Inactivation of Escherichia coli by ozone treatment of apple juice at different pH levels

This research investigated the efficacy of gaseous ozone on the inactivation of Escherichia coli ATCC 25922 and NCTC 12900 strains in apple juice of a range of pH levels, using an ozone bubble column. The pH levels investigated were 3.0, 3.5, 4.0, 4.5 and 5.0. Apple juice inoculated with E. coli strains (10⁶ CFU/mL) was treated with ozone gas at a flow rate of 0.12 L/min and ozone concentration of 0.048 mg/min/mL for up to 18 min. Results show that inactivation kinetics of E. coli by ozone were affected by pH of the juice. The ozone treatment duration required for achieving a 5-log reduction was faster (4 min) at the lowest pH than at the highest pH (18 min) studied. The relationship between time required to achieve 5 log reduction (t_5d) and pH for both strains was described mathematically by two exponential equations. Ozone treatment appears to be an effective process for reducing bacteria in apple juice and the required applied treatment for producing a safe apple juice is dependant on its acidity level.     

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.     

Safety and quality assessment during the ozonation of cloudy apple juice

Traditionally, ozone processing within the food industry has focused on solid foods by either gaseous treatment or washing with ozonized water. However, with the FDA's approval of ozone as a direct additive to food, the potential for liquid applications has emerged. This study investigates the effect of ozone processing on microbial inactivation (E. coli ATCC 25922 and NCTC 12900) and quality parameters (color, phenolic content) of cloudy apple juice. Apple juice samples were ozonated at room temperature (20 ± 1.5 °C) with a generated ozone concentration of 0.048 mg O(3) at a constant flow rate of 0.12 L/min and treatment time of 0 to 10 min. E. coli inactivation kinetics in apple juice were described quantitatively by using the Shoulder log-linear and the Weibull model. Ozone treatment of E. coli in apple juice demonstrate that a desired 5 log reduction can be achieved within 5 min. Apple juice color (L*, a*, and b*) and total phenols were significantly affected by ozone concentration and treatment time.     

Factors affecting growth of foodborne pathogens on minimally processed apples

Escherichia coli O157:H7, Salmonella and Listeria innocua increased by more than 2 log₁₀ units over a 24 h period on fresh-cut ‘Golden Delicious’ apple plugs stored at 25 and 20 °C. L. innocua reached the same final population level at 10 °C meanwhile E. coli and Salmonella only increased 1.3 log₁₀ units after 6 days. Only L. innocua was able to grow at 5 °C. No significant differences were observed between the growth of foodborne pathogens on fresh-cut ‘Golden Delicious’, ‘Granny Smith’ and ‘Shampion’ apples stored at 25 and 5 °C. The treatment of ‘Golden Delicious’ and ‘Granny Smith’ apple plugs with the antioxidants, ascorbic acid (2%) and NatureSeal^® (6%), did not affect pathogen growth. The effect of passive modified atmosphere packaging (MAP) on the growth of E. coli, Salmonella and L. innocua on ‘Golden Delicious’ apple slices was also tested. There were no significant differences in growth of pathogens in MAP conditions compared with air packaging of ‘Golden Delicious’ apple plugs, but the growth of mesophilic and psychrotrophic microorganisms was inhibited. These results highlight the importance of avoiding contamination of fresh-cut fruit with foodborne pathogens and the maintenance of the cold chain during storage until consumption.     

Modeling the growth rate and lag time of different strains of Salmonella enterica and Listeria monocytogenes in ready-to-eat lettuce

The growth parameters (growth rate, μ and lag time, λ) of three different strains each of Salmonella enterica and Listeria monocytogenes in minimally processed lettuce (MPL) and their changes as a function of temperature were modeled. MPL were packed under modified atmosphere (5% O₂, 15% CO₂ and 80% N₂), stored at 7–30 °C and samples collected at different time intervals were enumerated for S. enterica and L. monocytogenes. Growth curves and equations describing the relationship between μ and λ as a function of temperature were constructed using the DMFit Excel add-in and through linear regression, respectively. The predicted growth parameters for the pathogens observed in this study were compared to ComBase, Pathogen modeling program (PMP) and data from the literature. High R² values (0.97 and 0.93) were observed for average growth curves of different strains of pathogens grown on MPL. Secondary models of μ and λ for both pathogens followed a linear trend with high R² values (>0.90). Root mean square error (RMSE) showed that the models obtained are accurate and suitable for modeling the growth of S. enterica and L. monocytogenes in MP lettuce. The current study provides growth models for these foodborne pathogens that can be used in microbial risk assessment.     

