Combined effect of temperature and pulsed electric fields on pectin methyl esterase inactivation in red grapefruit juice (<Emphasis Type="Italic">Citrus paradisi</Emphasis>)

Pulsed electric fields (PEF) were applied to freshly prepared grapefruit juice using a laboratory scale continuous PEF system to study the feasibility of inactivating pectin methyl esterase (PME). Square wave PEF using different combinations of pre-treatment temperature, electric field strength and treatment time were evaluated in this study. Inactivation curves for the enzyme were plotted for each parameter and inactivation kinetics were calculated. Results showed that the highest level of inactivation (96.8%) was obtained using a combination of preheating to 50 °C, and a PEF treatment time of 100 μs at 40 kV/cm. Inactivation of grapefruit PME activity could be described using an exponential decay model. Calculated D-values following a 50 °C preheat were 77.5, 76.0 and 70.4 μs at 20, 30 and 40 kV/cm, respectively. The activation energy for the inactivation of PME by PEF was 36.2 kJ/mol.     

Combined effect of temperature and pulsed electric fields on soya milk lipoxygenase inactivation

Pulsed electric fields (PEF) were applied to freshly prepared soya milk using a laboratory scale continuous PEF system to study the feasibility of inactivating lipoxygenase (LOX). Square wave PEF using different combinations of pre-treatment temperature, electric field strength and treatment time were evaluated in this study. Inactivation curves for the enzyme were plotted for each parameter and inactivation kinetics were calculated and modelled. Results showed the highest level of inactivation (84.5%) was obtained using a combination of preheating to 50 °C, and a PEF treatment time of 100 μs at 40 kV/cm. Inactivation of LOX activity as a function of treatment time could be described using a first order kinetic model. Calculated D values following pre-heating to 50 °C were 172.9, 141.6 and 126.1 μs at 20, 30 and 40 kV/cm, respectively.     

Effects on the carotenoid pattern and vitamin A of a pulsed electric field-treated orange juice–milk beverage and behavior during storage

The effect of pulsed electric fields (PEF; electric field intensities 15, 25, 35, and 40 kV/cm, and treatments times from 40 to 700 μs) on the carotenoid and vitamin A profile of an orange juice–milk beverage was evaluated and compared with a pasteurization treatment (90 °C, 20 s). Subsequently, the evolution of these parameters during storage of the beverage at 4 and 10 °C was studied. The results obtained indicate that the application of the electric field influenced the concentration of the extracted carotenoids producing a slight increase at 15 kV/cm and a slight decrease at 40 kV/cm. On the other hand, when pasteurization was applied, it was found a reduction in total carotenoid concentration. The reductions in carotenoids with provitamin A activity were very small after pasteurization, the decreases in lutein and zeaxanthin must be taken into account. The degradation kinetics of the carotenoids during storage were similar for the pasteurized and PEF-treated beverages. However, they were less affected initially in the beverages treated by the new technology and were therefore maintained in greater quantities throughout the storage period.     

Inactivation Kinetics of Tomato Juice Lipoxygenase by Pulsed Electric Fields

ABSTRACT: Pulsed electric field (PEF) inactivation models for tomato juice lipoxygenase (LOX) were studied. Tomato juice was treated by PEF with the combinations of electric field strength (0, 10, 15, 20, 30, 35 kV/cm), PEF treatment time (20, 30, 50, 60, 70 μs), and PEF treatment temperature (10, 20, 30, 40, 50 °C). The first-order inactivation models, Hulsheger's model, Fermi's model, and the 2nd-order polynomial equation adequately described the LOX inactivation. Calculated D values were 161.0, 112.9, 101.0, and 74.8 μs at 15, 20, 30, and 35 kV/cm, respectively, at 30 °C. The activation energy for the inactivation of LOX by PEF was 35.7 kJ/mol. Applied electric field strength was the primary variable for the inactivation of LOX.     

