Biohydrogen production from cattle wastewater by enriched anaerobic mixed consortia: influence of fermentation temperature and pH

Experiments were conducted to select a natural mixed microflora seed source and investigate the effect of temperature and pH on fermentative hydrogen (H2) production from cattle wastewater by sewage sludge. Sewage sludge was shown to have higher cumulative H2 production than other inoculum collected from cow dung compost, chicken manure compost, and river sludge. Experimental results show that H2 production from cattle wastewater was significantly affected by both pH and temperature of the culture. The maximum H2 yield was obtained at pH 5.5. H2 yield and the ratio of butyrate/acetate (Bu/Ac) followed a similar production trend, suggesting that butyrate formation might favor H2 production. The optimal temperature for H2 production from cattle wastewater was 45 degrees C with peak values of H2 production (368 ml), hydrogen yield of 319 ml H2/g chemical oxygen demand (COD) consumed, and butyrate/acetate ratio of 1.43. Presence of ethanol and propionic acid indicated decreased hydrogen production; their concentrations were also affected by pH and temperature. A modified Gompertz model adequately described H2 production and bacterial growth.     

Influence of nitrogen and carbon sources on the production of ochratoxin A by ochratoxigenic strains of Aspergillus spp. isolated from grapes

This work studies the influence of nitrogen and carbon source on ochratoxin A production by three Aspergillus isolates A. ochraceus (Aso2), A. carbonarius (Ac25) and A. tubingensis (Bo66), all isolated from grapes. A basal medium (0.01 g/l FeSO4.7H2O, 0.5 g/l MgSO4.7H2O, 0.5 g/l Na2HPO4.2H2O, 1.0 g/l KCl) was prepared. This medium was supplemented with different nitrogen sources, both inorganic [(NH4)3PO(4), 0.3 g/l plus NH4NO3, 0.2 g/l] and organic (histidine, proline, arginine, phenylalanine, tryptophan or tyrosine) at two concentrations (0.05 g/l or 0.3 g/l), and different carbon sources (sucrose, glucose, maltose, arabinose or fructose) at three concentrations (10 g/l, 50 g/l or 150 g/l). A medium with sucrose (18 g/l) and glucose (1 g/l) was also tested. After a 10-day incubation period at 25 degrees C the highest levels of OTA (44.0 ng/ml, 13.5 ng/ml and 0.49 ng/ml for A. ochraceus, A. carbonarius and A. tubingensis, respectively) were obtained in the cultures containing 150 g/l of arabinose and 0.05 g/l of phenylalanine. Analysis of variance of the data showed that there were significant differences (p-value 0.05) among the OTA levels in the cultures with regard to carbon source and isolate. No significant differences were detected in OTA production regarding nitrogen source, although 0.05 g/l of phenylalanine generally favoured OTA production in the cultures of the three isolates. The dynamics of toxin production in the cultures of each isolate using the optimized basal medium supplemented with 0.05 g/l of phenylalanine and 150 g/l of arabinose for a period of 42 days at 25 degrees C was also studied. The maximum level of OTA was detected on the 3rd day of incubation in A. tubingensis cultures and on the 35th and 43(rd) days of incubation in A. ochraceus and A. carbonarius, respectively. This is the first study in which defined media have been used to assess the influence of carbon and nitrogen sources on OTA production by isolates of OTA-producing species isolated from grapes and to analyse the dynamics of toxin production in these species in a defined culture medium. This optimized medium for OTA production is being used in current studies aimed at elucidating its biosynthetic pathway.     

Properties of Rhodotorula gracilis D-amino acid oxidase immobilized on magnetic beads through his-tag

