Effect of high pressure homogenization on lactic acid bacteria phages and probiotic bacteria phages

To investigate the effect of high pressure homogenization on virus inactivation, phages specific for Lactobacillus delbrueckii, Lactobacillus helveticus, Streptococcus thermophilus, Lactococcus lactis, Lactobacillus paracasei and Lactobacillus plantarum were studied. The influence of pressure, number of passes, suspension medium and phage concentration were studied at 25 °C. Reductions in viability were proportional to pressure and number of passes, though the inactivation extent was phage-dependent. At 100 MPa, some bacteriophages were completely inactivated (6 log₁₀ reduction) after 3 or 5 passes, while others remained infective after 8 passes. For all phages, treatment at 60 MPa was insufficient for complete inactivation, even after 8 passes. No clear influence of suspending medium was observed. Inactivation seems to depend on phage concentration; the higher the initial load, the bigger the reduction achieved. Although these results showed that several phages studied are resistant to high-pressure homogenization, this strategy could be combined with others to control their presence in raw milk.     

Screening for and characterization of Lactococcus lactis bacteriophages with high thermal resistance

Fifty-six Lactococcus lactis phage isolates collected from different German dairies and obtained from a starter culture manufacturer were tested for their heat resistance. About 40% of these isolates resisted treatment at 80 °C for 5 min when they were heated in milk. The most resistant phage isolate, P1532, was collected from sour cream. Plaque-formation was still detectable even after heating at 97 °C for 5 min. The second heat-resistant one, P680, showed some plaque-forming ability after heating at 95 °C for 5 min. Kinetic parameters for the thermal inactivation of these two resistant phages were determined for temperatures ranging from 70 to 97 °C. The inactivation of phage P1532 in skim milk and in buffer medium were found to follow first-order kinetics and did not exhibit tailing, whereas in the inactivation curves of phage P680 tailing was observed. The D-value of P1532 at pasteurization temperature of 72 °C was calculated as 112 min.     

Modeling the inactivation kinetics of fruit bromelain in pineapple during high-pressure and thermal treatments

The stability of fruit bromelain (FBM) in pineapple pulp was studied within a high-pressure domain of 0.1–600 MPa/30–70 °C/1 s–30 min. The pulse effect was quantified as a function of pressure, temperature, pressure build-up and decompression times. A maximum of 60% reduction in FBM activity was obtained after a single pulse of 600 MPa/70 °C. Upon applying nth order model, the obtained reaction order (n) for thermal (0.1 MPa/30–70 °C) and high-pressure (100–600 MPa/30–70 °C) inactivation was 1.1 and 1.2, respectively. The inactivation rate constant (k) ranged from 1.2 to 45.0 × 10^− 3 U^n − 1 min^− 1. The activation energy was nonlinearly dependent on pressure (P); whereas, the activation volume was linearly related to temperature (T). The nonlinear dependence of k on P and T was modeled by an empirical equation. The D-values obtained from the empirical model appeared to be more realistic than those from the log-linear kinetics.Industrial relevancePineapple fruit bromelain (FBM) has numerous health benefits and therapeutic effects. It is a protease enzyme that helps in digestion. Processing of pineapple pulp needs attention towards retaining the maximum FBM activity in it. A detailed kinetic study of FBM within a broad range of pressure–temperature–time domain will help in designing a high-pressure process for the pineapple pulp with respect to its bromelain stability.     

Application of high power ultrasound in the supercritical carbon dioxide inactivation of Saccharomyces cerevisiae

The objective of the study was to analyze the influence of high power ultrasound (HPU) on the supercritical carbon dioxide (SC-CO₂) inactivation kinetics of Saccharomyces cerevisiae and to determine the effect of the temperature (31–41 °C), pressure (100–350 bar) and composition of the medium (YPD Broth, apple and orange juice) on the process of inactivation. Using a batch-mode SC-CO₂ at 350 bar and 36 °C, a reduction of 6.7 log-cycles was obtained after 140 min of treatment. However, when HPU (40 W ± 5 W and 30 kHz) was applied during the SC-CO₂ treatments, a reduction of 7 log-cycles was achieved after 2 min of treatment for all pressures and temperatures applied. The effect of increasing pressure (from 100 to 350 bar, 36 °C) or temperature (from 31 to 41 °C, 225 bar) did not significantly influence this inactivation level. The application of ultrasound leads to a vigorous agitation and cavitation which could accelerate the SC-CO₂ dissolving in the medium. This accelerates the penetration of CO₂ into cells and its inactivation mechanisms. In batch operations the application of HPU increases the speed of reaching saturation solubility of CO₂ in many liquid media and significantly reduces microbial inactivation times.     