Weibull distribution function based on an empirical mathematical model for inactivation of Escherichia coli by pulsed electric fields

The pulsed electric field inactivation kinetics of Escherichia coli suspended in orange juices with three different concentrations of carrot juice (0, 20, and 60%) was studied. Electric field strengths ranged from 25 to 40 kV/cm, and treatment times ranged from 40 to 340 micros. Experimental data were fitted to Bigelow, Hülsheger, and Weibull distribution functions, and the Weibull function provided the best fit (with the lowest mean square error). The dependency of each model's kinetic constant on electric field strength and carrot juice concentration was studied. A secondary model was developed to describe the relationship of Weibull parameters a and n to electric field strength and carrot juice concentration. An empirical mathematical model based on the Weibull distribution function, relating the natural logarithm of the survival fraction to treatment time, electric field strength, and carrot juice concentration, was developed. Parameters were estimated by a nonlinear regression. The results of this study indicate that the error rate for the model's predictions was 6.5% and that the model was suitable for describing E. coli inactivation.     

Inactivation of soybean lipoxygenase in soymilk by pulsed electric fields

The inactivation of soybean lipoxygenase by pulsed electric fields (PEF) was studied. Effects of PEF parameters (treatment time, pulse strength, pulse frequency and pulse width) were evaluated. Soymilk was exposed to pulsed strengths from 20 to 42 kV/cm for up to 1036 μs treatment time in square wave pulse of bipolar mode. Moreover, pulse frequency (100–600 Hz) and pulse width (1–5 μs) was also tested at constant pulsed treatment time of 345 μs and strength of 30 kV/cm. Residual activity of soybean lipoxygenase decreased with the increase of treatment time, pulse strength, pulse frequency and pulse width. The maximum inactivation of soybean lipoxygenase by PEF achieved 88% at 42 kV/cm for 1036 μs with 400 Hz of pulse frequency and 2 μs of pulse width at 25 °C. Inactivation of soybean lipoxygenase by pulsed electric fields was modeled using several kinetic models. Weibull distribution function was most suitable model describing the inactivation of soybean LOX as a function of pulsed electric fields process parameters. Moreover, reduction of soybean LOX activity related to the electric field strength could be well described by the Fermi model.     

Inactivation of Yersinia enterocolitica by pulsed electric fields

The influence of growth conditions, treatment medium characteristics and PEF process parameters on the lethal effect on Yersinia enterocolitica of pulsed electric fields (PEF) treatments in batch has been investigated. Growth phase, temperature of growth, pH, conductivity of the treatment medium, pulse width and frequency of pulses did not influence the sensitivity of Y. enterocolitica to PEF. However, an A_w decrease from >0.99 to 0.93 of the treatment medium increased the PEF resistance of Y. enterocolitica with 3.5 log₁₀ cycles after a treatment of 22 kV/cm, 800 μs and 880 kJ/kg. Inactivation of Y. enterocolitica increased with the field strength, treatment time and total specific energy up to a maximum of 6 log₁₀ cycles after 28 kV/cm, 2000 μs and 3559 kJ/kg. A nonlinear relationship was found among the survival fraction and the treatment time or the specific energy that was accurately described by a mathematical model based on the Weibull distribution. The inactivation of Y. enterocolitica by PEF was characterized by maximum field strength thresholds. Above these thresholds, specific energy necessary to obtain a given level of inactivation scarcely decreased by increasing the electric field strength, and inactivation of Y. enterocolitica only depended on the specific energy applied.     

Inactivation of Oxidative Enzymes by High-Intensity Pulsed Electric Field for Retention of Color in Carrot Juice

The effects of high-intensity pulsed electric fields (HIPEF) on oxidative enzymes and color of fresh carrot juice were studied. A response surface methodology (RSM) was used to evaluate the effect of pulse polarity (mono or bipolar mode), pulse width (from 1 to 7 μs), and pulse frequency (from 50 to 250 Hz) on color and peroxidase (POD) inactivation of carrot juice treated by HIPEF. The total treatment time and the electric field strength were set at 1,000 μs and 35 kV/cm, respectively, at a temperature below 35°C. The physicochemical characteristics of carrot juice were measured. There was a linear relationship between electrical conductivity and temperature of the carrot juice. The results showed that HIPEF-treated carrot juice at 35 kV/cm for 1,000 μs applying 6 μs pulse width at 200 Hz in bipolar mode led to 73.0% inactivation of POD. The color coordinates did not change significantly. Therefore, HIPEF was effective in POD inactivation and carrot juice color preservation.     