D-amino acid oxidase catalyzes one of the key steps in the production of semisynthetic cephalosporins. We expressed and purified recombinant Rhodotorula gracilis D-amino acid oxidase with C-terminal his-tags. This engineered enzyme was immobilized onto Ni(2+)-chelated nitrilotriacetic acid magnetic beads through the interaction between his-tag and Ni(2+). The kinetic constants, storage properties, and the reusability of the immobilized d-amino acid oxidase were determined. The effects of temperature, pH, and hydrogen peroxide on the activity of immobilized d-amino acid oxidase were also studied. The highest activity recovery was 75%. Thermal stability was improved after immobilization; the relative activity of the immobilized enzyme was 56% whereas the free enzyme was completely inactivated after incubation at 50 degrees C for 1 h. In the presence of 10 mM hydrogen peroxide, the immobilized enzyme did not show a rapid loss of activity during the first 2 h of incubation, which was observed in the case of the free enzyme; the residual activity of the immobilized enzyme after 9 h was 72% compared with 22% of the free form. The long-term storage stability was improved; the residual activity of the immobilized enzyme was 74% compared with 20% of the free enzyme when stored at room temperature for 10 d. The immobilized form retained 37% of its initial activity after 20 consecutive reaction cycles.     

Coculture-inducible bacteriocin activity of Lactobacillus plantarum strain J23 isolated from grape must

Detection and characterization of bacteriocin production by Lactobacillus plantarum strain J23, recovered from a grape must sample in Spain, have been carried out. Bacteriocin activity was degraded by proteolytic enzymes (trypsin, alfa-chymotrypsin, papaine, protease, proteinase K and acid proteases), and it was stable at high temperatures (121 degrees C, 20min), in a wide range of pH (1-12), and after treatment with organic solvents. L. plantarum J23 showed antimicrobial activity against Oenococcus oeni, and a range of Lactobacillus and Pediococcus species. Bacteriocin production was detected in liquid media only when J23 was cocultivated with some inducing bacteria, and induction took place when intact cells or 55 degrees C heated cells of the inducer were cocultivated with J23, but not with their autoclaved cells. Bacteriocin activity of J23 was not induced by high initial J23 inocula, and it was detected in cocultures during the exponential phase. The presence of ethanol or acidic pH in the media reduced bacteriocin production in the cocultures of J23 with the inducing bacteria. The presence of plantaricin-related plnEF and plnJ genes was detected by PCR and sequencing. Nevertheless, negative results were obtained for plnA, plnK, plNC8, plS and plW genes.     

Influence of agitation,inoculum density,pH, and strain on the growth parameters of Escherichia coli O157:H7--relevance to risk assessment

Foods may differ in at least two key variables from broth culture systems typically used to measure growth kinetics of enteropathogens: initial population density of the pathogen and agitation of the culture. The present study used nine Escherichia coli O157:H7 strains isolated from beef and associated with human illness. Initial kinetic experiments with one E. coli O157:H7 strain in brain-heart infusion (BHI) broth at pH 5.5 were performed in a 2 x 2 x 3 factorial design, testing the effects of a low (ca. 1-10 colony-forming units [CFU]/ml) or high (ca. 1000 CFU/ml) initial population density, culture agitation or no culture agitation, and incubation temperatures of 10, 19, and 37 degrees C. Kinetic data were modeled using simple linear regression and the Baranyi model. Both model forms provided good statistical fit to the data (adjusted r(2)>0.95). Significant effects of agitation and initial population density were identified at 10 degrees C but not at 19 or 37 degrees C. Similar growth patterns were observed for two additional strains tested under the same experimental design. The lag, slope, and maximum population density (MPD) parameters were significantly different by treatment. Further tests were conducted in a 96-well microtiter plate system to determine the effect of initial population density and low pH (4.6-5.5) on the growth of E. coli O157:H7 strains in BHI at 10, 19, and 37 degrees C. Strain variability was more apparent at the boundary conditions of growth of low pH and low temperature. This study demonstrates the need for growth models that are specific to food products and environments for plausible extrapolation to risk assessment models.     

Effect of water activity on growth and enterotoxin D production of Staphylococcus aureus

Four enterotoxin D-producing strains of Staphylococcus aureus, were cultured at 37 degrees C in brain heart infusion broth adjusted to various water activity (aw) levels by means of NaCl. For high cell inocula growth of all strains and enterotoxin D production were observed within 6 days at aw levels down to 0.86.     