A numerical approach for predicting volumetric inactivation of food borne microorganisms in liquid substrates by pulsed light treatment

Pulsed light (PL) technology is able to effectively destroy a wide variety of food spoilage and pathogenic microorganisms. However, the effectiveness of PL treatment depends on direct exposure of the target microorganisms to the short, high energy pulses of light. The complex physical and chemical properties of foods affect the way light propagates through a given food substrate, and thus there is a real potential for insufficiently or non-uniformly treated products. The objective of this work was to develop a method for predicting levels and spatial distribution of microbial inactivation in PL treatment of liquid substrates, and to validate the predictions with experimental data. Three liquids with different composition and optical properties (BPB, TSB, apple juice) were inoculated with either Escherichia coli ATCC 25922 or Listeria innocua FSL C2-008 and treated with PL, in two different geometries. The Weibull model was used to describe the microbial inactivation kinetics for each organism. The kinetic equations were coupled with previously determined equations describing either the total fluence (F_total) or UV fluence (F_UV) distribution in each of the liquids, for either cylindrical or rectangular prismatic geometries. COMSOL simulation software was used to generate maps of spatial distribution of microbial inactivation and to predict the average volumetric inactivation for each substrate. The model that used F_total provided gross over-estimations for microbial inactivation, while using F_UV as the treatment dose yielded reasonably good predictions of microbial inactivation, especially for the more opaque and turbid substrates. This approach can help processors determine which substrates would be suitable for PL treatment, and to design highly effective and uniform PL treatments.     

UV-C treatment of juices to inactivate microorganisms using Dean vortex technology

A laboratory-scale UV-C treatment device based on Dean vortex technology was tested for its potential to inactivate spoilage microorganisms in cloudy fruit juices. A log 5 and log 6 reduction could be achieved by inactivating Lactobacillus plantarum BFE 5092 and Escherichia coli DH5α in naturally cloudy apple juice at 1.9 and 7.7 kJ/L, respectively. A treatment with 9.6 kJ/L led to an approximately log 4 inactivation of Saccharomyces cerevisiae DSM 70478 and Alicyclobacillus acidoterrestris DSM 2498. The effects of possible influencing parameters such as optical density, turbidity and viscosity were analyzed with regard to the efficiency of the UV-C treatment. The optical density based on dissolved compounds appeared to be the most important factor which influenced the bacterial inactivation efficiency. Cell counts of L. plantarum BFE 5092 could be reduced in quarter-strength Ringer’s solution adjusted with dye from an initial level of approximately 1 × 10⁸-1 × 10¹ cfu/mL at an optical density (254 nm) of 20 at 9.6 kJ/L. Only a log 1.5 reduction, however, could be achieved at an optical density (254 nm) of 140 using the same UV-C treatment. Furthermore, no noticeable effect on inactivation could be determined by varying the turbidity or the viscosity of the juices investigated. An increasing flow rate and the consequently higher Dean number clearly improved the efficacy of the UV-C treatment. Thus, the inactivation of L. plantarum BFE 5092 in blood orange juice could be enhanced by an approximately 2.5-log reduction by increasing the Dean number from 32 to 256 at 7.7 kJ/L. The UV-C treatment using Dean vortex technology was shown in this study to effectively inactivate microorganisms even in cloudy juices. The optical density value seemed to be the exclusive determining factor on the efficiency of the UV-C inactivation of microorganisms based on Dean vortex technology, while the effect of suspended solids was negligible as a result of the efficient mixing by Dean vortices.     

