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.     

An investigation on pulsed electric fields technology using new treatment chamber design

A pulsed electric field (PEF) system was designed and constructed using modern IGBT technology. The main focus of this work was to design a new PEF treatment chamber that operate at high electric field intensities with limited increase in liquid temperature and limited fouling of electrodes. Four multi-pass treatment chambers were designed consisting of two stainless steel mesh electrodes in each chamber, with the treated fluid flowing through the openings of the mesh electrodes. The two electrodes are electrically isolated from each other by an insulator element designed to form a small orifice where most of the electric field is concentrated. Dielectric breakdown inside the chambers was prevented by removing the electrodes far from the narrow gap. The effect of PEF treatment on the inactivation of gram-negative Escherichia coli ATCC 25922 suspended in simulated milk ultra-filtrate (SMUF) of 100%, 66.67% and 50% w/w was investigated. Treatments with the same electrical input power but with higher electric field strengths provided larger degree of killing. The effect of PEF treatment using suspensions at different flow rates and different pulse frequencies was also investigated. In general, the inactivation rate of E. coli increased with increasing electric field strength, treatment time and processing temperature. More than 6 log reductions in E. coli suspended in SMUF was achieved using electric field intensity in the range of (37.2–49.6 kV/cm) with a treatment temperature not exceeding 38 °C.Industrial relevanceThis paper presents an innovative pulsed electric field system for non-thermal pasteurisation of liquid food. The system design provides uniform distribution of electric field and minimum fouling of electrodes. This PEF system can be scaled up to any industrial size, making it attractive for industrial applications.     

Inactivation of Escherichia coli O157:H7 in liquid whole egg using combined pulsed electric field and thermal treatments

Inactivation of Escherichia coli O157:H7 in liquid whole egg using thermal and pulsed electric field (PEF) batch treatments, alone and in combination with each other, was investigated. Electric field intensities in the range from 9 to 15 kV/cm were used in the study. The threshold temperature for thermal inactivation alone was 50 °C. PEF enhanced the inactivation of E. coli O157:H7 when the sample temperature was higher than the thermal threshold temperature. The maximum inactivation of E. coli O157:H7 obtained using thermal treatment alone was ∼2 logs at 60 °C. However, combined heat and PEF treatments resulted in up to 4 log reduction of the pathogen. The kinetic rate constants k_TE for combined treatments at 55 °C varied from 0.025 to 0.119 pulse⁻¹ whereas the rate constants at 60 °C ranged from 0.034 to 0.228 pulse⁻¹. These results indicated a synergy between temperature and electric field on the inactivation of E. coli O157:H7 within a given temperature range.     

Kinetics of destruction ofEscherichia coliandPseudomonas fluorescensinoculated in ewe's milk by high hydrostatic pressure

Ewe's1999199919991999 Academic PressAcademic PressAcademic PressAcademic Pressmilk standardized to 6% fat was inoculated with Escherichia coliand Pseudomonas fluorescensat a concentration of 10⁷and 10⁸ cfu ml⁻¹respectively, and treated by high hydrostatic pressure. Treatments consisted of combinations of pressure (50-300 MPa), temperature (2, 10, 25 and 50°C), and time (5, 10 and 15 min). Violet red bile agar and crystal-violet-tetrazolium count were used to determineE. coliandP. fluorescensrespectively. Pressurization at low and moderately high temperatures produced higher P. fluorescensinactivation than treatments at room temperature, while pressurization at only moderately high temperature produced high E. coliinactivation; low and room temperatures produced similar reductions. On E. coli,reductions of 6.055 log units were produced with 300 MPa for 15 min at 50°C, while on P. fluorescens,reductions of 5.059 log units were produced with 250 MPa for 15 min at 50°C. Both micro-organisms showed a first-order kinetics of destruction in the range 0-30 min, with D-values (at different temperatures and pressures from 150 to 300 MPa) between 2.055 and 18.058 min for E. coli,and 2.058-23.053 min for P. fluorescens.Abaroprotective effect of ewe's milk (6% fat) on both micro-organisms was observed, in comparison with other studies using different means and similar pressurization conditions.     