Effect of monoglycerides on the thermal inactivation kinetics ofBacillus cereusF4165/75 spores

The combined effect of monoglycerides and heat on the inactivation of Bacillus cereusF4165/75 spores was investigated. Linear survivor curves were obtained for spores suspended in buffer with or without added monoglyceride, and were consistent with first-order inactivation kinetics. In the presence of 3.056 mmol l^–1monoglyceride, the heat resistance of spores was affected differently depending upon the monoglyceride. Monolaurin and monolinolenin significantly lowered the spores' heat resistance, whereas monomyristin and monolinolein increased it but less dramatically. TheD_91°Cvalues obtained increased in the following order: monolaurin (3.056 mmol l^–1)< monolinolenin (3.056 mmol l^–1)< phosphate buffer (pH 7)< monomyristin (3.056 mmol l^–1)< monolinolein (3.056 mmol l^–1). Monolaurin was the most effective monoglyceride, possibly because of its lipophilic nature and great emulsifying properties. Increasing its concentration from 1.058 to 5 mmol l^–1significantly enhanced the heat inactivation rate ofB. cereusF4165/75 spores. The thermodynamic data were used to speculate on the mechanism of heat inactivation of the spores. These thermodynamic values were not significantly different in the presence or absence of monolaurin. This suggested that the enhancing effect due to increasing concentrations of monolaurin on the thermal inactivation of spores is essentially identical at any sterilization temperature, although it allows the heating temperature for sterilization to be reduced.     

Coliphage as pressure surrogates for enteric viruses in foods

In this study the potential of using selected bacteriophages as pressure surrogates for hepatitis A virus (HAV) and Aichi virus (AiV) was investigated. The coliphages included, T₄, MS2, Qβ, λ imm 434, λ cI 857 and λ cI 857A. T₄ displayed similar pressure responses as HAV and was chosen for further study. The most pressure-resistant phage, MS2, was selected as a possible surrogate to estimate AiV inactivation by high pressure processing (HPP). HAV, AiV and their selected bacteriophage surrogates were treated at a range of pressures and times in three different media. All four were treated in phosphate-buffered saline (PBS), artificial seawater (ASW) or oyster slurry (OS) at 250, 400 or 500 MPa for 1, 5 or 10 min at 20 °C. While T₄ had similar pressure resistance to HAV under conditions of high (500 MPa) and lower pressure (250 MPa), inactivation trends were very different following treatment at 400 MPa and when the viruses were suspended in OS. MS2 showed similar resistance as AiV but at ambient treatment temperatures only. The highest levels of inactivation of MS2 were achieved at 60 °C and 500 MPa. AiV was eliminated at 60 °C for 5 min at ambient pressure, but > 3 log survived exposure to 60 °C at 500 MPa. This degree of protection by pressure may be important in determining the mechanisms of pressure and heat resistances in other viruses.Industrial relevanceGreater knowledge of the responses of viruses and their surrogates to high pressure will aid in the validation of new high pressure-processed food that may be at risk to contamination from HAV or other enteric viruses.     

Inactivation of spoilage yeasts in apple juice by UV–C light and in combination with mild heat

The UV–C resistance of yeasts (Saccharomyces cerevisiae, Saccharomyces bayanus, Zygosaccharomyces bailii, Dekkera anomala and Dekkera bruxellensis) commonly involved in juice spoilage was investigated. Saccharomyces spp. showed the greatest UV resistance and yeast inactivation decreased with absorptivity of the medium. To improve the UV lethal effect on yeasts in juice, UV treatment was combined with heat (UV–H) (45–60 °C). The inactivation of S. cerevisiae by UV–H treatments between 52.5 and 57.5 °C was greater than use of both technologies individually, suggesting a synergistic effect. Modeling of inactivation data found that a combination of UV–C light with mild temperatures (50–60 °C) produced a 5-log₁₀ reduction of S. cerevisiae in clarified apple juice with lower UV doses (up to a 89.3% of reduction at 57.5 °C) and treatment times (up to 63% of reduction at 52.5 °C) than those needed for UV treatments or heat alone.Industrial relevanceThis investigation demonstrated the lethal efficacy of the combination of UV–C radiation and heat to improve the inactivation of spoilage yeasts in juice. Combined UV–H treatments synergistically increased yeast inactivation, with this approach being less effective than for bacteria. In any case, UV–H at mild temperatures would be an alternative to heat pasteurization treatments, enabling the inactivation of pathogenic bacteria and reducing the concentration of spoilage microorganisms like yeasts, obtaining a safe and stable product at lower temperatures.     

