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

Abstract

Application of high hydrostatic pressure (200, 300, 350 and 400 MPa) at 5 °C for 30 min to different micro‐organisms, including Gram‐positive and Gram‐negative bacteria, moulds and yeasts, proved to be more effective in inactivating these organisms than treatments at 20 °C for 10 min and at 10 °C for 20 min. Moulds, yeasts, Gram‐negative bacteria and Listeria monocytogenes were most sensitive, and their populations were completely inactivated at pressures between 300 and 350 MPa. The same conditions of pressure, temperature, and time were applied to different vegetables (lettuce, tomato, asparagus, spinach, cauliflower and onion), achieving reductions of from 2–4 log units in both viable mesophiles and moulds and yeasts at pressures of between 300 and 400 MPa. Sensory characteristics were unaltered, especially in asparagus, onion, tomato and cauliflower, though slight browning was observed in cauliflower at 350 MPa. Flow cytometry was applied to certain of the microbial populations used in the above experiment before 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 undetectable using the plate count method, suggesting that micro‐organisms suffering from pressure stress were metabolically inactive though alive. The pressurization treatments did not inactivate the peroxidase responsible for browning in vegetables. Confocal microscopic examination of epidermal tissue from onion 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 the vegetables, and should lower industrial costs.

Effect of temperature and compression/decompression rates on high pressure inactivation of Listeria

“…The results of the present work indicate that the inactivation of L. innocua in TSB was highest when pressurization was conducted below room temperature (4-10 °C). Similar results have been obtained for Listeria species inoculated in meat (CARLEZ et al, 1993), milk (GERVILLA et al, 2000), and vegetables (ARROYO et al, 1999). In some cases, the NDR obtained at 4 °C can only be exceeded when pressurization temperature is higher than 50 °C (CARLEZ et al, 1993).…”

Resistance of Bacillus Endospores to Extreme Terrestrial and Extraterrestrial Environments

“…However, because the particles of cosmic radiation interact with the shielding material by creating secondary radiation, the dose rates go through a maximum with increasing shielding thickness. The dose rate has been calculated behind different shielding thicknesses, based on (i) inactivation cross sections for B. subtilis spores from experimental data obtained at heavy ion accelerators (9), (ii) an estimated density for the meteorite of 1.8 g/cm 3 (taken from data for a Martian regolith), and (iii) the National Aeronautics and Space Administration HZETRN transport code using distribution of cosmic ray particles, their interaction with matter, and the track structure model for cell inactivation, which has been developed to calculate the radiation risks for humans in space missions (32). The dose rate reaches its maximum behind a shielding layer of about 10 cm; behind about 130 cm, the value is the same as obtained without any shielding, and only for higher shielding thickness is the dose rate reduced significantly (Fig.…”

Effect of High Pressure Homogenization on Improving the Quality of Milk and Sensory Properties of Yogurt: A Review

“…High pressures damage the cell membrane and denature proteins [24], [25]. High pressures (>200 MPa) are needed to kill Saccharomyces cerevisiae at (45-60 °C) due to the membrane damage and inactivation of cell metabolism [26]. HPH treatment is also reported to inactivate milk microorganisms logarithmically such as; L. monocytogenes in milk at 375 MPa and S. aureus at 600 MPa [17].…”

“…The size and shape of bacteria and also the nature of cell membrane affect their resistance to HPH. The order of microorganism's lethality after high pressure treatment is [8], [26]:…”