Alkaline phosphatase and microbial inactivation by pulsed electric field in bovine milk

The effects of pulsed electric field (PEF) treatments at field intensities of 25–37 kV cm^− 1 and final PEF treatment temperatures of 15 °C and 60 °C on the inactivation of alkaline phosphatase (ALP), Total Plate Count (TPC), Pseudomonas and Enterobacteriaceae counts were determined in raw skim milk. At 15 °C, PEF treatments of 28 to 37 kV cm^− 1 resulted in 24–42% inactivation in ALP activity and < 1 log reduction in TPC and Pseudomonas count, while the Enterobacteriaceae count was reduced by at least 2.1 log units to below the detection limit of 1 CFU mL^− 1. PEF treatments of 25 to 35 kV cm^− 1 at 60 °C resulted in 29–67% inactivation in ALP activity and up to 2.4 log reduction in TPC, while the Pseudomonas and Enterobacteriaceae counts were reduced by at least 5.9 and 2.1 logs, respectively, to below the detection limit of 1 CFU mL^− 1. Kinetic studies suggested that the effect of field intensity on ALP inactivation at the final PEF treatment temperature of 60 °C was more than twice that at 15 °C. A combined effect was observed between the field intensity and temperature in the inactivation of both ALP enzyme and the natural microbial flora in raw skim milk.Industrial relevanceMilk has been pasteurised to ensure its safety and extend its shelf life. However, the need for retaining heat-sensitive nutrient and sensory properties of milk has resulted in interest in the application of alternative technologies. The results of the current study suggest that PEF as a non-thermal process can be employed for the treatment of raw milk in mild temperature to achieve adequate safety and shelf life while preserving the heat-sensitive enzymes, nutrients and bioactive compounds.     

Microbiological and physicochemical stability of raw,pasteurised or pulsed electric field-treated milk

Pulsed electric field (PEF) processing was investigated as an alternative dairy preservation technology that would not compromise quality yet maintain safety. PEF processing of raw whole milk (4% fat) was conducted at two processing conditions (30 kV/cm, 22 μs, at either 53 or 63 °C outlet temperature) and compared with two thermal treatments (15 s, at either 63 or 72 °C) and a raw milk control and replicated twice. Milk bottles (2 L) from each treatment were incubated for two weeks, at 4 and 8 °C, and assessed for total plate count, pH, colour, rennetability, plasmin activity and lipid oxidation. The microbial quality of the thermal (72 °C/15 s) and PEF (63 °C) were similar. A drop in pH occurred after a change in counts was observed. Rennetability was not different between the treatments. Short chain acids dominated the volatile profile of the milk samples. Concentration of volatiles derived from microbial activity, namely 2,3-butanedione, acetic acid and other milk lipid derived short chain free fatty acids (e.g. butanoic and hexanoic acids), followed the trend of microbial activity in milk samples.Industrial relevanceResearch on the application of PEF to control spoilage and pathogenic microorganisms and enzyme systems in milk spans a wide array of processing equipment and reaction conditions. While industrial scale PEF processing of liquid milk for preservation and improved quality seems generally possible, substantiation of lower thermal damage under safe and scalable PEF conditions is required to enable economic feasibility.     

Effect of temperature,pH and presence of nisin on inactivation of Salmonella Typhimurium and Escherichia coli O157:H7 by pulsed electric fields

