Total Bacterial Inactivation Research Articles

Solar disinfection of drinking water at alkaline pH, assisted by ashes of spices and herbs containing catalytic metals, in a novel “phyto-Fenton” process

In this manuscript, the improvement of solar disinfection (SODIS) for water treatment through the photo-Fenton reaction using iron replacements, namely calcined herbs and spices (thyme, cumin, cinnamon, ginger, and cocoa), naturally containing high content of metals. The bactericidal performance of SODIS, H2O2/light and solar photo-Fenton (light/H2O2/Fe2+) were assessed in pure and Lake Leman water. Sodium percarbonate (SPC) and ashes were successfully employed as substitutes of H2O2 and Fe2+, respectively. Low spices‘/herbs’ addition improved the H2O2/light process, especially when ginger, cumin, thyme, or cocoa were used, inducing effective heterogeneous photo-Fenton process even at alkaline pH. In ultrapure water, solar photo-Fenton with ashes was only slightly slower than the corresponding Fe2+-driven reaction, while in lake water total bacterial inactivation was achieved by cumin, thyme or ginger ashes. SPC enhanced SODIS performance, while the addition of ashes resulted in faster kinetics than light/SPC only. Overall, this work succeeded in converting the classic photo-Fenton process into a low-cost variant, the “phyto-Fenton” process, driven by the metals contained in the organic-free ashes and the H2O2 content of SPC.

The efficacy of the VUV/O3 process run in a continuous-flow fluidized bed reactor for simultaneous elimination of favipiravir and bacteria in aqueous matrices.

The efficacy of the Vacuum UV/Ozonation (VUV/O3) process was evaluated for the degradation of favipiravir (FAV). It was found that coupling O3 and VUV resulted in a considerable synergistic catalytic effect on FAV removal. The VUV/O3 process performed better in moderately alkaline conditions than in acidic ones; complete FAV degradation and 99.4% TOC removal were achieved within 10 and 60min, respectively. HO• played the dominant role in FAV degradation, with a second-order reaction rate constant with HO• at 1.05×1010M-1s-1. The VUV/O3 process could effectively treat tap water spiked with FAV. Efficient FAV and TOC removal, as well as total bacterial inactivation, was attained when treating municipal secondary effluent by the VUV/O3 process. Finally, the VUV/O3 process was operated in a continuous-flow mode in a fluidized-bed (FBR) reactor for treating FAV-spiked tap water. Complete degradation and 75.1% mineralization of 10mg/L FAV were obtained at a hydraulic retention time of 1 and 8min, respectively. The findings clearly suggest that the VUV/O3 process operated in a continuous-flow FBR is a promising, efficient technology for the removal of novel and emerging contaminants, such as the antiviral FAV.

Effect of Pulsed Electric Field (PEF) on Bacterial Viability and Whey Protein in the Processing of Raw Milk

There is growing concern regarding the nutritional value of processed food products. Although thermal pasteurization, used in food processing, is a safe method and is widely applied in the food industry, food products lack quality and nutritional value because of the high temperatures used during pasteurization. In this study, the effect of pulsed electric field (PEF) processing on whey protein content and bacterial viability in raw milk was evaluated by changing the PEF strength and number of pulses. For comparison, traditional pasteurization techniques, such as low-temperature long-time (LTLT), ultra-high temperature (UHT), and microfiltration (MF), were also tested for total whey protein content, bacterial activity, and coliforms. We found that, after treatment with PEF, a significant decrease in total bacterial viability of 2.43 log and coliforms of 0.9 log was achieved, although undenatured whey protein content was not affected at 4.98 mg/mL. While traditional pasteurization techniques showed total bacterial inactivation, they were detrimental for whey protein content: β-lactoglobulin was not detected using HPLC in samples treated with UHT. LTLT treatment led to a significant decrease of 75% in β-lactoglobulin concentration; β-lactoglobulin content in milk samples treated with MF was the lowest compared to LTLT and UHT pasteurization, and ~10% and 27% reduction was observed.

