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.     

Efficacy of pulsed light for shelf-life extension and inactivation of Listeria monocytogenes on ready-to-eat cooked meat products

Pulsed light (PL) was tested for its utility to improve the microbial quality and safety of ready-to-eat cooked meat products. Vacuum-packaged ham and bologna slices were superficially inoculated with Listeria monocytogenes and treated with 0.7, 2.1, 4.2 and 8.4 J/cm². PL treatment at 8.4 J/cm² reduced L. monocytogenes by 1.78 cfu/cm² in cooked ham and by 1.11 cfu/cm² in bologna. The effect of PL on lipid oxidation and sensory properties was also investigated. The 2-thiobarbituric acid values were very low and chromaticity parameters were within the normal values reported for cooked meat products. PL at 8.4 J/cm² did not affect the sensory quality of cooked ham, while treatments above 2.1 J/cm² negatively influenced the sensory properties of bologna. The combination of PL and vacuum packaging provided ham with an additional shelf-life extension of 30 days compared with only vacuum packaging. The shelf-life of bologna was not extended by PL.

Industrial relevance

The efficacy of pulsed light for the decontamination of surfaces offers excellent possibilities to ensure food safety and to extend shelf-life of ready-to-eat (RTE) products. The results of this study indicate that Listeria monocytogenes can be reduced by approximately 2 log cfu/cm² in RTE cooked ham and 1 log cfu/cm² in bologna using a fluence of 8.4 J/cm². This dose does not affect the sensory properties of ham and triples its shelf-life when compared with conventional RTE products. On the contrary, fluences above 2.1 J/cm² are not suitable for the treatment of bologna since sensory quality is modified.     

Analytical data for Acacia senegal var. senegal gum samples collected between 1993 and 1995 from Sudan

Over 1500 authentic and commercial A. senegal var. senegal gum samples were analysed to evaluate existing quality control parameters and to assess the potential of new parameters such as pH, viscosity, viscosity average molecular weight (M_V) equivalent weight and total uronic acid content as additional qualifying indices. The data obtained indicate the following mean values: moisture (10.75%), ash (3.77%), nitrogen (0.328%), specific rotation (−31.3°), pH (4.66), equivalent weight (1436) and total uronic acid content (13.71%). The results also indicate wide variations in intrinsic viscosity and viscosity average molecular weight (mean values 16.4 ml g⁻¹ and 9.0×10⁵, respectively. Accordingly these parameters, therefore, cannot be recommended as qualifying indices.     

Effect of aeration and dilution rate on nisin Z production during continuous fermentation with free and immobilized Lactococcus lactis UL719 in supplemented whey permeate

The influence of dilution rate (D) and aeration on soluble and cell-bound nisin Z production was investigated during continuous free (FC) and immobilized cell (IC) cultures with Lactococcus lactis subsp. lactis biovar diacetylactis UL719 in supplemented whey permeate. Maximum total bacteriocin titres during non-aerated continuous FC and IC cultures were obtained for low D, with 1490 and 1090 IU mL⁻¹ for 0.15 h⁻¹ or 0.25 and 0.5 h⁻¹, respectively. For both systems, aeration increased nisin total production with maximum titres of 2560 and 2430 IU mL⁻¹ for low D, respectively, as well as specific production. Volumetric productivity was the highest for an intermediate D of 0.4 h⁻¹ during FC cultures (460 IU mL⁻¹ h⁻¹ for both aerated and non-aerated cultures), while it increased continuously with D during IC cultures, reaching high values of 1090 and 1760 IU mL⁻¹ h⁻¹ at 2.0 h⁻¹ without and with aeration, respectively. In comparison with previous data for FC batch cultures, data from this study may indicate that during continuous fermentations at steady state, some steps in nisin biosynthesis are limiting. In these conditions, nisin production by immobilized cells is reduced.     