The aim of this investigation is to evaluate the concurrent influence of temperature (4–50 °C), pH (3.5–7.0), and the presence of nisin (up to 200 μg/mL) on the inactivation of two PEF-resistant Gram-negative, pathogenic bacteria, Salmonella Typhimurium STCC 878 and Escherichia coli O157:H7, using a PEF treatment of 30 kV/cm and 99 μs. A response surface model using a central composite design was developed for the purpose of understanding the individual effects and interactions of these factors.The models showed that temperature was the factor with the greatest influence on the PEF inactivation in the two strains investigated. Increasing the treatment temperature from 4 to 50 °C increased the lethality of PEF up to at least 4 Log₁₀ reductions for both microorganisms at all pH levels investigated. PEF lethality varied with the square of the pH observing the highest microbial PEF sensitivity at pH 5.25 at all temperatures. The addition of nisin to the treatment medium did not influence the PEF lethality independently of the temperature.PEF induced 1.0–1.5 Log₁₀ cycles of damaged cells at pH 3.5 for Salmonella Typhimurium STCC 878 and at pH 5.25 for E. coli O157:H7, independently of the temperature or the presence of nisin in the treatment medium.The application of PEF at 50 °C permitted the achievement of 5 Log₁₀ reductions of Salmonella Typhimurium STCC 878 and E. coli O157:H7 in a range of pH from 4.2 to 6.7 and from 4.5 to 6.0, respectively. Therefore, the application of PEF at moderate temperatures has great potential for achieving effective control of Gram-negative pathogenic microorganisms in the range of pH found in most foods.     

The inactivation of Listeria monocytogenes by pulsed electric field (PEF) treatment in a static chamber

An experimental analysis of the effect of pulsed electric field (PEF) energy on the inactivation of Listeria monocytogenes was conducted using a custom-designed static chamber and a gel suspension medium for treatment. This allowed PEF energy to be delivered to the suspension under near isothermal conditions. The effects of variations in the number of pulses (5–50 pulses), electric field strength (15–30 kV/cm), temperature (0–60°C) and media bases (water and skim milk) on the inactivation of L. monocytogenes were examined. At temperatures less than 50°C a maximum of 1 log reduction was obtained for L. monocytogenes regardless of pulse number or electric field strength within the ranges examined. In skim milk no reduction occurred. At 50°C and 55°C synergy between PEF and thermal energy was observed. The experimental approach separated the contribution of PEF and thermal energy to total kill and thus allowed this synergy to be quantified. At 55°C the kill due to PEF energy increased to 4.5 logs with another 4.5 logs reduction attributable to thermal energy. It appears that under the conditions of this study PEF alone has a very limited effect on the reduction of L. monocytogenes. However, the addition of thermal energy not only contributed to the kill, but also increased the susceptibility of L. monocytogenes to PEF energy.     

Modelling the inactivation of Escherichia coli ATCC 25922 using pulsed electric field

Microbial tests were conducted to determine the effect of pulsed electric field treatment on microbial inactivation of gram negative Escherichia coli ATCC 25922 suspended in simulated milk ultra filtrate (SMUF). Kinetic analysis of microbial inactivation due to combined pulsed electric field (PEF) and thermal treatments of E. coli was investigated. A generalized correlation for the inactivation rate constant as a function of both electric field intensity and treatment temperature was derived. Comparison between experimental and theoretical variation of E. coli concentration with time after PEF treatment in a complete recirculation mode was conducted using the inactivation kinetics developed from the single pass measurements.

Industrial relevance

PEF technology has a tremendous potential to replace thermal pasteurisation for products which are sensitive to temperature. In this work a pulsed electric field process was mathematically modelled and a generalized correlation for the inactivation rate constant as a function of electric field intensity and treatment temperature was derived. The correlation is needed for the design of industrial PEF systems.     

Microbiological and enzymatic activity of bovine whole milk treated by pulsed electric fields

Pulsed electric field (PEF)‐treated milk (25.7 kV/cm for 34 μs after preheating to 55 °C and holding for 24 s) was microbiologically stable for 21 days at 4 °C, and similar to thermally treated milk (63 °C for 30 min or 73 °C for 15 s). Alkaline phosphatase inactivation was comparable after PEF (preheating followed by PEF) and both thermal treatments. PEF treatment initially reduced xanthine oxidase (30%) and plasmin (7%) activities, but after 21 days of refrigerated storage these activities were similar to the initial untreated milk. During refrigerated storage of PEF (preheating followed by PEF) and thermally treated milk, lipolytic activity increased and pH levels decreased.     