The effect of commercial production and product formulation stresses on the heat resistance of Escherichia coli O157:H7 (NCTC 12900) in beef burgers

The effects of commercial beef burger production and product formulation on the heat resistance of Escherichia coli O157:H7 (NCTC 12900) in beef burgers were investigated. Fresh beef trimmings were inoculated with E. coli O157:H7 to approximately log10 7.0 cfu g(-1) and subjected to standard beef burger production processes, including freezing, frozen storage and tempering. The tempered trimmings were processed in line with commercial practice to produce burgers of two formulations, a 'Quality' burger containing 100% beef and an 'Economy' burger containing 70% beef and 30% other ingredients (salt, seasoning, soya, onion and water). The burgers were then frozen and stored. Control 'unprocessed' burgers were produced to each of the above formulations using fresh beef trimmings. All burger types were heat-treated at 55, 60 or 65 degrees C. Samples were examined by plating on Tryptone Soya Agar (TSA), incubated at 37 degrees C for 2 h, before overlaying with SMAC (TSA/SMAC) and incubation at 37 degrees C. The resultant counts were used to derive D-values for E. coli O1 57:H7. At each treatment temperature, the D-values from each burger formulation using frozen tempered trimmings were significantly lower (P < 0.001) than the D-values from that formulation using fresh trimmings. At each treatment temperature, the D-values from Economy burgers using processed trimmings were significantly higher (P < 0.001) than the D-values from Quality burgers using processed trimmings. A similar trend of significantly higher (P<0.001) D-values for Economy burgers was observed using fresh trimmings. This study found that commercial processing and product formulation have profound effects on the heat resistance of E. coli O157:H7 in beef burgers.     

Production of hydrogen sulfide from tetrathionate by the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1

When incubated under anaerobic conditions, five strains of Thiobacillus ferrooxidans tested produced hydrogen sulfide (H2S) from elemental sulfur at pH 1.5. However, among the strains, T. ferrooxidans NASF-1 and AP19-3 were able to use both elemental sulfur and tetrathionate as electron acceptors for H2S production at pH 1.5. The mechanism of H2S production from tetrathionate was studied with intact cells of strain NASF-1. Strain NASF-1 was unable to use dithionate, trithionate, or pentathionate as an electron acceptor. After 12 h of incubation under anaerobic conditions at 30 degrees C, 1.3 micromol of tetrathionate in the reaction mixture was decomposed, and 0.78 micromol of H2S and 0.6 micromol of trithionate were produced. Thiosulfate and sulfite were not detected in the reaction mixture. From these results, we propose that H2S is produced at pH 1.5 from tetrathionate by T. ferrooxidans NASF-1, via the following two-step reaction, in which AH2 represents an unknown electron donor in NASF-1 cells. Namely, tetrathionate is decomposed by tetrathionate-decomposing enzyme to give trithionate and elemental sulfur (S4O6(2-)-->S3O6(2-) + S(o), Eq. 1), and the elemental sulfur thus produced is reduced by sulfur reductase using electrons from AH2 to give H2S (S(o) + AH2-->H2S + A, Eq. 2). The optimum pH and temperature for H2S production from tetrathionate under argon gas were 1.5 and 30 degrees C, respectively. Under argon gas, the H2S production from tetrathionate stopped after 1 d of incubation, producing a total of 2.5 micromol of H2S/5 mg protein. In contrast, under H2 conditions, H2S production continued for 6 d, producing a total of 10.0 micromol of H2S/5 mg protein. These results suggest that electrons from H2 were used to reduce elemental sulfur produced as an intermediate to give H2S. Potassium cyanide at 0.5 mM slightly inhibited H2S production from tetrathionate, but increased that from elemental sulfur 3-fold. 2,4-Dinitrophenol at 0.05 mM, carbonylcyanide-m-chlorophenyl- hydrazone at 0.01 mM, mercury chloride at 0.05 mM, and sodium selenate at 1.0 mM almost completely inhibited H2S production from tetrathionate, but not from elemental sulfur.     