Identification and characterization of Dekkera bruxellensis, Candida pararugosa, and Pichia guilliermondii isolated from commercial red wines

Yeast isolates from commercial red wines were characterized with regards to tolerances to molecular SO₂, ethanol, and temperature as well as synthesis of 4-ethyl-phenol/4-ethyl-guaiacol in grape juice or wine. Based on rDNA sequencing, nine of the 11 isolates belonged to Dekkera bruxellensis (B1a, B1b, B2a, E1, F1a, F3, I1a, N2, and P2) while the other two were Candida pararugosa (Q2) and Pichia guilliermondii (Q3). Strains B1b, Q2, and Q3 were much more resistant to molecular SO₂ in comparison to the other strains of Dekkera. These strains were inoculated (10³–10⁴ cfu/ml) along with lower populations of Saccharomyces (<500 cfu/ml) into red grape juice and red wine incubated at two temperatures, 15 °C and 21 °C. Although Saccharomyces quickly dominated fermentations in grape juice, B1b and Q2 grew and eventually reached populations >10⁵ cfu/ml. In wine, Q3 never entered logarithmic growth and quickly died in contrast to Q2 which survived >40 days after inoculation. B1b grew well in wine incubated at 21 °C while slower growth was observed at 15 °C. Neither Q2 nor Q3 produced 4-ethyl-phenol or 4-ethyl-guaiacol, unlike B1b. However, lower concentrations of volatile phenols were present in wine incubated at 15 °C compared to 21 °C.     

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.     

Inhibitory effect of clove oil on Listeria monocytogenes in meat and cheese

The antilisteric activity of clove oil was examined in meat and cheese at both 30°C and 7°C. At concentrations of 0·5% and 1% clove oil restricted the growth of Listeria monocytogenes in the food items at both temperatures. The inhibitory activity of clove oil was more pronounced at a concentration of 1%.Listeria counts in treated samples were 1–3 log₁₀cfu g⁻¹less compared to controls at different intervals during storage. The results revealed the potential of clove oil as a natural preservative in meat and cheese.     

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.     

High nisin-Z production during repeated-cycle batch cultures in supplemented whey permeate using immobilized Lactococcus lactis UL719

Nisin-Z production was studied during repeated-cycle pH-controlled batch (RCB) cultures using Lactococcus lactis subsp. lactis biovar. diacetylactis UL719 immobilized in κ-carrageenan/locust bean gum gel beads in supplemented whey permeate. After an initial colonization of gel beads during the first two cycles, nisin-Z production in bulk medium and gel beads was very similar for 1-h and 2-h cycle RCB cultures. A very high nisin-Z production (8200 IU mL⁻¹) was measured in the broth after the 1-h cycles, with a corresponding volumetric productivity of 5730 IU mL⁻¹ h⁻¹. This productivity is much higher than maximum nisin productivities reported in literature or maximum productivities obtained previously for free-cell batch cultures (850 IU mL⁻¹ h⁻¹), and free-cell (460 IU mL⁻¹ h⁻¹) or immobilized-cell (1760 IU mL⁻¹ h⁻¹) continuous cultures, using the same strain and fermentation conditions. The stability of RCB cultures was demonstrated for 24 and 36 1-h cycles carried out over 3 and 6-day periods, respectively. Changing environmental conditions during batch cultures resulted high nisin production.     

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).     

Uneven application can influence the efficacy of s-methoprene against Rhyzopertha dominica (F.) in wheat

The Juvenile Hormone analogue s-methoprene is used to protect stored grain from pests such as the lesser grain borer, Rhyzopertha dominica (F.). The possibility that uneven application influences s-methoprene efficacy against this species was investigated in the laboratory. Adults of methoprene-susceptible strains were exposed for 14 days to wheat treated at doses of up to 0.6 mg kg⁻¹, or to mixtures of treated and untreated wheat giving equivalent average doses. Adult mortality after exposure to treated wheat was negligible in all cases (≤3.3%) and there was no significant effect of either average dose or evenness of application. In contrast, the number of adult progeny depended on both the average dose and evenness of application. Average doses of 0.3 and 0.6 mg kg⁻¹ reduced the number of live F₁ adults by 99–100% relative to the untreated wheat and no effect of evenness of application was detected. At lower doses, however, efficacy tended to decrease with increasing unevenness of application. When adults from the parental generation were transferred to untreated wheat for another 14 days neither the average dose nor evenness of application in the wheat from which they came had any significant effect on reproduction of these adults. This study demonstrates that uneven application can reduce the efficacy of s-methoprene against R. dominica, but that this is unlikely to influence the performance of s-methoprene against susceptible populations at target doses likely to be used in practice (e.g. 0.6 mg kg⁻¹ in Australia). However, the possibility that uneven application leads to underdosing and selects for resistance should be investigated.     