Equivalent processing for pasteurization of a pineapple juice–coconut milk blend by selected nonthermal technologies

Identifying equivalent processing conditions is critical for the relevant comparison of food quality attributes. This study investigates equivalent processes for at least 5-log reduction of Escherichia coli and Listeria innocua in pineapple juice–coconut milk (PC) blends by high-pressure processing (HPP), pulsed electric fields (PEF), and ultrasound (US) either alone or combined with other preservation factors (pH, nisin, and/or heat). The two blends (pH 4 and 5) and coconut milk (pH 7) as a reference were subjected to HPP at 300–600 MPa, 20°C for 0.5–30 min; PEF at an electric field strength of 10–21 kV/cm, 40°C for 24 µs; and US at 120 µm amplitude, 25 or 45°C for 6 or 10 min. At least a 5-log reduction of E. coli was achieved at pH 4 by HPP at 400 MPa, 20°C for 1 min; PEF at 21 kV/cm, 235 Hz, 40°C for 24 µs; and US at 120 µm, 45°C for 6 min. As L. innocua showed greater resistance, a synergistic lethal effect was provided at pH 4 by HPP with 75 ppm nisin at 600 MPa, 20°C for 5 min; PEF with 50 ppm nisin at 18 kV/cm, 588 Hz, 40°C for 24 µs; and US at 45°C, 120 µm for 10 min. The total soluble solids (11.2–12.4°Bx), acidity (0.47%–0.51% citric acid), pH (3.91–4.16), and viscosity (3.55 × 10⁻³–4.0 × 10⁻³ Pa s) were not significantly affected under the identified equivalent conditions. HPP was superior to PEF and US, achieving higher ascorbic acid retention and lower color difference in PC blend compared to the untreated sample.     

Effect of thermosonication,pulsed electric field and their combination on inactivation of Listeria innocua in milk

The impact of thermosonication (TS) and pulsed electric field (PEF), individually and combined, on the survival of Listeria innocua 11288 (NCTC) in milk was investigated. TS (400 W, 160 s) without pre-heating reduced L. innocua by 1.2 log₁₀ cfu mL^?1, while shorter treatment times produced negligible inactivation, suggesting TS to be a hurdle rather than an effective standalone treatment. PEF (30 and 40 kV cm^?1, 50 μs) at 10 °C caused a reduction of L. innocua of 1.1 and 3.3 log cycles, respectively. The highest field strength (40 kV cm^?1) combined with TS (80 s) led to 6.8 log₁₀ cfu mL^?1 inactivation. Milk pre-heated to 55 °C (over 60 s) prior to TS followed by PEF (30 and 40 kV cm^?1) showed inactivation between 4.5 and 6.9 log₁₀ cfu mL^?1, the latter being comparable (P > 0.05) with thermal pasteurisation. The data indicate that TS followed by PEF represents a valid alternative for L. innocua inactivation in milk.     

Identification of equivalent processing conditions for pasteurization of strawberry juice by high pressure,ultrasound, and pulsed electric fields processing