Solar disinfection as a direct tertiary treatment of a wastewater plant using a photochemical-photovoltaic hybrid system

This work evaluates the SolWat hybrid system for solar water disinfection and photovoltaic energy generation, for its implementation in tertiary treatment plants, using real wastewater directly from the effluent after its secondary treatment. Solar disinfection of E. coli, Enterococcus faecalis and Clostridium perfringens microorganisms was evaluated over the course of a complete year. Four experiments in batch mode were conducted in autumn, winter, spring and summer, and microbiological and physicochemical parameters were analysed. In addition, the kinetics of solar disinfection during 4 h were analysed, and the dose of lethal ultraviolet radiation for microorganisms established. Results showed that E. coli, Enterococcus faecalis and C. perfringens did not complete total bacterial inactivation after 4 h of treatment in the SolWat system, but that the inactivation levels achieved were sufficient as to allow for the reuse of water for various uses (urban, agricultural, industrial, etc.). Clostridium perfringens continued to be the most resistant bacteria vs. E. coli and Enterococcus faecalis. The total photovoltaic energy production in the hybrid system compared to the reference system was the same, generating both the SolWat module and the reference module identical electrical power due to the compensating effect of module water cooling vs. radiation losses.

Combined Antimicrobial Effect of Bio-Waste Olive Leaf Extract and Remote Cold Atmospheric Plasma Effluent.

A novel strategy involving Olive Leaf Extract (OLE) and Cold Atmospheric Plasma (CAP) was developed as a green antimicrobial treatment. Specifically, we reported a preliminary investigation on the combined use of OLE + CAP against three pathogens, chosen to represent medical and food industries (i.e., E. coli, S. aureus and L. innocua). The results indicated that a concentration of 100 mg/mL (total polyphenols) in OLE can exert an antimicrobial activity, but still insufficient for a total bacterial inactivation. By using plain OLE, we significantly reduced the growth of Gram positive S. aureus and L. innocua, but not Gram-negative E. coli. Instead, we demonstrated a remarkable decontamination effect of OLE + CAP in E. coli, S. aureus and L. innocua samples after 6 h. This effect was optimally maintained up to 24 h in S. aureus strain. E. coli and L. innocua grew again in 24 h. In the latter strain, OLE alone was most effective to significantly reduce bacterial growth. By further adjusting the parameters of OLE + CAP technology, e.g., OLE amount and CAP exposure, it could be possible to prolong the initial powerful decontamination over a longer time. Since OLE derives from a bio-waste and CAP is a non-thermal technology based on ionized air, we propose OLE + CAP as a potential green platform for bacterial decontamination. As a combination, OLE and CAP can lead to better antimicrobial activity than individually and may replace or complement conventional thermal procedures in food and biomedical industries.

Organic fertilizer as a chelating agent in photo-Fenton at neutral pH with LEDs for agricultural wastewater reuse: Micropollutant abatement and bacterial inactivation

In a water scarcity scenario, the reused wastewater could be an essential source for agricultural irrigation considering that 60% of fresh water is destined to this area. In this study, an organic fertilizer (Diethylene triamine pentaacetic acid, DTPA) was used as a new chelating agent of iron for photo-Fenton’s application at neutral pH using LEDs. Secondary effluents with different characteristics were tested for propranolol removal and bacterial inactivation. With DTPA, the best results were achieved with MBR matrix: 94.0% of propranolol removal and total bacterial inactivation after 120 min. IFAS matrix showed the worst results: 63.2% of propranolol removal and only 2-log reduction for Total Coliforms. The performance of DTPA as chelating agent was compared with EDTA and EDDS with two matrices. In MBR matrix, propranolol removal with EDTA was 100% in 15 min, while DTPA and EDDS reached similar results at 120 min (94.0 and 91.3%), respectively. The iron precipitation was evaluated, and DTPA showed high stability with Fe2+ (only 10.4% of iron reduction instead 97.3% for EDDS). In addition, it looks like that the stability of iron chelates plays an important role in bacterial inactivation. Thus, the experiments with DTPA showed the lowest bacterial growth-on-the-plate after 72 h of the end of the experiment. Biodegradability and phytotoxicity were also evaluated and the experiments with DTPA had the lowest toxicity. The results of the experiments performed with DTPA were compared with the values in Proposal for agricultural water reuse suggesting that treated effluent accomplish the requirements for agriculture.

