Response to high-pressure, low-temperature treatment in vegetables: determination of survival rates of microbial populations using flow cytometry anddetection of peroxidase activity using confocal microscopy

Application of high hydrostatic pressure (200, 300, 350 and 400 MPa) at 5 d egrees C for 30 min to different micro-organisms, including Gram-positive a nd Gram-negative bacteria, moulds and yeasts, proved to be more effective i n inactivating these organisms than treatments at 20 degrees C for 10 min a nd at 10 degrees C for 20 min, Moulds, yeasts, Gram-negative bacteria and L isteria monocytogenes were most: sensitive, and their populations were comp letely inactivated at pressures between 300 and 350 MPa. The same condition s of pressure, temperature, and time were applied to different vegetables ( lettuce, tomato, asparagus, spinach, cauliflower and onion), achieving redu ctions of from 2.4 log units in both viable mesophiles and moulds and yeast s at pressures of between 300 and 400 MPa. Sensory characteristics were una ltered, especially in asparagus, onion, tomato and cauliflower, though slig ht browning was observed in cauliflower at 350 MPa. Flow cytometry was appl ied to certain of the microbial populations used in the above experiment be fore and after the pressurization treatment. The results were indicative of differing percentage survival rates depending on micro-organism type, with higher survival rates for Gram-positive bacteria, except L. monocytogenes, than in the other test micro-organisms. Growth of survivors was undetectab le using the plate count method, suggesting that microorganisms suffering f rom pressure stress were metabolically inactive though alive. The pressuriz ation treatments did not inactivate the peroxidase responsible for browning in vegetables. Confocal microscopic examination of epidermal tissue from o nion showed that the enzyme had been displaced to the cell interior. Use of low temperatures and moderately long pressurization times,yielded improved inactivation of micro-organisms and better sensorial characteristics of th e vegetables, and should lower industrial costs.