Release mechanisms of bioactive compounds in fruits submitted to high hydrostatic pressure: A dynamic microstructural analysis based on prickly pear cells

High hydrostatic pressure (HHP) promotes the release of bioactive compounds from their intracellular compartments making them more bioaccessible. Our aim was to propose a schematic tissue model to explain the release mechanisms of betalains and phenolic compounds in vegetable cells submitted to HHP by analyzing cell microstructure, cell morphology, cell viability and the localization of bioactive compounds in prickly pear fruits. Prickly pear slices were pressurized at 100, 350 and 600 MPa at 20 degrees C. Chlorenchyma cells (in peels) and parenchyma cells (in pulps) were analyzed by transmission electron microscopy, confocal laser scanning microscopy and optical microscopy. After pressurization, the respiration and ethylene production of processed fruits were measured every 6 h (during storage at 16 degrees C and 75% RH for 24 h). In chlorenchyma cells, HHP ruptured betalain-storing vesicles in the cytoplasm and possibly increased the activity of endogenous enzymes. Contrarily, HHP released betalains from the vacuoles of parenchyma cells due to breaking of the tonoplast where they presented higher stability. In both tissues, phenolic compounds were released from cell walls with increasing pressure and enhanced by cell wall ultrastructural modifications (100 MPa), rupture (350 MPa) and the rearrangement of microfibrillated cellulose (600 MPa). Prickly pears submitted to HHP presented advanced senescence marked by considerable ethylene increase and the gradual loss of CO2 production after 6 h. Cells were viable at 100 MPa by conserving intact cell membranes and after 24 h their respiration rates presented no significant differences compared to controls therefore indicating the possibility of synthesis of bioactive compound as a response to abiotic stress. We have proposed a new approach for analyzing the effects of HHP and have identified the storing of betalains in vesicles located in the cytoplasm of chlorenchyma cells for the first time. This study is the first to fathom the dynamic morphological changes and release mechanisms of bioactive compounds in vegetable cells subjected to HHP.