New Evidence for Ag-Sputtered Materials Inactivating Bacteria by Surface Contact without the Release of Ag Ions: End of a Long Controversy?

The study provides new evidence for Ag-coated polyester (PES) mediating Escherichia coli inactivation by way of genetically engineered E. coli (without porins, from now denoted porinless bacteria). This allows the quantification of the bactericidal kinetics induced by the Ag surface without the intervention of Ag ions. Bacterial inactivation mediated by Ag-PES was seen to be completed within 60 min. The samples were prepared by high-power impulse magnetron sputtering (HiPIMS) at different sputter powers. In anaerobic media, this process required 120 min. The amounts of ions (Ar+, Ag+, and Ag2+) generated during the deposition by direct current magnetron sputtering (DCMS) and HiPIMS were determined by mass spectrometry. The thickness of the Ag films sputtered on PES by DCMS (0.28 A) during 100 s was found to be 340 nm. Thicknesses of 250, 230, and 200 nm were found when sputtering with HiPIMS was tuned at 8, 17, and 30 A, respectively. By scanning transmission electron microscopy (STEM-HAADF), the atomic distribution of Ag and oxygen was detected. By X-ray photoelectron spectroscopy (XPS), a shift in the Ag oxidation state was observed within the bacterial inactivation period. This reveals redox catalysis within the time required for the total bacterial inactivation due to the interaction between the bacterial suspension and Ag-PES. Surface properties of the Ag-coated PES samples were additionally investigated by X-ray diffraction (XRD). The formation of Ag plasmon was detected by diffuse reflectance spectroscopy (DRS) and was a function of the applied sputtering energy. The indoor sunlight irradiation dose required to induce an accelerated bacterial inactivation was found to be 5-10 mW/cm2.

Deciphering the Mechanisms of Bacterial Inactivation on HiPIMS Sputtered CuxO-FeOx-PET Surfaces: From Light Absorption to Catalytic Bacterial Death.

The production of nontoxic, affordable, and efficient antibacterial surfaces is key to the well-being of our societies. In this aim, antibacterial thin films have been prepared using earth-abundant metals deposited using high-power impulse magnetron sputtering (HiPIMS). The sputtered FeOx, CuxO, and mixed CuxO-FeOx films exhibited fast bacterial inactivation properties under exposure to indoor light (340-720 nm) showing total bacterial inactivation within 180, 120, and 60 min, respectively. The photocatalytic mechanisms of these films were investigated, from the absorption of photons up to the bacteria's fate, by means of ultrafast transient spectroscopy, flow cytometry, and malondialdehyde (MDA) quantification justifying the cell wall disruption. The primary driving force leading to bacterial inactivation was found to be the oxidative stress at the interface between the sputtered thin films and the microorganism. This was justified by using engineered porinless bacteria disabling the possible ion diffusion leading to internal bacterial inactivation. Such stress is a direct consequence of the photogenerated electron-hole pairs at the interface of the sputtered layers. By diffuse reflectance spectroscopy, we found that both FeOx and CuxO present a band gap of ∼2.9 eV (>425 nm), while the mixed CuxO-FeOx thin film has a band gap below 2.3 eV (>540 nm). The structure and atomic composition of the films were characterized by energy-dispersive X-ray, X-ray photoelectron, and optical spectroscopy. While the composition and metal oxidation states are distinct in all three films, the difference in photocatalytic efficiency can, at first sight, be explained as the direct consequence of their absorbance and the unique interaction between Fe and Cu oxides in the composite film.

