led to irreversible changes in structure (growth of porosity, loss of tissue coherence, formation of microchannels, etc.) and cell composition (destruction of cell components, e.g., the nucleus). Aberrations in cell walls or cell membranes due to ultrasonic passage were not observed. Explanations of the observed phenomena were proposed.