Microstructural and morphological behaviors of asparagus lettuce cells subject to high pressure processing

• Precise cellular shape is visualized in vivo and in situ by CLSM.
• Lower pressure caused changes to rounder shape and smaller size cells.
• Cell morphology retained under higher pressure.
• Pectin restricted to the corners of tricellular junctions at 200 MPa and 300 MPa.
• Cell microstructural and morphological behaviors were displayed by a 7-cell tissue model.

To explore the mechanisms of texture change under high pressure (HP) processing, the effects of different pressures on cell microstructure and cell morphological behaviors of asparagus lettuce were investigated using different microscopy approaches. Pectin immunofluorescence labeling was adopted to analyze cell morphological change in vivo by in situ visualization of cell wall. Image analysis showed that cells in asparagus lettuce treated at lower pressures (100 MPa, 200 MPa and 300 MPa) exhibited rounder and smaller than the untreated samples. However, the shapes of cells in asparagus lettuce treated at 500 MPa were close to the control. The in situ analysis on pectin distribution by CLSM indicated that pectin was mainly restricted in the corners of tricellular junctions in the samples treated at 200 MPa and 300 MPa, whereas pectin uniformly distributed around the whole cell wall at 500 MPa. The results of cell activity staining were consistent with that of ultrastructure change of cell wall and membrane observed by TEM, which suggested that plasma membrane was ruptured at the pressure of 200 MPa, 300 MPa and 500 MPa. Meanwhile, the integrity of cell wall structure was retained under all HP conditions. Based on the above results, a 7-cell tissue model has been proposed to show the cell morphological behavior. On the one hand, moderate HP (100 MPa–300 MPa) caused an initial texture loss of asparagus lettuce tissue, probably due to the loss of turgor pressure and the loose skeleton of cell wall. On the other hand, HP (500 MPa) caused less apparent texture loss of asparagus lettuce due to the unchanged pectin distribution, which can maintain the rigid and flexible cell wall.