Solar-assisted bacterial disinfection and removal of contaminants of emerging concern by Fe2+-activated HSO5- vs. S2O82- in drinking water

This research demonstrates the feasibility to enhance solar disinfection (SODIS) treatment by addition of peroxymonosulfate (PMS) and peroxydisulfate (PDS) by the generation of sulfate (and hydroxyl) radicals through different activation routes. The different promoters were i) sunlight irradiation, ii) mild heat (40 °C), and iii) μM amounts of Fe2+, all present during actual field SODIS experiments, or voluntarily added alongside PMS/PDS. In a first approach, the promoters were studied separately, in pairs and finally all together in a combined process (CP). In all the cases, PMS showed a higher efficiency than PDS in E. coli removal, requiring lower concentration and a faster reaction time towards total bacterial inactivation. Therefore, the combined process (Oxidant/Fe2+/Sunlight/40 °C) reached total bacterial inactivation (6-logU) in 30 min when PMS was used, while it took twice as long with the PDS. These effects can be further enhanced when PMS with H2O2 is used, and barely 20 min are needed for complete bacterial removal. Besides total disinfection, the combined processes were suitable to eliminate micropollutants in μg/L concentration (drugs, pesticides, etc.) during solar treatment. Finally, the efficiency of the treatment methods was successfully tested in a lake water matrix, in a feasibility assay as a potential drinking water treatment method. The economic analysis highly supports the use of these oxidants. Although the use of PMS or PDS increases the cost of treatment, it is not mandatory to add other reagents or external activators; iron is ubiquitous in natural water and can act as activator, while during SODIS sunlight irradiation can provide UV and mild water heating, hence act as an effective disinfection method. Finally, a mechanistic proposal for the combined processes is given, as an overview of the occurring reactions leading to bacterial inactivation.

Bacterial adhesion and inactivation on Ag decorated TiO2-nanotubes under visible light: Effect of the nanotubes geometry on the photocatalytic activity.

This study investigates the effect of the diameter of TiO2 nanotubes and silver decorated nanotubes on optical properties and photocatalytic inactivation of Escherichia coli under visible light. The TiO2 nanotubes (TiO2-NTs) were prepared using the electrochemical method varying the anodization potential starting from 20 V until 70 V. The Ag nanoparticles were carried out using the photoreduction process under the same experimental conditions. The diameter size was determined using the scanning electronic microscopy (SEM). TiO2-NTs diameter reached ∼100 nm at 70 V. Transmission electronic microscopy (TEM) imaging confirmed the TiO2-NTs surface decoration by silver nanoparticles. The Ag-NPs average size was found to be equal to 8 nm. The X-Ray diffraction (XRD) analysis confirm that all TiO2-NTs crystallize in the anatase phases regardless the used anodization potential. The decrease of the photoluminescence (PL) intensity of Ag NPs decorated TiO2-NTs indicates the decrease of the specific area when the nanotubes diameter increases. The UV-vis absorbance show that the absorption edges was bleu shifted with the increasing of nanotubes diameter, which can be explained by the increase of the crystallites average size. The bacterial adhesion and inactivation tests were carried in the dark and under light. Bacteria were seen to adhere on TiO2-NTs in the dark; however, under light the bacteria were killed before they establish a strong contact with the TiO2-NTs and Ag/TiO2-NTs surfaces. Bacterial inactivation kinetics were faster when the anodizing potential of the NTs-preparation increases. A total bacterial inactivation was obtained on ∼100 nm nanotubes diameter within 90 min. This result was attributed to the enhancement of the TNTs crystallinity leading to reduced surface defects. Redox catalysis was seen to occur under light on the TiO2-NTs and Ag/TiO2-NTs. the photo-induced antibacterial activity on the AgO/Ag2O decorated TiO2-NTs was attributed to the interfacial charge transfer mechanism (IFCT).

