Contamination of infant powdered milk in use with enterotoxigenic Staphylococcus aureus

One-hundred and ten of the 114 samples of infant milk formulae collected from nursing mothers contained viable staphylococci, with the highest mean count of 1·0 × 10² cfu g⁻¹, from samples collected the day the tin was opened. The highest mean total aerobic plate count was 2·6 × 10³ cfu g⁻¹. Titratable acidity and pH of the reconstituted milk ranged from 0·06 to 0·8 and 6·40 to 6·52 respectively. 52·0% of the 123 isolates were S. aureus, 49·6% produced β-hemolysin and 17·1% produced α-hemolysin. Enterotoxin A was produced by 6·5%, B by 1·6%, C by 2·4%, D by 1·6% and E by 0·8% of the isolates. The total staphylococcal and aerobic plate counts were not affected by either the period elapsed from the opening of tin to sampling or the brand of milk.     

Production of a red–purple food colorant from Opuntia stricta fruits by spray drying and its application in food model systems

Opuntia stricta fruit juice is a potential source of betacyanin pigments which can be used as a natural red-purple food colorant. In this work a powder food colorant was obtained by co-current spray drying of O. stricta fruit juices with a bench-scale two fluid nozzle spray dryer. Glucose syrup (DE 29) was used as drying aid. Optimum conditions for spray drying were as follow: juice content (20% v/v; 1.2 °Brix), glucose syrup content (10% w/v), liquid feed rate (0.72 l/h), spray air flow-rate (0.47 m³/h), drying air flow-rate (36 m³/h), and inlet drying air temperature 160 °C. Color was retained during the drying process (>98%) and drying yield was high (58%). The powder colorant showed high color strength (4.0), being this color strength stable when stored at room temperature for one month. This colorant was successfully applied in two food model systems: a yogurt and a soft-drink. Food presented a vivid red-purple tonality very attractive for consumers that was maintained after one month under refrigeration (4 °C) (ΔE < 5).     

Enumeration, isolation and characterization of Bacillus cereus strains from Spanish raw rice

Bacillus cereus was present in 61 samples of raw rice analysed representing unhusked, husked and commercial origins. B. cereus in husked and white rice samples did not reach 10² cfu g⁻¹, while in the unhusked rice B. cereus densities exceeded 10³ cfu g⁻¹. Processing steps such as drying, husking and polishing reduced the number of B. cereus in the final product. Eight strains with typical morphology of B. cereus on Polymyxin–Mannitol–Egg Yolk–Phenol Red Agar (PMYPA) were isolated. According to ISO confirmatory tests, the API System tests and supplementary tests of motility, oxidase activity and enterotoxin production, these isolates were characterized and identified as B. cereus. All strains were motile, oxidase-negative and produced diarrheal enterotoxin in TSB. D and z -values were used to characterize heat resistance of spores obtained from the eight strains ofB. cereus characterized. A large diversity in heat resistance was observed among the isolates. At 90°C, D -values ranged from 2·23 to 23·26 min, with five groups of D -value means significantly different at the 95% confidence level. D₉₅- and D₁₀₀ values calculated for the eight strains ranged from 0·69 to 5·17 min and from 0·43 to 1·09 min, respectively. Statistical analysis revealed that there was significant difference between the D -value means obtained for the strains at each temperature. The z -values for the eight strains of B. cereus tested in this study ranged from 7·42°C to 8·20°C with an average of 7·7°C.     

Byssochlamys nivea inactivation in pineapple juice and nectar using high pressure cycles

The aim of this work was to assess the inactivation of Byssochlamys nivea ascospores in pineapple juice and nectar by combining pressure sequences involving high pressure cycles with relatively mild thermal processing. The effect of 550 and 600 MPa sustained pressures (holding time of 15 min), combinations of sustained pressures and pressure pulses (holding time of 10 s), and pressure cycles (two, three and five cycles of 550 and 600 MPa for 7.5, 5 and 3 min, respectively), at 20, 40, 60, 70, 80 and 90 °C were compared. B. nivea ascospores were inactivated by applying sustained a pressure of 600 MPa at 90 °C for 5 min (juice) and 15 min (nectar), and three and five cycles of pressure at 600 MPa and 80 °C for nectar with holding time of 5 and 3 min, respectively, and in all pressure cycles for juice. In general, pressure cycles were more effective for inactivating B. nivea ascospores than the application of sustained high pressures.     