High pressure carbon dioxide combined with high power ultrasound processing of dry cured ham spiked with Listeria monocytogenes

Traditionally, thermal treatments for the inactivation of Listeria monocytogenes in meat products involve undesirable changes of the product quality. In recent years, efforts have been carried out to develop alternative methods to inactivate L. monocytogenes without affecting the product quality attributes. In this context, the feasibility of combined high pressure carbon dioxide and high power ultrasound (HPCO₂ + HPU) treatment to inactivate L. monocytogenes inoculated on the surface of dry cured ham was investigated. Inactivation data were determined at 6, 8 and 12 MPa, as a function of temperature (22, 35, 45 °C) and treatment time (0.5 to 30 min), and compared to those obtained after thermal and HPCO₂ treatments.Color, pH and acidity changes of the samples after both thermal and HPCO₂ + HPU treatments were measured and compared, sensorial profile of the treated samples was evaluated by a sensory panel and shelf-life was determined by a storage study for 4 weeks at 4 °C.The results clearly revealed that HPU alone was not able to induce any microbial inactivation while HPCO₂ + HPU treatment always assured a certain level of inactivation, variable with the process temperature used: the inactivation efficiency was demonstrated higher at 35 °C rather than 22 °C and no enhancement was observed at 45 °C compared to 35 °C. Process conditions of 12 MPa, 35 °C, at 10 W for 5 min assured inactivation to undetectable level of L. monocytogenes spiked on the surface of the product with an initial concentration of about 10⁹ CFU/g. No differences were detected between acidity, pH, color and sensory attributes of the untreated and HPCO₂ + HPU treated dry cured ham surface, while slight differences were measured between the values obtained for the untreated and thermally treated samples. Additionally, the storage study demonstrated that a full microbial and quality shelf-life was assured for 4 weeks at 4 °C. The results obtained may open the doors to the application of such an innovative process at industrial level, in particular to treat ham-type or meat products.     

Resistance of Enterobacter sakazakii to pulsed electric fields

The influence of various environmental factors on Enterobacter sakazakii inactivation by pulsed electric fields was studied and the mechanisms underlying the changes in resistance were also explored. E. sakazakii PEF resistance was higher upon entering the stationary growth phase, but it did not significantly change with growth temperature. E. sakazakii cells were also more resistant to PEF in both acidified and low water activity media. Thus, for stationary-phase cells grown at 30 °C a treatment of 50 pulses at 31 kV/cm led to 5.1 log₁₀ cycles of inactivation in media of pH 7.0 (a_w > 0.99), 1.4 log₁₀ cycles in media of pH 4.0 (a_w > 0.99) and 0.3 log₁₀ cycles in media of a_w = 0.98 (pH 7.0). However, whereas the higher PEF tolerance in acid media was coincident with an increased number of cells capable of repairing their sublethally-injured cytoplasmic membranes, the higher resistance in media of lower water activity was not. To the best of our knowledge, this is the first time that sublethal injuries in outer membrane after PEF treatments have been found.Industrial relevanceThis work provides data about PEF inactivation kinetics and PEF resistance of E. sakazakii under several conditions that might be useful for designing food pasteurization processes by PEF technology. The occurrence of sublethal injuries in cytoplasmic and outer membranes under the most protective treatment conditions, gives the chance to develop combined processes that might increase the effectiveness of the PEF process.     

High-pressure calorimetric evaluation of ice crystal ratio formed by rapid depressurization during pressure-shift freezing of water and pork muscle

The amount of ice nuclei formed during the pressure release is important for the final formation and development of ice crystals in pressure shift freezing (PSF) frozen products. In this study, a high-pressure (HP) calorimeter was used to evaluate the ratio of ice crystals instantaneously formed by rapid depressurization during PSF of pure water and pork muscle tissue. Experiments were carried out initial pressure levels of 62, 115, 157 and 199 MPa, with corresponding phase change temperatures of −5, −10, −15 and −20 °C, respectively (slightly higher than phase change point of water–ice I). The ice crystal ratio was determined based on calorimetric peak measured and heat balance. The evaluated regression relationship between observed ice crystal ratio (R_ice in %) and pressure (P, MPa) was R_ice–water = 0.115P + 0.00013P² (R² = 0.96, n = 9) for pure water, and R_ice–pork = 0.080P + 0.00012P² (R² = 0.95, n = 11) for pork muscle. Compared to other methods, the calorimetric evaluation does not require any of the pressure-related properties of the test sample. HP calorimetry can thus be used to evaluate ice crystal ratio for PSF of foods even though their pressure related properties may be unknown.     