The objective of this study was to evaluate the effectiveness of high pressure processing (HPP), ultrasound (US) and pulsed electric fields (PEF) for the pasteurization of strawberry juice (SJ). Acid-adapted Escherichia coli was used to inoculate SJ prior to treatment with HPP, US, and PEF. HPP was applied at several pressures (200–400 MPa) up to 2 min while US (120 μm, 24 kHz) was conducted at 25, 40, and 55 °C up to 10 min in continuous pulsing mode. In order to avoid excessive use of SJ, PEF was performed using a model solution (MS) basically composed of citric acid (8 g/L), fructose (35 g/L), glucose (35 g/L), Na₂HPO₄ (0.2 M) and NaCl (5%) to simulate the SJ electrical conductivity, pH, and total soluble solid (TSS). A face-centered composite design was conducted for PEF processing at different electric field intensities (EFI) (25–35 kV/cm) and treatment times (5–27 μs). Processing conditions were selected that resulted in 5-log CFU/mL inactivation of E. coli. HPP at 300 MPa for 1 min, and US at 55 °C (thermosonication) for 3 min reduced E. coli in SJ by 5.75 ± 0.52 and 5.69 ± 0.61 log CFU/mL, respectively. PEF treatment at 35 kV/cm, 27 μs treatment time, 350 mL/min flow rate, and 2 μs pulse width in monopolar mode resulted in 5.53 ± 0.00 log reduction of E. coli in MS. Likewise, E. coli population in SJ was also reduced by 5.16 ± 0.15 log after applying the same PEF conditions to SJ. No E. coli was detected in SJ subjected to conventional thermal pasteurization at 72 °C for 15 s. All technologies reduced the natural microbiota below 2 log CFU/mL in terms of the total aerobic bacteria and yeast-mold counts. Thus, this study identified the equivalent conditions for the SJ pasteurization by three nonthermal processing technologies.Industrial relevanceConsumers have an increasing interest towards fresh-like food products with desirable nutritional and sensorial attributes. High pressure, ultrasound and pulsed electric field are three relevant novel nonthermal technologies as alternatives to conventional thermal treatments. This study identified the processing conditions of these three nonthermal technologies for the pasteurization of strawberry juice based on equivalent inactivation of acid-adapted E. coli. From an industrial point of view, the established processing conditions are useful references for the development of novel berry juices. In addition to microbiological safety, this study on equivalent processing allows direct efficacy and quality comparisons of a given juice pasteurized by the three nonthermal technologies under consideration.     

Inactivation of Escherichia coli in a Tropical Fruit Smoothie by a Combination of Heat and Pulsed Electric Fields

ABSTRACT: Moderate heat in combination with pulsed electric fields (PEF) was investigated as a potential alternative to thermal pasteurization of a tropical fruit smoothie based on pineapple, banana, and coconut milk, inoculated with Escherichia coli K12. The smoothie was heated from 25 °C to either 45 or 55 °C over 60 s and subsequently cooled to 10 °C. PEF was applied at electric field strengths of 24 and 34 kV/cm with specific energy inputs of 350, 500, and 650 kJ/L. Both processing technologies were combined using heat (45 or 55 °C) and the most effective set of PEF conditions. Bacterial inactivation was estimated on standard and NaCl‐supplemented tryptone soy agar (TSA) to enumerate sublethally injured cells. By increasing the temperature from 45 to 55 °C, a higher reduction in E. coli numbers (1 compared with 1.7 log₁₀ colony forming units {CFU} per milliliter, P < 0.05) was achieved. Similarly, as the field strength was increased during stand‐alone PEF treatment from 24 to 34 kV/cm, a greater number of E. coli cells were inactivated (2.8 compared with 4.2 log₁₀ CFU/mL, P < 0.05). An increase in heating temperature from 45 to 55 °C during a combined heat/PEF hurdle approach induced a higher inactivation (5.1 compared with 6.9 log₁₀ CFU/mL, respectively {P < 0.05}) with the latter value comparable to the bacterial reduction of 6.3 log₁₀ CFU/mL (P≥ 0.05) achieved by thermal pasteurization (72 °C, 15 s). A reversed hurdle processing sequence did not affect bacterial inactivation (P≥ 0.05). No differences were observed (P≥ 0.05) between the bacterial counts estimated on nonselective and selective TSA, suggesting that sublethal cell injury did not occur during single PEF treatments or combined heat/PEF treatments.     

The influence of lactoperoxidase,heat and low pH on survival of acid-adapted and non-adapted Escherichia coli O157:H7 in goat milk

In hot climates where quality of milk is difficult to control, a lactoperoxidase (LP) system can be applied in combination with conventional preservation treatments at sub-lethal levels to inhibit pathogenic microbes. This study investigated the effect of combined heat treatments (55 °C, 60 °C and 72 °C) and milk acidification (pH 5.0) on survival of acid-adapted and non-adapted Escherichia coli O157:H7 strains UP10 and 1062 in activated LP goat milk. Heat treatment at 72 °C eliminated E. coli O157:H7. Acid-adapted strains UP10 and 1062 cells showed resistance to combined LP and heat at 60 °C in fresh milk. The inhibition of acid-adapted and non-adapted E. coli O157:H7 in milk following combined LP-activation, heat (60 °C) and milk acidification (pH 5.0) suggests that these treatments can be applied to reduce E. coli O157:H7 cells in milk when they occur at low numbers (<5 log₁₀ cfu mL^?1) but does not eliminate E. coli O157:H7 to produce a safe product.     