Comparative analysis of acid resistance in Listeria monocytogenes and Salmonella enterica strains before and after exposure to poultry decontaminants. Role of the glutamate decarboxylase (GAD) system

Data on the ability of chemical poultry decontaminants to induce an acid stress response in pathogenic bacteria are lacking. This study was undertaken in order to compare the survival rates in acid broths of Listeria monocytogenes and Salmonella enterica strains, both exposed to and not exposed to decontaminants. The contribution of the glutamate decarboxylase (GAD) acid resistance system to the survival of bacteria in acid media was also examined. Four strains (L. monocytogenes serovar 1/2, L. monocytogenes serovar 4b, S. enterica serotype Typhymurium and S. enterica serotype Enteritidis) were tested before (control) and after exposure to trisodium phosphate, acidified sodium chlorite, citric acid, chlorine dioxide and peroxyacids (strains were repeatedly passed through media containing increasing concentrations of a compound). Stationary-phase cells (10⁸ cfu/ml) were inoculated into tryptic soy broth (TSB) acidified with citric acid (pH 2.7 and 5.0) with or without glutamate (10 mM) added, and incubated at 37 °C for 15 min. Survival percentages (calculated from viable colonies) varied from 2.47 ± 0.67% to 91.93 ± 5.83%. L. monocytogenes cells previously exposed to acid decontaminants (citric acid and peroxyacids) showed, when placed in acid TSB, a higher (P < 0.05) percentage of survival (average 38.80 ± 30.52%) than control and pre-exposed to non-acidic decontaminants strains (22.82 ± 23.80%). Similar (P > 0.05) survival percentages were observed in previously exposed to different decontaminants and control Salmonella strains. The GAD acid resistance system did not apparently play any role in the survival of L. monocytogenes or S. enterica at a low pH. This study demonstrates for the first time that prior exposure to acidic poultry decontaminants increases the percentage of survival of L. monocytogenes exposed to severe acid stress. These results have important implications for the meat industry when considering which decontaminant treatment to adopt.     

α-Anomer-selective glucosylation of (+)-catechin by the crude enzyme, showing glucosyl transfer activity, of Xanthomonas campestris WU-9701

α-Anomer-selective glucosylation of (+)-catechin was carried out using the crude enzyme, showing α-glucose transferring activity, of Xanthomonas campestris WU-9701 with maltose as a glucosyl donor. When 60 mg of (+)-catechin and 50 mg of the enzyme (5.25 units as maltose hydrolysing activity) were incubated in 10 ml of 10 mM citrate-Na₂HPO₄ buffer (pH 6.5) containing 1.2 M maltose at 45°C, only one (+)-catechin glucoside was selectively obtained as a product. The (+)-catechin glucoside was identified as (+)-catechin 3′-O-α- -glucopyranoside (α-C-G) by ¹³C-NMR, ¹H-NMR and two-dimensional HMBC analysis. The reaction at 45°C for 36 h under the optimum conditions gave 12 mM α-C-G, 5.4 mg/ml in the reaction mixture, and the maximum molar conversion yield based on the amount of (+)-catechin supplied reached 57.1%. At 20°C, the solubility in pure water of α-C-G, of 450 mg/ml, was approximately 100 fold higher than that of (+)-catechin, of 4.6 mg/ml. Since α-C-G has no bitter taste and a slight sweet taste compared with (+)-catechin which has a very bitter taste, α-C-G may be a desirable additive for foods, particularly sweet foods.     

Effect of lactic acid on Listeria monocytogenes and Edwardsiella tarda attached to catfish skin

This study evaluated the use of lactic acid to decontaminate Listeria monocytogenes andEdwardsiella tarda attached to catfish skin with or without mucus. At the highest inoculum levels (10⁴–10⁵cfu skin⁻¹), lactic acid (0·5–2·0%) exposure for 10 min reduced counts of L. monocytogenes firmly attached to catfish skin by 0·9–>1·9 log₁₀cfu skin⁻¹and cells loosely attached by 2·7–>3·7 logs. Counts of E. tarda firmly attached to catfish skin were reduced by 0·9–>3·0 logs and cells loosely attached by 1·5–>3·5 logs. Overall bacterial numbers of lactic acid-treated cells that were firmly attached to skin with mucus were higher than on skin without mucus. Firmly attached L. monocytogenes was more resistant to lactic acid than was firmly attached E. tarda. Catfish skin mucus decreased the antimicrobial effect of lactic acid against attached L. monocytogenes and E. tarda.     