High pressure and thermal inactivation kinetics of polyphenol oxidase and peroxidase in strawberry puree

The objective of this work was to study the thermal and high pressure inactivation kinetics of polyphenol oxidase (PPO) and peroxidase (POD) in strawberry puree. PPO from two strawberry cultivars (‘Festival’ and ‘Aroma’) was found to be highly thermostable in strawberry puree with no significant inactivation even after 30 min treatment at 100 °C. In contrast, POD from the two cultivars displayed very high thermosensitivity with complete inactivation in less than 5 min at 70 °C. The thermal inactivation kinetics of strawberry POD was described by a biphasic model. The activation energies for the inactivation of the stable and the labile fractions were estimated to be 254.9 and 221.6 kJ/mol respectively. Combined high pressure–thermal treatment at pressures ranging from 100 to 690 MPa, temperatures ranging from 24 to 90 °C and treatment times between 5 and 15 min did not have significant effect on PPO while substantial inactivation of POD was observed. The inactivation kinetics of POD during combined high pressure–thermal processing was well described by first-order kinetics probably due to the inactivation of the labile fraction during the pre-heating and the compression phase.Industrial relevanceThe oxidative enzymes polyphenol oxidase and peroxidase cause the degradation of anthocyanins and other polyphenols in strawberry products, leading to discoloration and loss of antioxidant activity. In this work the thermal and high pressure inactivation of strawberry polyphenol oxidase and peroxidase was investigated so as to assess the suitability of high pressure processing as an alternative to thermal processing. Strawberry polyphenol oxidase was found to be highly resistant to both thermal and high pressure inactivation. Thus in order to maintain the quality of processed strawberry products, high pressure processing should be accompanied by additional measures such as exclusion of oxygen, refrigerated storage and the use of natural enzyme inhibitors.     

Microbial decontamination of porous model food powders by Vacuum-Steam-Vacuum treatment

The objective of this work was to study the systematic inactivation of immobilized heat-resistant, validated wild-type bacteria (sporulated Bacillus subtilis D2 and vegetative Cronobacter sakazakii H30) in porous enlarged model food powder particles using the Vacuum-Steam-Vacuum (VSV) process. Developed bacterial sensors allowed a local contamination of model particles at a defined intraparticle position. Contradicting previous studies, a spore inactivation of 4 log ₁₀ CFU in high depths of up to 10 mm (particle core) was possible by VSV treatment despite a noticeable intraparticle attenuation. To mimic inactivation in smaller food powder particles, similar-sized bacterial sensors were treated and showed slightly curve-linear kinetics explained by a short warm-up phase (lag time). Measured inactivation was lower than that predicted by traditional capillary (z = 8.0  ^∘ C) treatment of suspensions due to non-linearity in the high temperature regime. This could be described by twice as high z values of 15.4 °C (no lag time) and 17.2 °C (lag time), respectively.Industrial relevance of present workThe emerging Vacuum-Steam-Vacuum (VSV) decontamination process aims at efficient heat transfer on solid surfaces and pores using saturated steam as the heat-transferring fluid. VSV was already used for treating vegetables, fruits, meats and spices. This work describes use of VSV for dry porous solids.     