Evaluation of increased incubation temperature and cefixime-tellurite treatment for the isolation of Escherichia coli O157:H7 from minced beef

To improve enrichment and isolation of Escherichia coli O157:H7, this study evaluated increased incubation temperature and cefixime-tellurite (CT) on five strains of each of the following bacteria, E. coli, Hafnia alvei, Enterobacter spp., Citrobacter freundii and E. coli O157:H7, and two strains of E. coli O157:nH7. These were grown in pure culture in LST broth with varying cefixime-tellurite concentrations. A range of incubation temperatures from 37 to 46 degrees C was investigated for the inhibition of cohabitant microorganisms. Minced beef, spiked with E. coli O157:H7 and cohabitant microorganisms was investigated. Increased incubation temperature (42 degrees C) and treatment with half of the prescribed amount of cefixime-tellurite by BAM for SMAC agar in enrichment step were the most effective in selectively growing E. coli O157:H7. The results show that E. coli O157:H7 is more resistant to these two conditions than the other cohabitant bacteria.     

Fumonisin production on irradiated corn kernels: effect of inoculum size.

Production of fumonisins B1, B2, and B3 by Fusarium moniliforme was evaluated on irradiated corn kernels inoculated with different spore concentrations (10, 10(2), 10(3), 10(5), and 10(6)), a water activity of 0.97, and a temperature of 25 degrees C. There was a direct relationship between the level of toxin produced and inoculum size. The highest levels of total fumonisin produced after 35 days of incubation were 5,028 and 9,063 ng/g at 10(5) and 10(6) spores per ml, respectively. The pattern of fumonisin production (FB1 > FB2 > FB3) in cultures growing from different inocula was not affected during the 35 days of incubation. The ratio between FB2 and FB1 varied from 0.15 to 0.42, whereas the ratio between FB3 and FB1 varied from 0.34 to 0.87.     

Inactivation of Escherichia coli O157: H7 and Listeria monocytogenes by PR-26, a synthetic antibacterial peptide.

This study reports the antibacterial effect of PR-26, a synthetic peptide derived from the first 26 amino acid sequence of PR-39, an antimicrobial peptide isolated from porcine neutrophils. A three-strain mixture of Escherichia coli O157:H7 or Listeria monocytogenes of approximately 10(8) CFU was inoculated to a final concentration of 10(7) CFU/ml in 1% peptone water (pH 7.0), containing 50 or 75 microg/ml of PR-26, and incubated at 37 degrees C for 0, 6, 12, and 24 h; at 24 degrees C for 0, 12, 24, and 36 h; or at 10 or 4 degrees C for 0, 24, 72, and 120 h. Control samples included 1% peptone water inoculated with each pathogen mixture but containing no PR-26. The surviving population of each pathogen at each sampling time was determined by plating on tryptic soy agar with incubation at 37 degrees C for 24 h. At 37 degrees C, PR-26 decreased E. coli O157:H7 and L. monocytogenes populations by >5.0 log CFU/ml at 12 h, with complete inactivation at 24 h. At 24 degrees C, PR-26 reduced E. coli O157:H7 and L. monocytogenes by approximately 3.5, 4.0, and 4.5 log CFU/ml at the end of 12-, 24-, and 36-h incubations, respectively. At 4 and 10 degrees C, the inhibitory effect of PR-26 on E. coli O157:H7 and L. monocytogenes was significantly lower (P < 0.05) than that at 37 and 24 degrees C: a 2- to 3-log CFU/ml reduction was observed at 120-h incubation. Results indicate that PR-26 could potentially be used as an antimicrobial agent, but applications in appropriate foods need to be validated.     

Recovery of Escherichia coli ATCC 25922 in phosphate buffered saline after treatment with high hydrostatic pressure