Enhancement of the solid-liquid extraction of sucrose from sugar beet (Beta vulgaris) by pulsed electric fields

A systematic study of the impact of pulsed electric fields (PEF) parameters (1–7 kV/cm, 5–40 pulses, specific energy of 0.006–0.19 kJ/kg per pulse, pulse frequency of 1–10 Hz, pulse width of 2–5 μs, square and exponential decay pulses) on the kinetics of the sucrose extraction from sugar beet at different temperatures (20–70 °C) has been carried out in this investigation. The efficiency of the solid-liquid extraction was independent of the frequency, as well as of the pulse width, and the pulse shape at 7 kV/cm, and it was influenced by the electric field strength applied and by the temperature of the extracting medium. Sucrose yield increased with both field strength, time of extraction, and temperature. The effect of the field strength was higher the lower the temperature. The application of 20 pulses at 7 kV/cm (3.9 kJ/kg) increased the maximum yield by 7 and 1.6 times, compared to non-PEF-treated samples, at 20 and 40 °C, respectively. A mathematical expression was generated, which enabled to evaluate the influence of the electric field strength (from 0 to 7 kV/cm) and temperature (from 20 to 70 °C) on the sucrose extraction efficiency and the extracting time in a solid-liquid PEF-assisted sucrose extraction process. Based on this equation, for 80%-sucrose extraction in 60 min, the temperature could be reduced from 70 °C to 40 °C, when 20 pulses of 7 kV/cm were applied.     

Inactivation and kinetic model for the <Emphasis Type="Italic">Escherichia coli</Emphasis> treated by a co-axial pulsed electric field

Inactivation of Escherichia coli CGMCC1.90 inoculated into carrot juice by a co-axial pulsed electric field was investigated and the fitting of its inactivation kinetics was performed by the Hülsheger and Peleg models. The average electric field strength ranged from 5 to 25 kV/cm and the number of pulses was from 207 to 1449 pulses. The level of E. coli inactivation increased with the increment of the electric field strength and the number of pulses. At the same specific energy input level, higher electric field strength caused higher microbial reduction. As the number of pulses increased, the kinetic constants b _E and calculated based on Hülsheger model varied from 0.3116 to 0.3790 and from 4.0565 to 1.6121 kV/cm, obtained by Peleg model from 8.0412 to 2.5676 kV/cm, but the k value changed little from 2.4213 to 2.6624, respectively. The model performance evaluation was assessed by using a series of indices including accuracy factor, bias factor, the sum of the squares of the differences of the natural logarithm of observed and predicted values, root mean square error and correlation coefficient R ² between observed and predicted values. According to these parameters, Hülsheger model fitted better to the inactivation by PEF than Peleg model in the study.     

INACTIVATION KINETICS OF SALMONELLA DUBLIN BY PULSED ELECTRIC FIELD

Microbial inactivation kinetic models are needed to predict treatment dosage in food pasteurization processes. In this study, we determined inactivation kinetic models of Salmonella dublin in skim milk with a co-field flow high voltage pulsed electric field (PEF) treatment system. Electric field strength of 15–40 kV/cm, treatment time of 12–127 μs, medium temperatures of 10–50C were tested. A new inactivation kinetic model that combines the effect of treatment time to electric field strength or medium temperature was developed.     

The influence of process parameters for the inactivation of Listeria monocytogenes by pulsed electric fields

The influence of the electric field strength, the treatment time, the total specific energy and the conductivity of the treatment medium on the Listeria monocytogenes inactivation by pulsed electric fields (PEF) has been investigated. L. monocytogenes inactivation increased with the field strength, treatment time and specific energy. A maximum inactivation of 4.77 log(10) cycles was observed after a treatment of 28 kV/cm, 2000 micros and 3490 kJ/kg. The lethal effect of PEF treatments on L. monocytogenes was not influenced by the conductivity of the treatment medium in a range of 2, 3 and 4 mS/cm when the total specific energy was used as a PEF control parameter. A mathematical model based on the Weibull distribution was fitted to the experimental data when the field strength (15-28 kV/cm), treatment time (0-2000 micros) and specific energy (0-3490 kJ/kg) were used as PEF control parameters. A linear relationship was obtained between the log(10) of the scale factor (b) and the electric field strength when the treatment time and the total specific energy were used to control the process. The total specific energy, in addition to the electric field strength and the treatment time, should be reported in order to evaluate the microbial inactivation by PEF.     