Microbial, physical-chemical and sensory spoilage during the refrigerated storage of cooked pork loin processed by the sous vide method

The aim was to study spoilage during the refrigerated storage of cooked pork loin processed by the sous vide method. Samples were packaged under vacuum into polyamide-polypropylene pouches, cooked at an oven temperature/time of 70 °C/12 h, chilled at 3 °C and stored at 2 °C for 0, 5 or 10 weeks. Microbial (psychrotrophs, lactic acid bacteria, Enterobacteriaceae, moulds and yeasts), physical–chemical (pH, water activity, TBARS, acidity, L^*a^*b^* colour, texture profile analysis and shear force) and sensory (appearance, odour, flavour, texture and acceptance) parameters were determined. The results showed that sensory spoilage preceded microbiological spoilage of sous vide pork loin. Counts bellow 1 log cfu/g of psychrotrophs, anaerobic psychrotrophs, Enterobacteriaceae and lactic acid bacteria were detected in any control week, while moderate counts (2–3 log cfu/g) of moulds and yeasts were found. Minor changes in water activity, lipid oxidation, CIELab colour, hardness, cohesiveness or gumminess were associated with spoilage of pork loin, only decreases of lactic acid, springiness and shear force were observed. The pork loin was unacceptable after 10 weeks. This loss of acceptance was mainly due to the deterioration of meaty flavour and odour, although the loss of appearance, juiciness and firmness also contributed. Moderate warmed-over and rancidity were detected. The sensory analysis was the most effective method for determining the shelf life of the sous vide pork-based dishes.     

Inactivation of Escherichia coli in orange juice using ozone

This research investigated the efficacy of gaseous ozone for the inactivation of Escherichia coli ATCC 25922 and NCTC 12900 strains in orange juice. Orange juice inoculated with E. coli (10⁶ CFU mL^− 1) as a challenge microorganism was treated with ozone at 75–78 µg mL^− 1 for different time periods (0–18 min). The efficacy of ozone for inactivation of both strains of E. coli was evaluated as a function of different juice types: model orange juice, fresh unfiltered juice, juice without pulp, and juice filtered through 500 µm or 1 mm sieves. Fast inactivation rates for total reduction of E. coli were achieved in model orange juice (60 s) and in juice with low pulp content (6 min). However, in unfiltered juice inactivation was achieved after 15–18 min. This indicated that juice organic matter interferes with antibacterial activity of gaseous ozone. The effect of prior acid (pH 5.0) exposure of E. coli strains on the inactivation efficacy of ozone treatment was also investigated. There was a strain effect observed, where prior acid exposure resulted in higher inactivation times in some cases by comparison with the control cells. However, the overarching influence on inactivation efficacy of ozone was related to the pulp content. Generally, the applied gaseous ozone treatment of orange juice resulted in a population reduction of 5 log cycles.

Industrial relevance

To facilitate the preservation of unstable nutrients many juice processors have investigated alternatives to thermal pasteurisation, including un-pasteurised short shelf life juices with high retail value. This trend has continued within the European Union. However within the US recent regulations by the FDA have required processors to achieve a 5-log reduction in the numbers of the most resistant pathogens in their finished products. Pathogenic E. coli may survive in acid environments such as fruit juices for long periods. This study demonstrates that the use of ozone as a non-thermal technology is effective for inactivation of E. coli and acid exposed E. coli in orange juice. Information on the design of the ozone treatment for inactivation of E. coli which results into safe juice products is also among the main outputs of this work. Ozone auto-decomposition makes this technology safe for fruit juice processing.     