Escherichia coli ATCC 25922 in phosphate buffered saline solution (PBS, pH 7.1, 10(8) CFU/ml) was inactivated by high hydrostatic pressure (HHP, 400 to 600 MPa) treatment at 25 degrees C for 10 min. Colonies of E. coli were not detected on non-selective plate count agar immediately after a HHP-treatment of at least 550 MPa. E. coli subjected to at least 500 MPa in PBS were incubated at 4, 25, and 37 degrees C for 120 h. No colonies were detected on plate count agar throughout the 120-h incubation period at 4 or 37 degrees C. In contrast, the number of E. coli during storage at 25 degrees C increased from an undetectable level (< 1 CFU/ml) to the level of initial cell counts regardless of the treatment pressure level. The recovery in PBS required a maximum time of 48 h, while the period during which cell numbers remained at an undetectable level increased from 24 to 72 h as the treatment pressure increased. E. coli treated at 550 and 600 MPa in PBS were inoculated into trypticase soy broth (TSB) and stored at 4, 25, and 37 degrees C for 120 h. No recovery was recorded in TSB during the 120-h storage at 37 degrees C. In contrast, the number of E.coli during storage at 25 degrees C in TSB increased beyond the level of initial cell counts regardless of the treatment pressure level. The recovery of cell numbers observed in TSB was faster than that in PBS samples, as bacterial growth in TSB assisted faster recovery. When the incubation temperature in PBS was shifted to 25 degrees C after 120-h at 4 or 37 degrees C, recovery of E. coli was observed in samples shifted from 4 to 25 degrees C regardless of the treatment pressure. However, the time during which cell numbers remained at an undetectable level was extended by increasing the level of treatment pressure, and recovery required a maximum time of 48 h. On the other hand, no recovery was observed with HHP-treated E. coli subjected to an incubation temperature shift from 37 to 25 degrees C. This study indicates that an appropriate incubation temperature after HHP-treatment is needed to optimize the recovery of HHP-injured bacteria and thus prevent overestimation of the lethal effect of HHP-treatment.     

Zerovalent irons: styles of corrosion and inorganic control on hydrogen pressure buildup

Apparent corrosion rates have been measured for several commercially available zerovalent irons by monitoring hydrogen evolution in closed cells. Sievert-type rate constants (ks) were determined to account for hydrogen entering the iron lattice. Thus corrected corrosion rates (Rcorr) are provided for all irons tested in this study. Because the rate of hydrogen entering the iron lattice increases with PH2(1/2), and the rate of hydrogen production from corrosion, far from equilibrium conditions, is independent of PH2, at some time under closed system conditions the two rates become equal and a steady-state PH2 is attained. A relation describing this condition has been derived: PH2SS = [Rcorr/ ks]2 For the granular irons considered in this study, PH2SS values vary from less than one to eight bars, in contrast to the calculated thermodynamic equilibrium PH2 values for anaerobic corrosion, which range from 138 to 631 bar depending on the assumed product of corrosion. Because groundwater flow at an iron permeable reactive barrier removes hydrogen gas in the dissolved state, PH2SS values will be less than calculated using the relation above. A method is presented to calculate PH2 values along the flow direction in a PRB, and thus the maximum PH2 value that can possibly develop, assuming no bacterial utilization of the produced hydrogen.     

Elimination by ozone of Shigella sonnei in shredded lettuce and water

Several outbreaks of shigellosis have been attributed to the consumption of contaminated fresh-cut vegetables. The minimal processing of these products make it difficult to ensure that fresh produce is safe for consumer. Chlorine-based agents have been often used to sanitize produce and reduce microbial populations in water applied during processing operations. However, the limited efficacy of chlorine-based agents and the production of chlorinated organic compounds with potential carcinogenic action have created the need to investigate the effectiveness of new decontamination techniques. In this study, the ability of ozone to inactivate S. sonnei inoculated on shredded lettuce and in water was evaluated. Furthermore, several disinfection kinetic models were considered to predict S. sonnei inactivation with ozone. Treatments with ozone (1.6 and 2.2 ppm) for 1 min decreased S. sonnei population in water by 3.7 and 5.6 log cfu mL(-1), respectively. Additionally, it was found that S. sonnei growth in nutrient broth was affected by ozone treatments. After 5.4 ppm ozone dose, lag-phases were longer for injured cells recovered at 10 degrees C than 37 degrees C. Furthermore, treated cells recovered in nutrient broth at 10 degrees C were unable to grow after 16.5 ppm ozone dose. Finally, after 5 min, S. sonnei counts were reduced by 0.9 and 1.4 log units in those shredded lettuce samples washed with 2 ppm of ozonated water with or without UV-C activation, respectively. In addition, S. sonnei counts were reduced by 1.8 log units in lettuce treated with 5 ppm for 5 min. Therefore, ozone can be an alternative treatment to chlorine for disinfection of wash water and for reduction of microbial population on fresh produce due to it decomposes to nontoxic products.     