KINETICS OF INACTIVATION OF ESCHERICHIA COLI IN CARROT JUICE BY PULSED ELECTRIC FIELD

The inactivation kinetics of Escherichia coli inoculated into carrot juice by pulsed electric field (PEF) was investigated, and the experimental data were fitted to Hülsheger and Peleg models. The electric field strength ranged from 5 to 20 kV/cm, and the number of pulses was from 207 to 1449. The level of E. coli inactivation increased with the increment of the electric field strength and the number of pulses. As the number of pulses increased, the kinetic constants b_E and E_c^a (Hülsheger model) varied from 0.2429 to 0.5778 cm/kV and from 7.1301 to 5.7842 kV/cm, respectively. The k and E_c^b obtained using the Peleg model varied from 2.3277 to 1.4725 kV/cm and from 12.2523 to 7.4755 kV/cm, respectively. The fitting performance of the two models was evaluated by using a series of indices including accuracy factor, bias factor, sum of the squares of the differences of the natural logarithm of the observed and predicted data, correlation coefficient and the root mean square error between the observed and the predicted data. A comparison among these corresponding parameters indicates that the Peleg model better describes the inactivation kinetics of E. coli by PEF than the Hülsheger model.     

Evaluation of selected mathematical models to predict the inactivation of Listeria innocua by pulsed electric fields

The inactivation of Listeria innocua ATCC 51742 by pulsed electric fields was investigated at 35, 40 and 45 kV/cm. Results indicate that at treatment times shorter than 37 μs at 40 and 45 kV/cm, and 49 μs at 35 kV/cm, there is a linear relationship between the logarithm of the survivor fraction and the treatment time. However, longer times result in an abrupt increase in the slope of the inactivation curve and in inactivation values greater than six logarithmic cycles. A model based on Weibull's survival function was used to describe microbial inactivation and then compared to a first-order kinetic model. Distribution parameters of Weibull's survival function and kinetic constant for the first-order kinetic model were calculated by fitting experimental data. Calculated mean times for microbial inactivation from Weibull's distribution were 11.55, 8.65 and 5.39 μs at 35, 40 and 45 kV/cm, respectively. The goodness-of-fit between experimental and predicted values was determined using an accuracy factor. The model based on the Weibull survival distribution provided better accuracy factors than first-order kinetics. The model based on Weibull's survival function seems promising for describing survival curves that exhibit concavity.     

Effect of refrigerated storage on ascorbic acid content of orange juice treated by pulsed electric fields and thermal pasteurization

Application of pulsed electric fields (PEF) can lead to longer shelf life of fruit juices with minimal product quality loss and good retention of fresh-like flavour. The aim of this study was to evaluate the effect of PEF and conventional pasteurization (90 °C, 20 s) on ascorbic acid content of orange juice, and to assess modifications in ascorbic acid concentration of orange juice stored in refrigeration at 2 and 10 °C for 7 weeks. The ascorbic acid degradation rate was −0.0003, −0.0006, −0.0009 and −0.0010 μs⁻¹ for fields of 25, 30, 35 and 40 kV/cm, respectively. With selected PEF treatment (30 kV/cm and 100 μs) the shelf life based on 50% ascorbic acid losses was 277 days for the PEF-treated orange juice stored at 2 °C, while for the pasteurized juice was 90 days.     

The Impact of Thermosonication and Pulsed Electric Fields on Staphylococcus aureus Inactivation and Selected Quality Parameters in Orange Juice

The effect of thermosonication (TS) and pulsed electric fields (PEF) on inactivation of Staphylococcus aureus (SST 2.4) and selected quality aspects in orange juice was investigated. Conventional pasteurization (HTST, 94 °C for 26 s) was used as a control. TS (10 min at 55 °C) applied in combination with PEF (40 kV/cm for 150 μs) resulted in a comparable inactivation of S. aureus to that achieved by conventional HTST. TS/PEF did not affect the pH, conductivity, or °Brix and had a milder impact on the juice color than thermal treatment. Furthermore, the non-enzymatic browning index was significantly affected by HTST (P < 0.05) but not by TS and PEF. Ascorbic acid retention was almost complete after TS and PEF (96.0%), but it was substantially lower (P < 0.05) after HTST (80.5%). Residual activity of pectin methyl esterase (PME) decreased as PEF field strength and treatment time increased; however, applying TS and PEF in combination left a greater residual PME activity than HTST (12.9 vs 5.0%, respectively).     