Prebiotic effect of fermented cashew apple (Anacardium occidentale L) juice

Leuconostc mesenteroides B-512F and L. mesenteroides B-742 were cultivated in clarified cashew apple juice to produce prebiotic oligosaccharides. Yeast extract (20 g/L); K₂HPO₄ (g/L) and sucrose (50 g/L) were added to the juice to promote the microbial growth and dextransucrase production. Initial pH was adjusted to 6.5 with H₃PO₄. Fermentations were carried out at 30 °C and 150 rpm for 24 h. The prebiotic effect of the fermented cashew apple juice, containing oligosaccharides, was evaluated through the Lactobacillus johnsonii B-2178 growth. L. johnsonii was incubated for 48 h using fermented cashew apple juice as substrate. Lactobacillus growth was compared to the microbial growth in non-fermented juice and in MRS broth. L. johnsonii growth in the fermented cashew apple juice was threefolds the observed growth in the non-fermented juice.     

Inactivation of different strains of Escherichia coli O157:H7 in various apple ciders treated with dimethyl dicarbonate (DMDC) and sulfur dioxide (SO2) as an alternative method

Escherichia coli has been identified as the causative agent in numerous foodborne illness outbreaks associated with the consumption of fresh apple cider. Apple cider has a pH which is normally below 4.0 and would not be considered a medium capable of supporting the growth of foodborne pathogens. The association of unpasteurized apple cider with foodborne illness due to E. coli O157:H7 has however, led to increased interest in potential alternative methods to produce pathogen free cider. Apple cider was prepared from eight different apple cultivars, inoculated with approximately 10⁶–10⁷ CFU of three strains of E. coli O157:H7 per ml (933, ATCC 43889, and ATCC 43895) and tested to determine the effectiveness of sulfur dioxide (SO₂) and dimethyl dicarbonate (DMDC). Bacterial populations for treated and untreated samples were then enumerated by using non-selective media. Eight different ciders were treated with DMDC (125 and 250 ppm) and SO₂ (25, 50, 75, 100 ppm). Greater than a 5-log reduction was achieved at room temperature with 250 ppm of DMDC and 50 ppm of SO₂ after the incubation time of 6 h and 24 h, respectively. Addition of DMDC and/or SO₂ may offer an inexpensive alternative to thermal pasteurization for the production of safe apple cider for small apple cider producers.     

Inactivation kinetics of peroxidase and polyphenol oxidase in peach juice treated with gaseous ozone

The effectiveness of gaseous ozone for inactivating peroxidase (POD) and polyphenoloxidase (PPO) in peach juice was investigated. The suitability of first‐order and Weibull models to describe inactivation kinetics was also analysed. Peach juice was exposed to ozone (0.11 and 0.20 mg O₃ min^?1 mL^?1) in a bubble column up to 12 min at 20 ± 1 °C. Enzyme activities were reduced due to treatments. The magnitude of the inactivation increased with ozone level and exposure time. Reductions in activity after 12 min of treatment ranged between 99.5% and 99.8% for POD and between 93.9% and 97.3% for PPO, depending on ozone concentration. Inactivation curves were successfully fitted with the first‐order and Weibull models; although, based on the root‐mean‐square error, the corrected Akaike and the Bayesian Schwarz criterion, the Weibull model showed stronger capability in all cases.     

Kinetic modelling of non-enzymatic browning of apple juice concentrates differing in water activity under isothermal and dynamic heating conditions

The kinetics of non-enzymatic browning in apple juice concentrates were investigated. The effect of a_w (in the range of 0.74–0.99) and/or reactant concentration on brown pigment formation was monitored under isothermal heat treatment at four temperatures (60, 70, 80 and 90 °C) in apple juice solutions having either the same or different concentrations of reactant solutes. The extent of the Maillard reaction was evaluated by spectrophotometric measurements at 420 nm (A₄₂₀). The absorbance–time curves were fitted to five different kinetic models (zero and first order, weibull, logistic and the parabolic model) and estimates of browning rate constants and other model parameters were obtained. Regression analysis revealed that the logistic model was the most appropriate for describing browning in apple juice. The initial reactant concentration, but not water activity, had a significant effect on the colour change of apple juice. The processing temperature also had a strong impact on browning kinetics. Secondary models, expressing the dependence of the best fitted primary model parameters on temperature and a_w, were further developed and validated by comparing the predicted model parameters with the values observed in independent isothermal experiments. Finally, the derived model was further evaluated against the observed browning responses of apple juice under dynamic heating conditions, underlining the applicability of the developed model as a practical prediction tool for the study of non-enzymatic browning.     