Evaluation of the effect of defrosting practices of ground beef on the heat tolerance of Listeria monocytogenes and Salmonella Enteritidis

The effect of common defrosting practices of ground beef, including (i) defrosting in the refrigerator (5 °C for 15 h), (ii) defrosting at room temperature (25 °C for 12 h) and (iii) defrosting in the microwave, on the heat tolerance of artificially inoculated Listeria monocytogenes and Salmonella Enteritidis, was studied. The thermal inactivation of S. Enteritidis was not, overall, affected by defrosting practices. In contrast, defrosting at room temperature resulted, overall, in an increased heat tolerance of L. monocytogenes compared to the rest tested defrosting practices. Inactivation kinetics of the two pathogens for the different defrosting practices were determined by fitting the data to the Weibull model. The δ parameter of the Weibull model (heat challenge time (min) required for the first 1-log reduction) for S. Enteritidis and for defrosting at 25 °C, microwave defrosting, defrosting at 5 °C and for the control (fresh ground beef inoculated with the pathogens just before the heat challenge trials) was 1.13, 1.62, 1.60 and 0.96, respectively, while the corresponding values for L. monocytogenes were 20.13, 10.82, 9.95 and 9.47, respectively. The findings of this study should be useful in risk assessments and in developing food handling guidelines for the consumers.     

Inhibition of Clostridium sporogenes growth in mascarpone cheese by co-inoculation with Streptococcus thermophilus under conditions of temperature abuse

The ability of a biological control system to inhibit the outgrowth of Clostridium sporogenes spores during storage of mascarpone cheese under temperature-abuse conditions was investigated. Challenge studies were carried out on mascarpone cheese artificially contaminated with spores of C. sporogenes (10 cfu g⁻¹), and with or without the coinoculum of a Streptococcus thermophilus strain (10⁵cfu g⁻¹). During storage at 4, 12, and 25°C, the outgrowth of clostridia spores, the growth of S. thermophilus, and the pH changes were evaluated at 10, 20, 30, and 40 days. In mascarpone cheese stored at 4° and 12°C, S. thermophilus and C. sporogenes did not show any growth. The initial pH (6·14) of the product also remained unchanged. During storage at 25°C S. thermophilus grew up to about 10⁷cfu g⁻¹after 10 days, resulting in a pH decrease of mascarpone cheese to values close to 4·5. The cell number decreased progressively during storage reaching values near to 10¹cfu g⁻¹after 40 days, whereas product acidity remained constant. C. sporogenes, when inoculated alone, also grew at 25°C. The cell number increased to levels of about 10⁷cfu g⁻¹after 20–40 days of storage according to the different mascarpone cheese lots used. No growth was found when C. sporogenes was co-inoculated in mascarpone cheese with S. thermophilus and stored at 25°C. The study on the behaviour of C. sporogenes, known as a non-toxigenic variant of Clostridium botulinum, allowed us to obtain useful information for setting up an effective biological control system to inhibit growth of the toxigenic species as well. The use of an additional barrier, besides refrigerated storage, may help to maintain the safety of mascarpone cheese in the event it was exposed to elevated temperatures.     

Purification, characterization, and gene cloning of sphingomyelinase C from Streptomyces griseocarneus NBRC13471

Sphingomyelinase C (SMC) was purified to homogeneity from the culture supernatant of Streptomyces griseocarneus NBRC13471. The purified enzyme appeared as a single band of 38 kDa by using an electropherogram trace. The molecular mass of the enzyme as determined by MALDI-TOF MS was 32,102 Da, indicating that SMC is monomeric in nature. Under experimental conditions, the highest enzyme activity was found at pH 9.0 and 50–55 °C, and the enzyme was stable from pH 5 to 10 and up to 37 °C. The SMC activity requires Mg²⁺ or Mn²⁺ and the order of potency to enhance the activity was Zn²⁺ ≥ Mn²⁺ > Cu²⁺ ≥ Fe²⁺. Phenylmethylsulfonyl fluoride and EDTA inhibited the enzyme activity, showing that SMC belongs to a group of metalloenzymes and a class of serine hydrolases. The enzyme activity was inhibited by DTT, but not by mercaptoethanol and iodoacetamide. SDS inhibited the enzyme activity; by contrast, Triton X-100 stimulated the activity. The N-terminal and internal amino-acid sequences were determined as H₂N-APAAATPSLK, AREIAAAGFFQGND, and NTVVQETSAP. The gene encoding SMC consisted of 1020 bp encoding a signal peptide of 42 amino acids and a mature protein of 297 amino acids with a calculated molecular mass of 32,125 Da. The conserved region of DNase I-like family enzymes and the amino acid residues that are highly conserved in the active center of other bacterial SMCs were also found in the deduced amino acid sequence of S. griseocarneus SMC.     