Combined high hydrostatic pressure and thermal treatments fully inactivate trypsin inhibitors and lipoxygenase and improve protein solubility and physical stability of calcium-added soymilk

The objective of this work was to assess the possibility of obtaining calcium-added soymilk with acceptable characteristics regarding protein solubility and physical stability, and inactivation of trypsin inhibitors and lipoxygenase, through a combined thermal-high hydrostatic pressure treatment. A Doehlert design was applied to study the effect of combining pressure levels (500–700 MPa), initial temperatures (45–65 °C) and CaCl₂ concentrations (5–15 mmol L^− 1). Results showed that protein solubility was a function of CaCl₂ concentration (p < 0.005), and that inactivation of trypsin inhibitors was a function of pressure level and temperature (p < 0.005). Lipoxygenase activity was fully inactivated in most of the conditions tested. Physical stability was improved by the combined treatments: depending on calcium concentration, either no settling was detected in a 5-day period or a less conspicuous phase separation was observed. Our results indicate that some combined thermal-high hydrostatic pressure treatments allow the preparation of calcium-enriched soymilks with improved physical stability without additives such as chelating agents, and acceptable in terms of full inactivation of trypsin inhibitors and lipoxygenase.Industrial relevanceAn analysis of the effect of the combination of high hydrostatic and thermal treatments on calcium-added soymilk is introduced in the present study. Some processing conditions were found to allow the obtention of a product with improved protein solubility and physical stability, and with inactivated trypsin inhibitors and lipoxygenase enzyme. This information could be beneficial considering the reduction of process time and energetic costs, because the assayed combination of thermal and high pressure treatments takes advantage of the instantaneous compression heating. Thus, the results provided in the present work could be useful to prepare an acceptable calcium-added soymilk.     

Pressure–ohmic–thermal sterilization: A feasible approach for the inactivation of Bacillus amyloliquefaciens and Geobacillus stearothermophilus spores

The efficacy of a pressure–ohmic–thermal sterilization (POTS) for Bacillus amyloliquefaciens and Geobacillus stearothermophilus spore inactivation was investigated. Spores (2.5 × 10⁸ cfu/ml) were inoculated in 0.1% NaCl solution (pH 5.0 and 7.0), green pea puree (pH 6.1), carrot puree (pH 5.0) or tomato juice (pH 4.1). Samples were ohmically (50 V/cm) treated at 600 MPa and 105 °C for various holding times using a laboratory-scale high-pressure processor. B. amyloliquefaciens and G. stearothermophilus spores suspended in 0.1% NaCl solution (pH 7.0) were inactivated by 4.6 and 5.6 log, respectively, for a 30-min holding time. B. amyloliquefaciens and G. stearothermophilus spores in tomato juice were reduced by 3.1 and 4.8 log, respectively, for a 10-min holding time. Spore germination was highest in the G. stearothermophilus suspended in 0.1% NaCl solution (pH 7.0). POTS treatment appears to be a potent method for inactivating pressure–thermal resistant bacterial spores.Industrial RelevanceFood industry is interested in developing superior quality low-acid shelf-stable foods. This study evaluated the pressure–ohmic–thermal sterilization (POTS) for the inactivation of Bacillus amyloliquefaciens and Bacillus stearothermophilus endospores. The impact of food matrices and acidity on the spore resistance was also investigated. Knowledge gained from the study will help the food processors for understanding the importance of various POTS treatment parameters for sterilization of low-acid foods.     

Application of electro-activated potassium acetate and potassium citrate solutions combined with moderate heat treatment on the inactivation of Clostridium sporogenes PA 3679 spores

Combined effects of moderate temperatures and the electro-activated aqueous solutions of potassium acetate and potassium citrate on the inactivation of C. sporogenes PA 3679 spores (D_121°C = 1.18 min) were studied. Four types of solutions (potassium acetate with/without KCl and potassium citrate with/without KCl) were activated at 400 mA for 60 min. The oxidation reduction potential (ORP) and pH values ranged from + 417.50 to + 1043.33 mV and 3.18 to 3.47, respectively. The combination of these solutions with a moderate heat treatment (95 °C, 105 °C, and 115 °C) for different time (5, 10, 20, and 30 min) was sufficient to reach a 100% of spore destruction (inactivation) in a medium with an initial contamination level comprised between 7.0 and 7.8 log CFU/mL. The sporicidal effect of solutions was also present even if activated solutions were applied alone against spores without being combined with heat treatment. Spore morphology was determined under transmission electron microscopy and showed that there were important damages, such as rupture of spores and release of spore components in all of the treated spores. Thus, the sporicidal effect detected was the result of inactivation mechanisms of electro-activated solutions on spores. In almost all of observed micrographs, there were coreless spores, deformed spores, or debris of spores. The current investigation can be used for achieving further studies in order to better understand the mechanisms of inactivation of C. sporogenes spores by electro-activated solutions.Industrial relevanceThis research article aims to study the combined effect of electro-activated potassium acetate and citrate solutions and moderate heat treatment on the viability of Clostridium sporogenes in model solutions as a non-pathogenic surrogate of Clostridium botulinum. The objective was to use hurdle technology to produce nutritious, minimally processed foods while ensuring food safety. Moreover, this approach allowed for a reduced level of sodium in canned foods since the solutions were sodium-free.     

