Insights into the mechanisms of ultrasound treatment on plant tissue as revealed by TD-NMR, microstructural analysis, and electrical properties measurements

The aim of this study has been to investigate the impact of ultrasound (US) treatment on the structure, water distribution, and electrical properties of apple and carrot tissues. The influence of sonication power and duration was analysed. Slices of apple and carrot were subjected to ultrasound treatment at 21 kHz for 10, 20, 30, 45, and 60 min, using power levels of 180 and 300 W. Water distribution and its physical state were evaluated by means of the TD-NMR technique. Furthermore, the microstructure, electrical capacitance, and conductivity of the samples were studied.

In fresh (untreated) apple tissue, electrical conductivity increased significantly from 33.1 ± 3.1 to 53.8 μS/cm after 60 min of US treatment at 300 W. These findings were supported by NMR results and SEM imaging, which revealed increased porosity and the formation of microchannels, indicating disruption of cell membrane integrity in the apple. Free water from the vacuole was relocated to the extracellular spaces and cytoplasm, and to a lesser extent was retained by the cell wall.

In contrast, carrot tissue exhibited stable electrical conductivity (56.8 ± 5.0 and 52.8–64.3 μS/cm for fresh and US-treated samples, respectively), suggesting only subtle changes in cell wall organisation without membrane rupture. Moreover, based on both the electrical properties and NMR results, it may be concluded that water and ions were likely transmitted from the vacuole and retained by the cell wall in carrot tissue to a greater extent than in apple.

US-treated carrot samples were characterized by greater distances between cell residues and the formation of microscopic channels, with average cross-sectional cell areas increasing from 0.006 mm² in untreated samples to 0.013 mm² after 20 min at 180 W, and up to 0.015 mm² after 30 min at 300 W.