Bacterial spore inactivation at 45-65 °C using high pressure processing: Study of Alicyclobacillus acidoterrestris in orange juice

High pressure processing (HPP) is a new non-thermal technology commercially used to pasteurize fruit juices and extend shelf-life, while preserving delicate aromas/flavours and bioactive constituents. Given the spoilage incidents and economic losses due to Alicyclobacillus acidoterrestris in the fruit juice industry, the use of high pressure (200 MPa – 600 MPa) in combination with mild temperature (45 °C–65 °C) for 1–15 min, to inactivate these spores in orange juice were investigated. As expected, the higher the temperature, pressure and time, the larger was the A. acidoterrestris inactivation. The survival curves were described by the first order Bigelow model. For 200 MPa, D_45 °C = 43.9 min, D_55 °C = 28.8 min, D_65 °C = 5.0 min and z-value = 21.3 °C. At 600 MPa, D_45 °C = 12.9 min, D_55 °C = 7.0 min, D_65 °C = 3.4 min and z-value = 34.4 °C. Spores were inactivated at 45 °C and 600 MPa, and at 65 °C only 200 MPa was needed to achieve reduction in spore numbers.     

Antagonistic effect of Pseudomonas graminis CPA-7 against foodborne pathogens in fresh-cut apples under simulated commercial conditions

Recently, we reported that the application of the strain CPA-7 of Pseudomonas graminis, previously isolated from apple, could reduce the population of foodborne pathogens on minimally processed (MP) apples and peaches under laboratory conditions. Therefore, the objective of the present work was to find an antioxidant treatment and a packaging atmosphere condition to improve CPA-7 efficacy in reducing a cocktail of four Salmonella and five Listeria monocytogenes strains on MP apples under simulated commercial processing. The effect of CPA-7 application on apple quality and its survival to simulated gastric stress were also evaluated. Ascorbic acid (2%, w/v) and N-acetyl-l-cysteine (1%, w/v) as antioxidant treatments reduced Salmonella, L. monocytogenes and CPA-7 recovery, meanwhile no reduction was observed with NatureSeal^® AS1 (NS, 6%, w/v). The antagonistic strain was effective on NS-treated apple wedges stored at 10 °C with or without modified atmosphere packaging (MAP). Then, in a semi-commercial assay, efficacy of CPA-7 inoculated at 10⁵ and 10⁷ cfu mL⁻¹ against Salmonella and L. monocytogenes strains on MP apples with NS and MAP and stored at 5 and 10 °C was evaluated. Although high CPA-7 concentrations/populations avoided Salmonella growth at 10 °C and lowered L. monocytogenes population increases were observed at both temperatures, the effect was not instantaneous. No effect on apple quality was detected and CPA-7 did not survived to simulated gastric stress throughout storage. Therefore, CPA-7 could avoid pathogens growth on MP apples during storage when use as part of a hurdle technology in combination with disinfection techniques, low storage temperature and MAP.     

Modeling inactivation kinetics of food borne pathogens at a constant temperature

Four food borne pathogens (Listeria monocytogenes CA and Ohio₂, Salmonella enteritidis FDA and Salmonella typhimurium E21274, Escherichia coli O157:H7 931 and 933, Staphylococcus aureus 485 and 765) were inactivated under mild temperature (60 °C) and their survival curves determined at selected time intervals. Tailing was observed in all survival curves as a monotonic upward concavity. The resulting survival curves were either described by the Weibull or traditional first-order model and goodness of fit of these models was investigated. Regression coefficients (R²), root mean square error (RMSE) and correlation plots suggested that Weibull model produced a better fit to the data than the traditional model. Hazard plots suggested that the Weibull model was fully appropriate for the data being analysed. Although more studies should be carried out to evaluate the applicability of the nonlinear models, the present study has shown that thermal process calculations should most probably be reconsidered. This could lead to a reduction in under- and over-processing of thermally treated foods     