Nano-TiO2 enhanced photofermentative hydrogen produced from the dark fermentation liquid of waste activated sludge

After anaerobic dark fermentation of waste activated sludge (WAS) for hydrogen production, there are a large number of organic compounds including protein, polysaccharide, and volatile fatty acids left in the dark fermentation liquid, which can be further bioconverted to hydrogen by photofermentation techniquea. In this study, the enhancement of photofermentative hydrogen produced from WAS dark fermentation liquid by using nano-TiO2 is reported. First, high concentration of NH(4)(+)-N in the dark fermentation liquid was observed to inhibit the photofermentative hydrogen production, and its removal was essential. Then the effect of nano-TiO2 on photofermentative hydrogen generation was investigated, and the addition of 100 mg/L nano-TiO2 increased hydrogen by 46.1%. Finally, the mechanisms for nano-TiO2 improving hydrogen production were investigated. It was found that nano-TiO2 improved the decomposition of protein and polysaccharide to small-molecule organic compounds and promoted the growth of photosynthetic bacteria and the activity of nitrogenase but decreased the H2-uptake hydrogenase activity.     

Carbon sequestration in soil by in situ catalyzed photo-oxidative polymerization of soil organic matter

Here we describe an innovative mechanism for carbon sequestration in soil by in situ photopolymerization of soil organic matter under biomimetic catalysis. Three different Mediterranean soils were added with a synthetic water-soluble iron-porphyrin, irradiated by solar light, and subjected first to 5 days incubation and, then, 15, and 30 wetting and drying (w/d) cycles. The in situ catalyst-assisted photopolymerization of soil organic carbon (SOC) increased water stability of soil aggregates both after 5 days incubation and 15 w/d cycles, but not after 30 w/d cycles. Particle-size distribution of all treated soils confirmed the induced soil physical improvement, by showing a concomitant lower yield of the clay-sized fraction and larger yields of either coarse sand- or fine sand-size fractions, depending on soil texture, though only after 5 days incubation. The gain in soil physical quality was reflected by the shift of OC content from small to large soil aggregates, thereby suggesting that photopolymerization stabilized OC by both chemical and physical processes. A further evidence of the carbon sequestration capacity of the photocatalytic treatment was provided by the significant reduction of CO(2) respired by all soils after both incubation and w/d cycles. Our findings suggest that "green" catalytic technologies may potentially be the bases for future practices to increase soil carbon stabilization and mitigate CO(2) emissions from arable soils.     

Viability of Clostridium perfringens,Escherichia coli,and Listeria monocytogenes surviving mild heat or aqueous ozone treatment on beef followed by heat,alkali, or salt stress

The threat of pathogen survival following ozone treatment of meat necessitates careful evaluation of the microorganisms surviving under such circumstances. The objective of this study was to determine whether sublethal aqueous ozone treatment (3 ppm of O3 for 5 min) of microorganisms on beef surfaces would result in increased or decreased survival with respect to subsequent heat, alkali, or NaCl stress. A mild heat treatment (55 degrees C for 30 min) was used for comparison. Reductions in three-strain cocktails of Clostridium perfringens, Escherichia coli O157:H7, and Listeria monocytogenes on beef following the heat treatment were 0.14, 0.77, and 1.47 log10 CFU/g, respectively, whereas reductions following ozone treatment were 1.28, 0.85, and 1.09 log10 CFU/g, respectively. C. perfringens cells exhibited elevated heat resistance at 60 degrees C (D60 [time at 60 degrees C required to reduce the viable cell population by 1 log10 units or 90%] = 17.76 min) following heat treatment of beef (55 degrees C for 30 min) but exhibited reduced viability at 60 degrees C following ozone treatment (D60 = 7.64 min) compared with the viability of untreated control cells (D60 = 13.84 min). The D60-values for L. monocytogenes and E. coli O157:H7 following heat and ozone exposures were not significantly different (P > 0.05). C. perfringens cells that survived ozone treatment did not exhibit increased resistance to pH (pH 6 to 12) relative to non-ozone-treated cells when grown at 37 degrees C for 24 h. The heat treatment also resulted in decreased numbers of surviving cells above and below neutral pH values for both E. coli O157:H7 and L. monocytogenes relative to those of non-heat-treated cells grown at 37 degrees C for 24 h. There were significant differences (P < 0.05) in C. perfringens reductions with increasing NaCl concentrations. The effects of NaCl were less apparent for E. coli and L. monocytogenes survivors. It is concluded that pathogens surviving ozone treatment of beef are less likely to endanger food safety than are those surviving sublethal heat treatments.     