A green solar photo-Fenton process for the elimination of bacteria and micropollutants in municipal wastewater treatment using mineral iron and natural organic acids

In this investigation, a new, green photo-Fenton process for wastewater treatment is proposed, involving the use of a natural iron source and natural additives, deriving from wastes, acting as iron chelators. The use of mineral iron as precursor in the photo-Fenton process, instead of iron salts, was still able to promote E. coli inactivation. Furthermore, the addition of four low weight organic acids (citric, tartaric, ascorbic and caffeic) showed a significant enhancement of the process, reaching total inactivation except for caffeic acid, which showed no significant effects. Two natural products, rich in the promising organic acids were tested as additives, lime and orange juice, plus their infusion. Lime-based additives showed better results compared to orange-based ones, which could be attributed to the excessive addition of organic matter in the orange systems. The formation of photoactive complexes with the mineral iron and the organic acids from the natural products induced the production of reactive species and ferrous ion, sustaining a homogeneous Fenton reaction. Finally, the proposed modified process was tested against different secondary effluents from a municipal wastewater treatment plant. Total bacterial inactivation was reached in the lime-based system, with no visible microorganism regrowth after 48h. Additionally, the process was able to eliminate 40% of the total identified micropollutants of the secondary effluent reaching almost 50% of removal of the total effluent organic matter.

Disinfection of urban wastewater by solar driven and UV lamp – TiO2 photocatalysis: Effect on a multi drug resistant Escherichia coli strain

The effect of TiO2 photocatalysis on the inactivation of an antibiotic resistant Escherichia coli strain selected from an urban wastewater treatment plant (UWWTP) effluent was investigated. Different light sources including a 250 W wide spectrum lamp, a 125 W UV-A lamp and solar radiation, as well as, photocatalysts loadings (TiO2 Degussa P25) in the range from 0.05 to 2.00 g TiO2 L−1 were evaluated. The higher efficiency (total bacterial inactivation after 10 min of irradiation) was observed in the absence of TiO2 when the wastewater was irradiated using the 250 W lamp. In the presence of TiO2 a decreasing inactivation trend was observed (99.76% and 72.22% inactivation after 10 min irradiation at 0.10 and 2.00 g TiO2 L−1 respectively). Under solar simulated conditions the highest inactivation efficiency (93.17%) after 10 min of irradiation was achieved at the lower photocatalyst loading (0.05 g TiO2 L−1). The concept of “reactor optical thickness” was introduced to explain the rates of disinfection observed. The optimum photocatalyst loading estimated by radiation absorption-scattering modeling was found to be 0.1 g TiO2 L−1 for all lamps. The difference between experimental tests and modeling may be due to TiO2 particles aggregation. Comparative kinetic tests between solar and solar simulated photocatalytic (SSP) processes using 0.05 g TiO2 L−1 in suspension showed a quite similar inactivation behavior up to 30 min of irradiation, but only the SSP process resulted in a total inactivation of bacteria after 60 min of exposure. Antibiotic resistant test (Kirby–Bauer) on survived colonies showed that the SSP and SP processes affected in different ways the resistance of E. coli strain to the target antibiotics.

Removal of faecal indicator pathogens from waters and wastewaters by photoelectrocatalytic oxidation on TiO2/Ti films under simulated solar radiation

The disinfection efficiency of water and secondary treated wastewater by means of photoelectrocatalytic oxidation (PEC) using reference strains of Enterococcus faecalis and Escherichia coli as faecal indicators was evaluated. Operating parameters such as applied potential (2-10V), initial bacterial concentration (10(3)-10(7)CFU/mL), treatment time (up to 90min) and aqueous matrix (pure water and treated effluent) were assessed concerning their impact on disinfection. PEC experiments were carried out using a TiO(2)/Ti film anode and a zirconium cathode in the presence of simulated solar radiation. Bacterial inactivation was monitored by the culture method and real-time SYBR green PCR. A 6.2 log reduction in E. faecalis population was achieved after 15min of PEC treatment in water at 10V of applied potential and an initial concentration of 10(7)CFU/mL; pure photocatalysis (PC) led to only about 4.3 log reduction, whilst negligible inactivation was recorded when the respective electrochemical oxidation process was applied (i.e. without radiation). PEC efficiency was generally improved increasing the applied potential and decreasing initial bacterial concentration. Regarding real wastewater, E. coli was more susceptible than E. faecalis during treatment at a potential of 5V. Wastewater disinfection was affected by its complex composition and the contained mixed bacterial populations, yielding lower inactivation rates compared to water treatment. Screening the results obtained from both applied techniques (culture method and real-time PCR), there was a discrepancy regarding the recorded time periods of total bacterial inactivation, with qPCR revealing longer periods for complete bacterial reduction. PEC is superior to PC in terms of E. faecalis inactivation presumably due to a more efficient separation and utilization of the photogenerated charge carriers, and it is mainly affected by the applied potential, initial bacterial concentration and the aqueous matrix.