Effect of Zataria multiflora Boiss. essential oil and nisin on Salmonella typhimurium and Staphylococcus aureus in a food model system and on the bacterial cell membranes

The effects of different concentrations of Zataria multiflora Boiss. essential oil (EO: 0, 5, 15 and 30 μl 100 ml⁻¹) and nisin (N: 0, 0.25 and 0.5 μg ml⁻¹), temperatures (T: 25 and 8 °C), and storage times (up to 21 days) on growth of Salmonella typhimurium and Staphylococcus aureus in a commercial barley soup were evaluated in a factorial design study. The growth of S. typhimurium was significantly (P < 0.05) decreased by EO concentrations and their combinations with N concentrations at 8 °C. For S. aureus, the viable count was significantly (P < 0.05) inhibited by EO and N concentrations and their combinations, incubated at both storage temperatures. The mechanism of the antimicrobial action of EO, N, and their combinations against cell membranes of the tested organisms were also studied by measurement of the release of cell constituents and by the electronic microscopy observations of the cells. The significant increase of the cell constituents’ release of both organisms was observed as a result of treatments with EO and EO in combination with N. Electronic microscopy observations revealed that the cell membranes of S. typhimurium treated by EO and EO in combination with N were significantly damaged, while cells treated with only N looked similar to untreated cells. The electron micrographs of treated cells of S. aureus with EO, N, and their combination also showed important morphological damages and disrupted membranes.     

Effect of X-ray treatments on inoculated Escherichia coli O157: H7, Salmonella enterica, Shigella flexneri and Vibrio parahaemolyticus in ready-to-eat shrimp

This study was conducted to evaluate the inactivation effect of X-ray treatments on Escherichia coli O157: H7, Salmonella enteric (S. enterica), Shigella flexneri (S. flexneri) and Vibrio parahaemolyticus (V. parahaemolyticus) artificially inoculated in ready-to-eat (RTE) shrimp. A mixed culture of three strains of each tested pathogen was used to inoculate RTE shrimp. The shrimp samples were inoculated individually with selected pathogenic bacteria then aseptically placed in sterile plastic cups and air-dried at 22 °C for 30 min (to allow bacterial attachment) in the biosafety cabinet prior to X-ray treatments. The inoculated shrimp samples were then placed in sterilized bags and treated with 0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 2.0, 3.0 and 4.0 kGy X-ray at ambient temperature (22 °C and 60% relative humidity). Surviving bacterial populations were evaluated using a non-selective medium (TSA) with the appropriate selective medium overlay for each bacterium; CT-SMAC agar for E. coli O157: H7, XLD for S. enterica and S. flexneri and TCBS for V. parahaemolyticus. More than a 6 log CFU reduction of E. coli O157: H7, S. enterica, S. flexneri and V. parahaemolyticus was achieved with 2.0, 4.0, 3.0 and 3.0 kGy X-ray, respectively. Furthermore, treatment with 0.75 kGy X-ray significantly reduced the initial microflora on RTE shrimp samples from 3.8 ± 0.2 log CFU g⁻¹ to less than detectable limit (<1.0 log CFU g⁻¹).     

Influence of organic acid concentration on survival of Listeria monocytogenes and Escherichia coli 0157:H7 in beef carcass wash water and on model equipment surfaces

Acid-adapted cultures of Escherichia coli O157:H7 and Listeria monocytogenes were inoculated in meat decontamination spray-washing runoff fluids in order to evaluate their survival and potential to form biofilms on stainless steel coupons. The cultures (10⁷ cfu ml⁻¹) and stainless steel coupons were exposed to mixtures of water and organic acid washings (composites of each of 2% acetic acid or lactic acid washings with water washings from meat decontamination in proportions of 1/9, 1/49, 1/99 [vol/vol]) or to water washings for up to 14 days at 15°C. E. coli O157:H7 formed biofilms and remained detectable (1.3 log cfu cm⁻²) on stainless steel for up to 4 d in the 1/9 dilution (pH 3.17–3.77) of the organic acid washings, and persisted throughout storage (14 d) in the 1/49 (pH 3.96–4.33) and 1/99 (pH 4.34–6.86) dilution of the organic acid washings. L. monocytogenes populations were unable to form detectable (<1.3 log cfu cm⁻²) biofilms in the 1/9 and 1/49 dilutions of both organic acid washings for up to 14 d; however, by day-14 in the 1/99 dilution of the washings, the pathogen was able to attach at detectable levels (2.7 to 3.4 logs). The pH effects of lower concentrations (1/49 or 1/99) of acidic washings decreased over time due to the formation of amine compounds produced by the natural meat flora, allowing resuscitation of the acid-stressed pathogen survivors. The resuscitation of acid-stressed pathogens may potentially enhance their survival and prevalence in biofilms and thus more attention should be focused on avoiding or minimizing the collection of decontamination runoff fluids on food contact equipment surfaces.     