Mechanisms of hydrogen peroxide decomposition in soils

The rates and mechanisms of hydrogen peroxide (H2O2) decomposition were examined in a series of soil suspensions at H2O2 concentrations comparable to those found in rainwaters. The formation of hydroxyl radical (OH) as a possible decomposition intermediate was investigated using a new, highly sensitive method. In surface soils with higher organic matter or manganese content, H2O2 usually decayed rapidly, with disproportionation to water and dioxygen dominating the decomposition, whereas the formation of the hydroxyl radical (OH) represented <10% of the total H2O2 decomposed. In contrast, for soils with lower organic matter content, H2O2 usually decayed much more slowly, but OH was a major product of the H2O2 decomposed. The decomposition was principally associated with soil particles, not the soil supernatant. Different sterilization techniques indicated that decomposition of H2O2 was at least partly due to biological activity. Because the loss of H2O2 can largely be accommodated by the production of O2 and OH within these soils, our results suggest that disproportionation through a catalase-type mechanism and the production of OH through a Haber-Weiss mechanism represent the principal routes through which H2O2 is lost.     

Effect of pH on survival, thermotolerance, and verotoxin production of Escherichia coli O157:H7 during simulated fermentation and storage

Heat treatment is increasingly being introduced to fermented meat processing, since the acid tolerance properties of Escherichia coli O157:H7 can permit this organism to survive traditional processing procedures. This study investigated the effect of growth pH and fermentation on the thermotolerance at 55 degrees C of E. coli O157:H7 in a model fermented meat system. E. coli O157:H7 (strain 380-94) was grown at pH 5.6 or 7.4 (18 h at 37 degrees C), fermented to pH 4.8 or 4.4 in brain heart infusion broth, and stored for 96 h. Cells grown at pH 5.6 had higher D values at 55 degrees C (D55 degrees C) than cells grown at pH 7.4 (P < 0.001). Cells fermented to pH 4.8 had higher D55 degrees C than those fermented to pH 4.4 (P < 0.001). Cells fermented to pH 4.8 demonstrated an increase in D55 degrees C during storage (P < 0.001), whereas cells fermented to pH 4.4 showed a decrease in D55 degrees C during the same period (P < 0.001). The effect of growth pH on verotoxin production by E. coli O157:H7 was assessed using the verotoxin assay. Cells grown at pH 5.6 had lower verotoxin production then cells grown at pH 7.4. This effect was not sustained over storage. These results indicate that a lower growth pH can confer cross-protection against heat. This has implications for the production of acidic foods, such as fermented meat, during which a heating step may be used to improve product safety.     

Menadione-catalyzed luminol chemiluminescence assay for the rapid detection of viable bacteria in foods under aerobic conditions

A menadione-catalyzed luminol chemiluminescence assay was developed for the rapid detection and estimation of viable bacteria in foods. The principle of this assay is based on the extracellular menadione-catalyzed active oxygen spieces (O2- and H2O2) generated by the activity of NAD(P)H:menadione oxidoreductase in viable cells. This luminol chemiluminescence assay requires 10 min for the incubation of cells with menadione and then 2 s for the measurement of chemiluminescence intensity after an injection of luminol solution without the treatment of cell lysis. This method was evaluated using liquid food samples of milk, vegetable juice, green tea, and coffee spiked with Escherichia coli ATCC 25922. The study result revealed that E. coli contamination at 1 to 10 CFU/ml in these foods could be detected after incubation at 37 degrees C for 7 h in an enrichment medium; however, the green tea and coffee samples requires filtration. This method could be a useful tool for the rapid evaluation of microbial food contamination.