Inactivation by solar photo-Fenton in pet bottles of wild enteric bacteria of natural well water: Absence of re-growth after one week of subsequent storage

Iron photo-assisted inactivation of wild enteric bacteria (total coliforms/E. coli and Salmonella spp.) was carried out in water from the Sahelian wells having different pH (W1: 4.9 and W2: 6.3) and a natural iron content of 0.07mg/L. We evaluate the efficiency of the disinfection on different systems containing both or only one Fenton reagent (H2O2/Fe2+): (i) H2O2/Fe2+/hv, (ii) Fe2+/hv, (iii) H2O2/hv, and (iv) only light irradiation (hv) at lab and field scale. Generally, 0.6mg/L of Fe2+ and/or 8.5mg/L of H2O2 were used in the Fenton reagent. The systems H2O2/Fe2+/hv and H2O2/hv led to total inactivation of Salmonella and E. coli. The natural iron content (0.07mg/L) was enough to drive an efficient photo-Fenton process leading to total bacterial inactivation. Our results show that: (i) the iron salt present in Sahelian water is enough to perform a photo-Fenton disinfection of drinking water when adding H2O2, (ii) addition of external iron salts at near neutral pH has no additional effect on the bacterial photo-Fenton inactivation process. After one week of storage, no enteric bacteria re-growth was observed in treated waters. Mechanistic suggestions are presented to explain the observed results.

Inactivation of Enterococcus faecalis by TiO2-mediated UV and solar irradiation in water and wastewater: culture techniques never say the whole truth

In this work, the disinfection efficiency of water and secondary treated wastewater by means of UV-A, UV-C and solar irradiation in the presence or absence of TiO(2), using a reference strain of Enterococcus faecalis as faecal indicator, was evaluated. Operating parameters such as TiO(2) loading (0-1500 mg L(-1)), initial bacterial concentration (2 × 10(2)-10(8) CFU mL(-1)) and treatment time (up to 120 min) were assessed concerning their impact on disinfection. E. faecalis inactivation was monitored by the conventional culture method and real-time PCR. Regarding photocatalytic treatment, disinfection efficiency was improved by increasing TiO(2) concentration and bacterial inactivation took place in relatively short treatment times. Comparing the three disinfection methods, it was observed that UV-C irradiation yielded a better efficiency during water treatment than UV-A and solar irradiation. Furthermore, UV-A was more efficient than solar irradiation in the presence of the same loading of TiO(2). Regarding real wastewater, it was observed that only UV-C irradiation was capable of totally inactivating E. faecalis population in a short time. Screening the results obtained from both applied techniques (culture method and real-time PCR), there was a discrepancy, regarding the recorded time periods of total bacterial inactivation. Real-time PCR data revealed that longer periods are needed for 100% bacterial reduction during the treatments tested compared to the estimated time by culture method. This is probably attributed to the phenomenon of "viable but not culturable bacteria", caused by stressed conditions induced during disinfection experiments. Taking into account the contrast of results and in order to perform a thorough evaluation of disinfection techniques, conventional culture method should be accompanied by a DNA-based method. According to our findings, real-time PCR proved to be a reliable and accurate molecular tool for the identification and quantification of bacterial indicators, like E. faecalis, in aquatic samples after disinfection treatment.