Combination of warm water and hydrogen peroxide to reduce the numbers of Salmonella Typhimurium and Listeria innocua on field salad (Valerianella locusta)

The optimal conditions of decontamination of field salad by treatment with warm water containing hydrogen peroxide were determined with the aid of a central composite design. It was the aim to improve the hygienic status while keeping the sensory quality at the highest level possible. Two series of experiments were performed to study the effects of the following parameters: temperature (45–50 °C), H₂O₂ (1–4%) and treatment time (60–210 s). In series 1 the effect on sensory properties and reduction of total aerobic counts was investigated. To determine sensory quality, treated samples stored at 4 °C for 7 days were subjected to evaluation by 15 panelists. Overall visual quality (OVQ), color, texture and odor correlated well with product acceptability (R²=0.934, 0.946, 0.945 and 0.956, respectively), and OVQ was the most sensitive criterion. An acceptability of 75% in OVQ was chosen as the limit and an acceptance area was defined to deduce permitted conditions for decontamination. Within that area a reduction of total counts by 2.64–3.22 and 1.9–3.48 log₁₀ cfu/g was achieved after treatment and storage at 4 °C for 7 days, respectively. In series 2 field salad was inoculated with Listeria innocua and Salmonella Typhimurium and it was observed that within the area of acceptability a reduction of the numbers of the challenge organisms by 1.27–3.26 and 0.79–3.05 log₁₀ cfu/g, respectively, was achieved immediately after treatment, and by 1.64–3.09 (L. innocua) and 1.1–2.63 log₁₀ cfu/g (S. Typhimurium) after storage. The initial decontaminating effect on L. innocua and S. Typhimurium was sustained or even increased after storage, when the salad was treated below 48.9 °C for more than 68 s. Three independent experiments confirmed the reliability of the models of the count reduction of L. innocua and S. Typhimurium.     

Improving the enrichment procedure for Enterobacteriaceae detection

The current ISO standard method for detection of Enterobacteriaceae (21528-1:2004) includes enrichment in EE broth which has been shown to be inhibitory to some members of this family, notably Cronobacter spp. A shortened procedure omitting the EE broth has been proposed, however competition from Gram-positive flora may be detrimental to the effective recovery of low levels of target organisms in some sample matrices. In this study we investigated novel cost effective modifications, designed to improve ISO 21528-1:2004 for the detection of Enterobacteriaceae. Initial experiments used a worse-case scenario involving stressed Enterobacteriaceae strains known to grow poorly in laboratory media as well as representative background competitors from powdered milk. The interaction between the Enterobacteriaceae and their competitors was characterised and additives to enhance the growth of target strains over non-target strains were investigated.Supplementation of BPW with 40 μM 8-hydroxyquinoline, 0.5 g L⁻¹ ammonium iron(III) citrate, 0.1 g L⁻¹ sodium deoxycholate and 0.1 g L⁻¹ sodium pyruvate (BPW-S) improved the recovery of Enterobacteriaceae from artificially and naturally contaminated samples. This improvement of the pre-enrichment broth may also be of interest for methods designed to detect specific foodborne pathogens belonging to the Enterobacteriaceae (e.g. Salmonella spp., Cronobacter spp.) that require a pre-enrichment step in BPW.     

Evaluation of buoyant density centrifugation as a sample preparation method for NASBA-ELGA detection ofCampylobacter jejuni in foods

Buoyant densities of four Campylobacter jejuni strains were in the range of 1·084–1·087 g ml⁻¹. Milk (3% fat) and chicken skin homogenates had buoyant densities beneath 1·033 g ml⁻¹. Discontinuous buoyant density centrifugation (BDC) in 40% Standard Isotonic BactXTractor medium respectively succeeded in recovering C. jejuni (5×10³–5×10⁴cfu ml⁻¹) from spiked milk (3% fat) and chicken skin. NASBA–ELGA detection of C. jejuni (5×10²–5×10³cfu ml⁻¹) in 12 different food samples using four different sample preparation methods was performed: RNA extraction by heating (filter and non-filter stomacher bag), RNA extraction by BDC (non-filter stomacher bag), RNA extraction by GuSCN–Triton-X-100 lysis and silica-purification (non-filter stomacher bag). BDC succeeded in overcoming inhibition of the amplification reaction except for one of the soft cheese samples. It was noticed that for chicken skin, chicken meat, pork, chicken sausage, turkey meat, milk (3% fat) and skimmed milk a simple heat treatment at 96°C without any additional precautions to prevent inhibition accomplished NASBA–ELGA detection of the pathogen. The use of a filter stomacher bag improved the quality of the NASBA–ELGA detection signal for beef but did not prevent inhibition of the amplification reaction in the cases of ground beef, prepared minced meat, soft cheese and hard cheese. The silica purification of RNA (which was used as a control treatment) accomplished NASBA–ELGA detection of C. jejuni for all of the latter food homogenates.     