Destruction of Salmonella Enteriditis inoculated onto raw almonds by high hydrostatic pressure

The effects of continuous (50,000, 60,000 and 70,000 psi with holding times of 5 and 10 min) and discontinuous (oscillatory) (six cycles at 60,000 psi with a holding time of 20 s) high hydrostatic pressure (HHP) treatments on the viability of two Salmonella Enteriditis strains (FDA and PT30) inoculated onto raw almonds were evaluated at 25, 50, and 55 °C. Complete inactivation of the S. Enteriditis was achieved in 0.1% peptone water after continuous pressurization at 60,000 psi and 25 °C for 5 min. Continuous pressurization of raw almonds inoculated with S. Enteriditis at 60,000 psi and 50 °C for 5 min resulted in less than a log reduction (log₁₀ 0.83) of vegetative cells. The decimal reduction time using the continuous pressurization parameters was determined to be 9.78 min. A discontinuous process consisting of six cycles of pressurization at 60,000 psi and 50 °C for 20 s provided greater than a one log reduction (log₁₀ 1.27 for FDA and log₁₀ 1.16 for PT30) of the S. Enteriditis concentration. The low water activity (a_w) of the almonds was found to impart baroprotective attributes on the S. Enteriditis cells. When the almonds were directly suspended in water and then pressurized, a log₁₀ reduction of 3.37 was achieved. HHP of certain dry foods appears to be feasible if the food is directly suspended in the pressurizing medium (water).     

Modelling kinetics of watercress (Nasturtium officinale) colour changes due to heat and thermosonication treatments

Watercress (Nasturtium officinale) colour changes due to blanching by heat and a combined treatment of heat/ultrasound (thermosonication) were studied in the temperature range of 82.5 to 92.5 °C. The application of thermosonication was intended to enable less severe blanching treatments and, therefore, improve the quality of the blanched product. The thermosonication blanching processes promoted changes of the green colour (aⁿ parameter) at a higher rate (P < 0.05), when compared with the heat blanching processes. No significant differences (P > 0.05) were detected between heat and thermosonication blanching processes in terms of the colour parameters Lⁿ, bⁿ and TCD changes. In both treatments, a fractional first order model fitted well the experimental data for Lⁿ, aⁿ and bⁿ (R_H² = 0.99; R_Ts² = 0.99) and TCD (R_H² = 0.92; R_Ts² = 0.96) colour parameters.The chlorophylls content showed no significant differences (P > 0.05) between thermally treated and thermosonicated watercress samples.The present findings will help to evaluate the effectiveness of thermosonication as a novel process to replace the classical heat treatment.Industrial relevanceConventional blanching commonly results in severe losses or destruction of nutrients due to process intensity and extended process times. Consequently, the attempts to use the synergistic effects of heat and ultrasound (at least) for enzyme inactivation are of high relevance. The results, although not conclusive, indicate, that they may aid optimization of blanching processes.     

Evaluation of pulsed electric field and minimal heat treatments for inactivation of pseudomonads and enhancement of milk shelf-life

Pulsed Electric Field (PEF) treatment of milk provides the opportunity to increase the shelf-life of fresh milk for distribution to distant markets. PEF treatments were evaluated in sterile (UHT) milk to determine the inactivation of added spoilage Pseudomonas isolates and the subsequent gains in microbial shelf-life (time taken to reach 10⁷ CFU mL^− 1). Little inactivation of Pseudomonas was achieved at 15 or 40 °C compared with 50 or 55 °C. The greatest inactivation (> 5 logs) was achieved by processing at 55 °C with 31 kV cm^− 1 (139.4 kJ L^− 1). Heat treatment at the application temperature without PEF treatment caused minimal inactivation of Pseudomonas (only 0.2 logs), demonstrating that the inactivation of the Pseudomonas was due to the PEF treatment rather than the heat applied to the milk. At added Pseudomonas levels of 10³ and 10⁵ CFU mL^− 1, the microbial shelf-life of PEF-treated milk was extended by at least 8 days at 4 °C compared with untreated milk. The total microbial shelf-life of the PEF-treated milk was 13 and 11 days for inoculation levels of 10³ and 10⁵ CFU mL^− 1 respectively. The results indicate that PEF treatment is useful for the reduction of pseudomonads, the major spoilage bacteria of milk.Industrial relevancePseudomonads are the major psychrotrophic spoilage microflora of refrigerated, stored HTST pasteurised milk. Long-life (UHT) products are an important component of milk sales in South-East Asia, but in recent years there has been an increasing demand for less processed milk products with extended shelf-life. The recent practice of shipping fresh bulk milk from Australia to South-East Asian countries has necessitated additional heat treatment prior to export and on arrival, to achieve the required shelf-life. Pulsed electric field treatment of HTST milk, applied alone or in combination with mild heat under optimised conditions, offers the opportunity of shelf-life extension, while limiting the reduction in quality attributes of milk associated with more severe additional heat treatments.     