Bacterial inactivation in fruit juices using a continuous flow Pulsed Light (PL) system

In this work, the susceptibility to pulsed light (PL) treatments of both a Gram-positive (L. innocua 11288) and a Gram-negative (E. coli DH5-??) bacteria inoculated in apple (pH = 3.49, absorption coefficient 13.9 cm^− 1) and orange juices (pH = 3.78, absorption coefficient 52.4 cm^− 1) was investigated in a range of energy dosages from 1.8 to 5.5 J/cm². A laboratory scale continuous flow PL system was set up for the experiments, using a xenon flash-lamp emitting high intensity light in the range of 100-1100 nm. The flashes lasted 360 ??s at a constant frequency of 3 Hz.The results highlighted how the lethal effect of pulsed light depended on the energy dose supplied, the absorption properties of liquid food as well as the bacterial strain examined. The higher the quantity of the energy delivered to the juice stream, the greater the inactivation level. However, the absorbance of the inoculated juice strongly influenced the dose deliver and, therefore, the efficiency of the PL treatment. Among the bacteria tested, E. coli cells showed a greater susceptibility to the PL treatment than L. innocua cells in both apple and orange juices. Following treatment at 4 J/cm², microbial reductions in apple and orange juices were, respectively, 4.00 and 2.90 Log-cycles for E. coli and 2.98 and 0.93 Log-cycles for L. innocua.Sublethally injured cells were also detected for both bacterial strains, thus confirming that membrane damage is an important event in bacterial inactivation by PL.     

Inhibition of Listeria monocytogenes and Escherichia coli O157:H7 in liquid broth medium and during processing of fermented sausage using autochthonous starter cultures

The antimicrobial effect of two autochthonous starter cultures of Lactobacillus sakei was evaluated in vitro (in liquid broth medium) and in situ assays. The inactivation of foodborne pathogens Listeria monocytogenes (serotype 4ab No 10) and Escherichia coli O157:H7 ATCC 43888 was investigated during the production of fermented sausage according to a typical Greek recipe using L. sakei strains as starter cultures. The inactivation kinetics were modeled using GInaFiT, a freeware tool to assess microbial survival curves. By the end of the ripening period, the inhibition of L. monocytogenes was significant in treatments with L. sakei 8416 and L. sakei 4413 compared to the control treatment. A 2.2-log reduction of the population of E. coli O157:H7 resulted from the autochthonous starter culture L. sakei 4413 during sausage processing. The use of the autochthonous starter cultures constitutes an additional improvement to the microbial safety by reducing foodborne pathogens.     

Modeling the inactivation of Salmonella typhimurium by dense phase carbon dioxide in carrot juice

The inactivation of Salmonella typhimurium inoculated into acidified carrot juice subjected to dense phase carbon dioxide (DPCD) was investigated. The pressures in the study were 10, 20 and 30 MPa, the temperatures were 32, 37 and 42 °C, and the treatment time was 5–90 min. The inactivation effect of DPCD was enhanced by increasing pressure and temperature. The sigmoid inactivation curves were characterized with the lag phase, exponential inactivation phase, and resistant phase. The inactivation curves were fitted to the modified Gompertz equation and the modified Logistic equation, the modified Gompertz equation was superior since its lowest residual sum of squares (RSS) was lower although there was no significant difference of goodness-of-fit between both models as indicated by F-test. The λ (the duration of the lag phase) and t_4-D (the time necessary to achieve 4-log cycles reduction) decreased with increasing pressure or temperature. The k_dm (the maximum specific value of the inactivation rate, min⁻¹) increased with increasing temperatures, and decreased with increasing pressures. The activation energy (Ea) and the activation volume (Va) necessary for inactivating S. typhimurium by DPCD were 19.06–29.39 kJ mol⁻¹ and 18.89–58.27 cm³ mol⁻¹.     