INACTIVATION OF ESCHERICHIA COLI IN SKIM MILK BY HIGH INTENSITY PULSED ELECTRIC FIELDS

The inactivation of microorganisms is the most important function in the processing of milk and dairy products. Traditionally, this purpose is realized by thermal treatment, but heat produces alterations to flavor and taste in addition to nutrient loss. The high intensity pulsed electric field (PEF) treatment should be a good alternative to heat because demonstrations have shown PEF can reduce the Escherichia coli survival fraction in aqueous solutions and model foods. In this study, PEF treatment was found to inactivate E. coli in skim milk (inoculum 10⁹ CFU/mL) at 15C. The microorganism inactivation satisfied Hülsheger's model following a first order kinetic for both the electric field intensity and number of pulses when skim milk inoculated with E. coli was treated in a static or continuous flow chamber. PEF treatment in a continuous system when the critical electric field (E_c) and minimum number of pulses (n_min) were 12.34 kV/cm and 2.7 at 30 kV/cm and 30 pulses (0.7–1.8 μs pulse width) inactivated more microorganisms than in a static system. It has also been proven that increasing the pulse duration increases the E. coli inactivation. The inactivation of E. coli using PEF is more limited in skim milk than in a buffer solution when exposed to similar treatment conditions of field intensity and number of pulses due to the complex composition of skim milk, its lower electrical resistivity and the presence of proteins.     

Modeling inactivation kinetics and occurrence of sublethal injury of a pulsed electric field-resistant strain of Escherichia coli and Salmonella Typhimurium in media of different pH

A study of the effect of pulsed electric fields (PEF) on the kinetics of inactivation and the occurrence of cell damage in Escherichia coli O157:H7 and Salmonella Typhimurium 878 treated in McIlvaine buffer covering a range from pH 3.5 to 7.0 was conducted. Mathematical equations based on the Weibull distribution were developed to describe the influence of the electric field strength, treatment time and pH of the treatment medium on the lethality and generation of cell damage of both Gram negative pathogenic bacteria after the application of PEF treatments. E. coli O157:H7 was more PEF resistant than Salmonella Typhimurium at all pH investigated. PEF resistance of E. coli was influenced by the pH but the pH hardly affected the PEF resistance of Salmonella Typhimurium 878. After 150 μs at 35 kV/cm, 1 and 5 log₁₀ cycles of inactivation of E. coli O157:H7 were observed in the range of pH 3.5–4.5 and 5.5–6.5, respectively. Cell damage increased with the field strength and treatment time. A maximum cell damage level of 4.2 and 2.7 log₁₀ cycles for E. coli O157:H7 and Salmonella Typhimurium was observed respectively after a treatment of 30 kV/cm at pH 3.5. PEF induced cell damage was not detected at pH higher than 5.0 for both microorganisms. The developed equations can be applied to design combining processes which can increase the lethality of PEF or to reduce the intensity of PEF treatments to achieve a determine level of microbial inactivation.Industrial relevanceThis study demonstrates that when the influence of several factors on the microbial behavior is investigated, the development of mathematical models is a very useful tool to evaluate the influence of each parameter and their interactions. In this study, it has been mathematically described for first time the influence of the pH of the treatment medium and the occurrence of sublethal injury in a wide range of electric field strengths and treatment times in two Gram negative pathogenic bacteria, Escherichia coli O157:H7 and Salmonella Typhimurium 878. These models would also be of interest for engineering design, evaluation and optimization of PEF process as a new technique for food preservation.     