Influence of freeze-drying conditions on survival of Oenococcus oeni for malolactic fermentation

Malolactic fermentation (MLF) is an important process in wine production. Oenococcus oeni is most often responsible for MLF. Starter culture technology, involving the inoculation of O. oeni into wines, has been developed for inducing MLF. In this study, the effects of cell washing, pH of suspension medium, preincubation in sodium glutamate, initial cell concentration and freezing temperature on viability of freeze-dried O. oeni H-2 were investigated. The cell viability of samples without washing with potassium phosphate buffer was significantly lower than those samples undergone washing. When pH of suspension medium was 7.0 the cell survival was the highest. The cell viability was enhanced when the cells were preincubated at 25 °C before freezing. When 2.5% sodium glutamate was used as protective agent in suspension medium, the optimal initial cell concentration was 10⁹ CFU/ml. The cell viability increased by 21.6% as freezing temperature decreased from − 20 °C to − 65 °C. However, when the cells were frozen in liquid nitrogen (− 196 °C), the cell survival significantly decreased.     

Nisin treatments to control Listeria monocytogenes post-processing contamination on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages

Post-processing contamination and growth of Listeria monocytogenes in whey cheeses stored under refrigeration is an important safety concern. This study evaluated commercially available nisin (Nisaplin^®) as a biopreservative to control L. monocytogenes introduced post-processing on Anthotyros, a traditional Greek whey cheese, stored at 4°C in vacuum packages for up to 45 days. The whey used (pH 6.5–6.7) was from Feta cheese manufacture, and it was subjected either to natural acidification (pH 5.3, readjusted to 6.2 with 10% NaOH) prior to heating, or to direct acidification (pH 6.0–6.2) at 80°C with 10% citric acid. Nisin was added either to the whey (100 or 500 IU g⁻¹) prior to heating, or to the cheese (500 IU g⁻¹) prior to packaging, also inoculated with ca. 10⁴ cfu g⁻¹ of L. monocytogenes strain Scott A. In cheese samples without nisin, L. monocytogenes (PALCAM agar) exceeded 7 log cfu g⁻¹ after the first 10 days of storage, irrespective of the whey acidification method. All nisin treatments had an immediate lethal effect (0.7–2.2 log reduction) on L. monocytogenes populations at inoculation (day 0), which was more pronounced with 500 IU g⁻¹ added to the whey. This treatment also suppressed L. monocytogenes growth below the inoculation level for 30 and 45 days in naturally and directly acidified samples, respectively. All other treatments had weak antilisterial effects. Nisin reversed the natural spoilage flora of Anthotyros cheese from Gram-positive to Gram-negative, and this ecological alteration was far more pronounced in the most effective antilisterial treatments.     

Repair and growth of heat- and freeze-injured Escherichia coli O157:H7 in selective enrichment broths

Two Escherichia coli O157:H7 strains, ATCC 35150 and 43894, were heat injured in a beef infusion at 53°C for 40 and 50 min, respectively (1· 5–2·0 log₁₀cfu ml⁻¹of injury) and freeze injured at −25°C for 30 days (1 log₁₀cfu ml⁻¹of injury) as determined by plating on MacConkey agar with 0·60% bile salts #3 (Mac-BS) as the selective medium and on Brain Heart Infusion agar (BHIA) as the non-selective medium. Repair of injury was measured in five selective enrichment broths [buffered peptone water supplemented with vancomycin, cefsulodin, and cefixime (BPW-VCC), modified EC broth with novobiocin (mEC+n), enterohaemorrhagic E. coli enrichment broth (EEB), double modified TSB (dmTSB), and BCM^®E. coli enrichment broth (BCM^®-EB)] versus TSB as the non-selective control broth over 3 h incubation at 37°C and 42°C. Repair was measured as the increase in cfu ml⁻¹enumerated on Mac-BS with time vs the total cfu ml⁻¹(injured and uninjured cells) enumerated on BHIA. In mEC+n, EEB, and dmTSB some death of both heat- and freeze-injured cells occurred immediately during the 3 h incubation (decrease on BHIA plates), and there was either minimal or no repair of the injured cells at both temperatures. Efficient repair of heat injury was obtained with both BPW-VCC and BCM^®-EB, but the latter produced a growth rate and final cell concentration closer to TSB. In freeze-injury repair however, BPW-VCC gave poor results while repair in BCM^®-EB was equal to TSB. Both BCM^®-EB and BPW-VCC inhibited the growth of all Gram-positive and a select number of Gram-negative bacteria tested. The ability of the selective enrichment broth BCM^®-EB to resuscitate heat- and freeze-injured E. coli O157:H7 efficiently within 3 h, warrants further testing with other types of stress in both artificially and naturally contaminated foods.     