Bacterial inactivation and organic oxidation via immobilized photo-Fenton reagent on structured silica surfaces

A woven inorganic silica fabric loaded with Fe-ions (EGF-Fe) was tested under simulated solar light during hydroquinone degradation. The abatement of hydroquinone was observed to attain about ∼80% within 3h. The photo-catalyst was also tested to inactive Escherichia coli K12 at “natural” pH and in the presence of a low concentration of H2O2. Addition of H2O2 (10mgL−1) did not by itself to bacterial inactivation. Total bacterial inactivation was mediated by EGF-Fe fabrics in the presence of H2O2 (10mgL−1) under solar light irradiation. A sample containing active (culturable) bacteria (∼105CFUmL−1) decreased to values <1CFUmL−1 within 3h reaction. Fe-mediated homogeneous process decreased the bacterial CFU content by about two orders of magnitude within 4h. During the degradation of hydroquinone only a small amount of iron ions were found in solution of about 1.2mgL−1, within 90min decreasing to values ≤0.5mgL−1 after 180min. The leaching of Fe-ions did not affect the photo-catalyst performance since EGF-Fe fabric did not deactivate after five or more cycles. The Fe-ions founds in solution mineralized hydroquinone to levels below 37% of its initial content. The present study presents the first report on the beneficial role of a heterogeneous iron supported catalyst leading to efficient bacterial inactivation in aqueous solution with iron leaching <0.1mgL−1, the detection limit for Fe-analysis in solution. No bacterial re-growth was observed during a post-irradiation period up to 24h in the dark.

Inactivation of Salmonella Typhimurium and Listeria monocytogenes using high-pressure treatments: destruction or sublethal stress?

To investigate potential resuscitation of Listeria monocytogenes and Salmonella Typhimurium after high hydrostatic pressure treatments. Pressure treatments were applied at room temperature for 10 min on bacterial suspensions in buffers at pH 7 and 5.6. Total bacterial inactivation (8 log(10) CFU ml(-1) of bacterial reduction) obtained by conventional plating was achieved regarding both micro-organisms. Treatments at 400 MPa in pH 5.6 and 600 MPa in pH 7 for L. monocytogenes and at 350 MPa in pH 5.6 and 400 MPa in pH 7 for S. Typhimurium were required respectively. A 'direct viable count' method detected some viable cells in the apparently totally inactivated population. Resuscitation was observed for the two micro-organisms during storage (at 4 and 20 degrees C) after almost all treatments. In the S. Typhimurium population, 600 MPa, 10 min, was considered as the treatment achieving total destruction because no resuscitation was observed under these storage conditions. We suggest a delay before performing counts in treated samples in order to avoid the under-evaluation of surviving cells. The resuscitation of pathogen bacteria after physical treatments like high hydrostatic pressure has to be considered from the food safety point of view. Further studies should be performed in food products to study this resuscitation phenomenon.

Impact of Food Disinfection on Beneficial Biothiol Contents in Vegetables

In this work we investigated the impact of food disinfection on the beneficial biothiol contents in a suite of vegetables consumed daily, including spinach, green bean, asparagus, cucumber, and red pepper. Four disinfection technologies commonly studied and/or used in food processing and preservation, including hydrogen peroxide, free chlorine, and gaseous- and aqueous-phase ozone, were examined with common dosages and contact times. Results indicate that the common disinfection technologies may result in significant loss of beneficial biothiols in vegetables which are essentially important to human health. For example, as much as 70% of biothiols were lost when spinach was treated with hydrogen peroxide (5.0 wt %) for 30 min. Approximately 48-54% of biothiols were destroyed by free chlorine and gaseous- and aqueous-phase ozone under typical contacting conditions. In red pepper, about 60-71% of reduced glutathione was oxidized by the disinfectants. The potential decrease in biothiols during disinfection was dependent upon the biothiol type, the disinfectant, and the vegetable. The effectiveness of total bacterial inactivation by the four disinfection technologies was concurrently evaluated. Results show that free chlorine is most effective, achieving disinfection efficiencies of greater than 4 log for all study vegetables. This study may provide important information for the food industry to design optimum contacting methods for vegetables to simultaneously achieve sufficient bacterial disinfection while minimizing loss of beneficial biothiols.