Biochemical and thermal characterization of crude exo-polygalacturonase produced by Aspergillus sojae

Crude exo-polygalacturonase enzyme (produced by Aspergillus sojae), significant for industrial processes, was characterized with respect to its biochemical and thermal properties. The optimum pH and temperature for maximum crude exo-polygalacturonase activity were pH 5 and 55 °C, respectively. It retained 60–70% of its activity over a broad pH range and 80% of its initial activity at 65 °C for 1 h. The thermal stability study indicated an inactivation energy of E_d = 152 kJ mol⁻¹. The half lives at 75 and 85 °C were estimated as 3.6 and 1.02 h, respectively. Thermodynamic parameters, ΔH^*, ΔS^* and ΔG^*, were determined as a function of temperature. The kinetic constants K_m and V_max, using polygalacturonic acid as substrate, were determined as 0.424 g l⁻¹ and 80 μmol min⁻¹, respectively. SDS-PAGE profiling revealed three major bands with molecular weights of 36, 53 and 68 kDa. This enzyme can be considered as a potential candidate in various applications of waste treatment, in food, paper and textile industries.     

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.     

Nutrient composition and technological quality of meat from alpacas reared in Peru

The aim of this study was to increase the knowledge on alpaca meat quality characteristics. Twenty Huacaya breed alpacas, reared under a traditional unspecialized production system at the Andean region of Peru, were slaughtered at ages between 18 and 24 months. Analyses were carried out on Longissimus thoracis and lumborum muscle (LTLM), unless otherwise specified. These included composition parameters: moisture, fat, protein, ash, minerals, amino acids, fatty acid profile (of both LTLM and perirenal fat), retinol and tocopherol concentrations and myoglobin and collagen contents. Other meat quality parameters were evaluated: pH, colour, water holding capacity and Warner–Bratzler shear-force. Alpaca LTLM was characterized by a low intramuscular fat content and mineral and amino acid compositions, polyunsaturated to saturated fatty acids ratio and conjugated linoleic acid content comparable to those found for beef and sheep meat. However, specifically, alpaca meat showed a relatively high n−6 to n−3 (3.7) ratio and low vitamin E concentration. Values of alpaca meat technological quality parameters were in the ranges reported for more conventional red meats, the exception being a lower b^* value.     

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.     

Inactivation of Listeria monocytogenes during drying and storage of peach slices treated with acidic or sodium metabisulfite solutions

This study evaluated whether treating inoculated peach slices with metabisulfite or acidic solutions enhanced inactivation of Listeria monocytogenes during dehydration and storage. Inoculated (five strain mixture of L. monocytogenes, 7.9 log cfu/g) peach slices were treated, dried for 6 h at 60°C and stored aerobically at 25°C for 14 d. Predrying treatments of inoculated peach slices included: (1) no treatment (control); or 10 min immersion in: (2) sterile water, (3) 4.18% sodium metabisulfite, (4) 3.40% ascorbic acid, or (5) 0.21% citric acid solutions. Samples were plated on tryptic soy agar with 0.1% pyruvate (TSAP) and PALCAM agar for enumeration of surviving bacteria. Immersion in sterile water reduced bacterial populations on peach slices by 0.7 log cfu/g (TSAP and PALCAM). Immersion in the sodium metabisulfite solution reduced populations by 1.5–2.0 log cfu/g, while acidic pretreatments reduced populations by 0.5–0.8 log cfu/g. After 6 h of dehydration, populations on control or water immersed slices were reduced by 3.2–3.4 log cfu/g, whereas populations on slices treated with sodium metabisulfite or acidic solutions were reduced by 4.3–5.1 log cfu/g (TSAP) and 5.3–6.2 log cfu/g (PALCAM), respectively. Bacteria were detectable by direct plating at 14 d of storage, except on acid treated slices. Immersion in acidic or metabisulfite solutions, before dehydration, should enhance inactivation of L. monocytogenes contamination on peach slices during dehydration and storage.