Changes in microorganism, enzyme, aroma of hami melon (Cucumis melo L.) juice treated with dense phase carbon dioxide and stored at 4 °C

The effects of dense phase carbon dioxide (DP-CO₂) treatment of 8, 15, 22, 30 and 35 MPa for 5 min, 15 min, 30 min, 45 min, 60 min at 35 °C, 45 °C, 55 °C, 65 °C on microorganism, enzyme, and aroma compounds in hami melon juice during storage at 4 °C for 4-weeks were investigated. Meanwhile, the color, browning degree, and Vitamin C were also studied. The DP-CO₂ treatment had significant effects on inactivation of microorganism and enzyme. It was indicated that higher pressure caused more inactivation of microbial total count and enzyme activity. When it reached 35 Mpa, 55 °C, 60 min, the microorganism was totally inactivated. The least residual activity of polyphenol oxidase (PPO), peroxidase (POD), and lipoxygenase (LOX) was 25.26%, 38.46 and 0.02% at 35MP, respectively. The restoration of PPO, POD and LOX residual activity after DP-CO₂ treatment was also observed, which was dependent on the pressure level. The aroma compounds were less affected after being treated with DP-CO₂, and the flavor of the melon juice was close to the fresh juice after storage at 4 °C for 4 weeks and did not produced cook off-odor. The changes of lightness L and browning degree A during storage were well fitted to a first-order kinetic model. The Vitamin C concentration decreased by DP-CO₂ processing, but this loss was lower than of the untreated sample.

Industrial relevance

Hami melon is highly appreciated for its nutritional quality and special flavor. The flesh of melon is heat sensitive, the sensitive nutrients, color and aromatic profile will be spoiled greatly or off-odour when it was produced with high temperature treatment. Dense phase carbon dioxide processing (DP-CO₂) is important to find an innovative food process to inactivate the enzyme and microorganism and protect the nutrient and unique flavor. In this study, the data proved that DP-CO₂ processing is a promising non-thermal alternative pasteurization to preserved fresh-squeezed melon juice.     

The fate of Clostridium perfringens spores exposed to ozone and/or mild heat pretreatment on beef surfaces followed by modified atmosphere packaging

Clostridium perfringens is a natural contaminant of raw beef products that can proliferate to dangerous cell levels under conditions of temperature abuse. Spores of the bacterium were inoculated onto irradiated London broil beef at levels of 3 log₁₀ spores/g beef. Samples of beef (7.5×10.0×1.0 cm) were treated with aqueous ozone (5 ppm O₃ for 5 min), or heat (60°C for 30 min), or both and then vacuum-packaged to 2 kPa for up to 10 d storage at 37°C, 25°C, or 4°C. Storage at 37°C resulted in increases in viable counts after 1 d to over 7 log₁₀ cfu/g beef, whereas storage at 4°C prevented spore germination and growth for all treatments. At 25°C, heat-treated beef samples reached 6 log₁₀ cfu/g viable counts in 2 d and spores/vegetative cells on control or ozone-treated samples did not germinate or grow through the first day of vacuum-packaged storage. Modified atmospheres with increasing CO₂ concentration were also compared with regard to bacterial survival during beef storage at 25°C. C. perfringens spores remained dormant in control and ozone-treated beef during a 10-d storage at 25°C. Pretreatment with heat increased germination and outgrowth during storage of beef, whereas ozone treatment and no treatment controls were effective in inhibiting spore germination and outgrowth in combination with increasing CO₂ concentrations above 30% or refrigeration. These data support the avoidance of heat in the pretreatment of raw beef.