Cloning, characterization, expression and antifungal activity of an alkaline serine protease of Aureobasidium pullulans PL5 involved in the biological control of postharvest pathogens

An alkaline protease gene was amplified from genomic DNA and cDNA of the antagonistic yeast-like fungus Aureobasidium pullulans PL5, a biocontrol agent effective against Monilinia laxa on stone fruit and Botrytis cinerea and Penicillium expansum on pome fruits. An open reading frame of 1248 bp encoding a 415-amino acid (aa) protein with a calculated molecular weight (M_r) of 42.9 kDa and an isoelectric point (pI) of 4.5 was characterized. The cDNAALP5 gene had an 18-amino acid signal peptide, one N-gylcosylation, one histidine active site, and one serine active site. The ALP5 gene with a M_r of 1351 bp contained two introns. One intron was of 54 bp, while the other was of 50 bp. Protein BLAST and phylogenetic tree analysis of the deduced amino sequences from the cDNAALP5 gene showed that the encoded protein had 100% homology to a protease enzyme (ALP2) of a sea strain of A. pullulans, suggesting that the protein ALP5 was an alkaline serine protease. Expression of ALP5 in Escherichia coli BL21 (DE3), followed by identification with Western-blotting, purification with Ni-NTA and analysis of enzymatic activity, yielded an homogeneous recombinant ALP5 which hydrolysed the substrate casein and inhibited the mycelial growth of the pathogens. At its optimal pH of 10.0 and reaction temperature of 50 °C, the recombinant protease exhibited the highest activity towards the substrate casein, though the highest stability was at lower temperatures and pH between 7.0 and 9.0. This study provided the direct evidence that extracellular proteases secreted by the antagonist A. pullulans PL5 played a role in the biocontrol activities against some postharvest pathogens of apple and peach.     

Nonthermal inactivation and sublethal injury of Lactobacillus plantarum in apple cider by a pilot plant scale continuous supercritical carbon dioxide system

The objective of this study was to evaluate the efficacy of supercritical carbon dioxide (SCCO₂) for inactivating Lactobacillus plantarum in apple cider using a continuous system with a gas-liquid metal contactor. Pasteurized apple cider without preservatives was inoculated with L. plantarum and processed using a SCCO₂ system at a CO₂ concentration range of 0-12% (g CO₂/100 g product), outlet temperatures of 34, 38, and 42 °C, a system pressure of 7.6 MPa, and a flow rate of 1 L/min. Processing with SCCO₂ significantly (P < 0.05) enhanced inactivation of L. plantarum in apple cider, resulting in a 5 log reduction with 8% CO₂ at 42 °C. The response surface model indicated that both CO₂ concentration and temperature contributed to the microbial inactivation. The extent of sublethal injury in surviving cells in processed apple cider increased as CO₂ concentration and processing temperature increased, however the percent injury dramatically decreased during SCCO₂ processing at 42 °C. Structural damage in cell membranes after SCCO₂ processing was observed by SEM. Refrigeration (4 °C) after SCCO₂ processing effectively inhibited the re-growth of surviving L. plantarum during storage for 28 days. Thus this study suggests that SCCO₂ processing is effective in eliminating L. plantarum and could be applicable for nonthermal pasteurization of apple cider.     

Pasteurisation of Apple Juice by Using Microwaves

Exposure of Escherichia coli to microwave treatments results in a reduction of the microbial population in apple juice. This research determined the effect of pasteurisation at different power levels (270-900 W) on the microbial quality of apple juice, using a home 2450 MHz microwave. Data obtained were compared with conventional pasteurisation (83 °C for 30 s). Apple juice pasteurisation at 720-900 W for 60-90 s resulted in a 2-4 logs population reduction. Using a linear model, the D -values ranged from 0.42±0.03 min at 900 W to 3.88±0.26 min at 270 W. The value forz was 652.5±2.16 W (58.5±0.4 °C). These observations indicate that inactivation of E. coli is due to heat.