Changes in cell wall stiffness and microstructure in ultrasonically treated apple

•Apple tissue was subjected to ultrasound.•Cell wall stiffness was evaluated by atomic force microscope.•Cell wall stiffness decreases with time of ultrasound treatment.•Ultrasound causes pectin solubilization in cell walls.•Ultrasound creates intercellular spaces.

Ultrasounds (US) enhances mass transfer in fruit, however, evidence of changes triggered at cell wall level are still very scarce. Apple tissue was subjected to ultrasound for 7.5, 15 or 30 min in order to evaluate the effect on the cell wall. The distribution of galacturonic acid (GalA) in water (WSP), calcium chelator (CSP) and sodium carbonate (DASP) soluble pectin fractions extracted from cell walls was measured. Cell wall stiffness was evaluated using an atomic force microscope. The cellular structure was studied using a confocal microscope. Area, perimeter, roughness and circularity of objects in the images were analyzed. Cell wall stiffness gradually decreased from 8.97 MPa to 4.24 MPa after 30 min. Low content of GalA in WSP fraction for the control samples showed a relatively low level of pectin solubilization in the apples used. Most GalA was distributed in DASP. A significant increase in GalA content in WSP as a function of time of US treatment was observed. Similarly, US increased the GalA content in CSP and DASP fractions, but a significant effect only occurred after the longest treatment time (30 min). The content of GalA in the insoluble fraction finally increased after 30 min. This showed that ultrasounds increased the overall pectin solubilization in cell walls. The exposure to ultrasound caused a significant decrease in the average area and the mean perimeter of the objects due to an increase in the area contributed by small objects (<6000 μm2). This has confirmed that ultrasounds creates intercellular spaces in fruit tissue.