Inactivation of Salmonella Typhimurium and Staphylococcus aureus by pulsed electric fields in liquid whole egg

In this study, the lethal effectiveness of pulsed electric fields (PEF) on the inactivation of Salmonella enterica subs. enterica ser. Typhimurium and Staphylococcus aureus in liquid whole egg (LWE) has been investigated. Maximum inactivation levels of 4 and 3 Log₁₀ cycles of the population of Salmonella Typhimurium and S. aureus were achieved with treatments of 45 kV/cm, 30 μs and 419 kJ/kg, and 40 kV/cm for 15 μs and 166 kJ/kg, respectively. The non-linear kinetics of inactivation observed for both microorganisms at all the investigated electric field strengths were described by mathematical equations based on the Weibull distribution. The developed equations enabled to compare the microbial resistance to PEF and to establish the most suitable treatment conditions to achieve a determined level of microbial inactivation. PEF treatments varying from 30 kV/cm, 67 µs and 393 kJ/kg to 45 kV/cm, 19 µs and 285 kJ/kg allow to reduce 3 Log₁₀ cycles the population of the microorganism of concern in PEF food processing of LWE, Salmonella Typhimurium.Industrial relevanceThe data presented in this investigation in terms of electric field strength, specific energy and treatment time result of relevance to evaluate the possibilities of PEF technology to pasteurize LWE with this technology. The models developed in this study can be applied to engineering design, and for the evaluation and optimization of the PEF technology as a new technique to obtain Salmonella free LWE.Based on our results it is not recommended to apply treatments of energy levels higher than 250 kJ/kg, since PEF lethality hardly increased but markedly augmented the energetic costs. For these energy values, PEF technology by itself is not sufficient (3 Log10 cycles in the best case scenario) to assure the safe security of LWE. Therefore, intelligent combinations of PEF with other preservation technologies have to be developed in order to use pulsed electric fields as an alternative to heat pasteurization of LWE.     

High pressure thermal processing for the inactivation of Clostridium perfringens spores in beef slurry

The spores of Clostridium perfringens can survive and grow in cooked/pasteurized meat, especially during the cooling of large portions. In this study, 600 MPa high pressure thermal processing (HPTP) at 75 °C for the inactivation of C. perfringens spores was compared with 75 °C thermal processing alone. The HPTP enhanced the inactivation of C. perfringens spores in beef slurry, resulting in 2.2 log reductions for HPTP vs. no reductions for thermal processing after 20 min. Then, the HPTP resistance of two C. perfringens spore strains in beef slurry at 600 MPa was compared and modeled, and the effect of temperature investigated. The NZRM 898 and NZRM 2621 exhibited similar resistance, and Weibull modeled well the log spore survivor curves. The spore inactivation increased when HPTP temperature was raised from 38 to 75 °C. The results confirm the advantage of high pressure technology to increase the thermal inactivation of C. perfringens spores in beef slurry.Industrial relevanceC. perfringens spores may cause food/meat poisoning as a result of improperly handled and prepared foods in industrial kitchens. Thermal processes at 100 °C or higher are generally carried out to ensure the elimination of these pathogenic spores. High pressure processing (HPP) is a food pasteurization technique which would help to maintain the sensorial and nutritional properties of food. Preservation of foods with HPP in conjunction with mild heat (HPTP) would enhance the spore inactivation compared to thermal processing alone at the same temperature, due to a known germination–inactivation mechanism. This technology, together with the application of Good Manufacturing Practices, including rapid cooling, is a good alternative to the traditional methods for producing safe processed meat and poultry products with enhanced sensory and nutritional quality.