Pressure-induced inactivation of bacteria through pressure-assisted thawing using a thermal buffer zone.

Effect of thermal buffer zone was examined on the microbial inactivation through a pressure-assisted thawing. A plastic bag of bacterial suspension enclosed with a thermal buffer zone was frozen at − 50 °C, and treated for 20 min with a pressure-assisted thawing in water of 4 °C. A reduction of 8-log cycle was obtained at 200 MPa for the stationary growth phase cells of Escherichia coli that was suspended in 1% skim milk and enclosed with wheat flour/water paste and two polytetrafluoroethylene plates. When 100% ethanol was used as a thermal buffer and the samples were pressured at 194 MPa in 1% skim milk, levels of E. coli and Listeria monocytogenes were reduced by 6-log cycle and 7-log cycle, respectively. Staphylococcus aureus decreased by 4-log cycle.Industrial relevanceThis work will contribute to new developments in the pressure processing of foods, since the use of a thermal buffer zone in pressure-assisted thawing was very effective in enhancing the level of pressure-induced microbial inactivation.     

Characterization of metastable high pressure phase transition positions and its influence on the behavior of microbial destruction

Effect of perturbation factors on phase transition metastable positions of whole milk (4% fat content) and their influence on microbial destruction characteristics of non-pathogenic Escherichia coli inoculated in milk subjected to high pressure low temperature treatment were evaluated using a specially developed high pressure (HP) cooling system. Initially, the phase transition data of milk transitioning through the metastable phases were obtained and fitted successfully using Simon-like models as done in previous studies and polynomial formulas with R² of 0.997 & 0.996 for ice I, and 0.989 & 0.989 for ice III, respectively. The phase transition position of milk was explored with 5% and 10% sodium chloride solution as perturbation sources, respectively. Results showed that the 5% sodium chloride solution can reduce the transition pressure of milk by 43 MPa and increase the transition temperature by 4.1 °C, so that the milk can achieve phase transition at lower pressure and higher temperature. Phase transition microbial destruction was characterized by discontinuity, mutation and segmentation when the phase transition pressure interval 250– 300 MPa was carefully refined. The inactivation amount of E. coli before the phase transition (250 MPa) was 1.11 log and the phase transition process itself brought an additional 1.26 log destruction of E. coli population in milk.Industrial relevanceHigh pressure low temperature (HPLT) phase change kinetics were employed to enhance microbial destruction. HPLT was established based on a self-cooling unit positioned inside conventional HP chamber offering opportunities for scale up and commercialization. The effectiveness of HPLT phase transition for Escherichia coli destruction was demonstrated. The related research in metastable state provides a reference point for commercial application of high-pressure-low-temperature technology for microbial destruction and quality enhancement.     

Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes in milk by caprylic acid and monocaprylin

Listeria monocytogenes and Escherichia coli O157:H7 are major foodborne pathogens implicated in various outbreaks involving pasteurized or unpasteurized milk, and various dairy products. The objective of this study was to determine the antibacterial effect of caprylic acid (CA, C_8:0) and its monoglyceride, monocaprylin (MC) on L. monocytogenes and E. coli O157:H7 in whole milk. A five-strain mixture of E. coli O157:H7 or L. monocytogenes was inoculated in autoclaved milk (10⁶ CFU/ml) containing 0, 25, or 50 mM of CA or MC. At 37°C, all the treatments, excepting 25 mm CA, reduced the population of both pathogens by approximately 5.0 log CFU/ml in 6 h. At 24 h of storage at 8°C, MC at both levels and CA at 50 mM decreased L. monocytogenes and E. coli O157:H7, respectively by >5.0 log CFU/ml. At 48 h of 4°C storage, populations of L. monocytogenes and E. coli O157:H7 were decreased to below detection level (enrichment negative) by 50 mm of MC and CA, respectively. Results indicate that MC could potentially be used to inhibit L. monocytogenes and E. coli O157:H7 in milk and dairy products, but sensory studies need to be conducted before recommending their use.