Study on the Maillard Reaction Enhanced by Pulsed Electric Field in a Glycin�CGlucose Model System

Effects of pulsed electric field (PEF) on glycin and glucose content, browning value, and antioxidant activity of a glycin–glucose solution were explored. Results showed that at PEF intensity of 40 kV/cm, the solution’s absorbance at 420 nm was significantly increased from 0 to approximately 0.17 after 7.35-ms treatment. The temperature of PEF-treated samples was overall lower than 40 °C. It was also detected that the antioxidant activity of treated sample was increased to 39.36%. Moreover, 13.09% glycin and 50.76% glucose were consumed during the Maillard reaction in this experiment. This study indicates that pulsed electric field treatment, especially with higher intensity over 30 kV/cm, is a promising method to significantly promote the Maillard reaction in glycin–glucose solution.     

Modeling high-intensity pulsed electric field inactivation of a lipase from Pseudomonas fluorescens

The inactivation kinetics of a lipase from Pseudomonas fluorescens (EC 3.1.1.3.) were studied in a simulated skim milk ultrafiltrate treated with high-intensity pulsed electric fields. Samples were subjected to electric field intensities ranging from 16.4 to 27.4 kV/cm for up to 314.5 μS, thus achieving a maximum inactivation of 62.1%. The suitability of describing experimental data using mechanistic first-order kinetics and an empirical model based on the Weibull distribution function is discussed. In addition, different mathematical expressions relating the residual activity values to field strength and treatment time are supplied. A first-order fractional conversion model predicted residual activity with good accuracy (A_f = 1.018). A mechanistic insight of the model kinetics was that experimental values were the consequence of different structural organizations of the enzyme, with uneven resistance to the pulsed electric field treatments. The Weibull model was also useful in predicting the energy density necessary to achieve lipase inactivation.     

Optimizing critical high-intensity pulsed electric fields treatments for reducing pectolytic activity and viscosity changes in watermelon juice

A response surface methodology was used to determine the combined effect of high-intensity pulsed electric fields (HIPEF) variables such as frequency, pulse width and polarity on the inactivation of pectolytic enzymes involved in viscosity changes of juices. Pectin methylesterase (PME) and polygalacturonase (PG) activities as well as viscosity were determined in watermelon juices processed at pulse frequencies from 50 to 250 Hz and pulse widths ranging from 1.0 to 7.0 μs in monopolar or bipolar mode. Electric field strength and total treatment time were maintained constant in all treatments at 35 kV/cm and 1,000 μs. Second-order expressions were accurate enough to fit the experimental results. The great PME reduction contrasted with the low impact of HIPEF on the PG activity of watermelon juice within the range of assayed conditions. Minimal residual PME activity values (15%) were obtained by selecting pulse widths higher than 5.5 μs at 250 Hz in bipolar mode, whereas the lowest PG residual activities (60%) were achieved after applying 7.0-μs bipolar pulses at 250 Hz. Moreover, watermelon juice viscosity increased throughout the range of studied conditions. The highest viscosity observed in the juice after applying 7.0-μs bipolar pulses at 250 Hz was related to the lowest PME activities obtained in the product treated under those conditions. Hence, the HIPEF processing optimization through frequency, pulse width and polarity could contribute to assure enzymatic inactivation while keeping valuable attributes of juices.     

Inactivation of orange juice peroxidase by high‐intensity pulsed electric fields as influenced by process parameters

The inactivation of orange juice peroxidase (POD) under high‐intensity pulsed electric fields (HIPEF) was studied. The effects of HIPEF parameters (electric field strength, treatment time, pulse polarity, frequency and pulse width) were evaluated and compared with conventional heat pasteurization. Samples were exposed to electric field strengths from 5 to 35 kV cm^?1 for up to 1500 µs using square wave pulses in mono‐ and bipolar mode. Effect of pulse frequency (50–450 Hz), pulse width (1–10 µs) and electric energy on POD inactivation by HIPEF were also studied. Temperature was always below 40 °C. POD was totally inactivated by HIPEF and the treatment was more effective than thermal processing in inactivating orange juice POD. The extent of POD inactivation depended on HIPEF processing parameters. Orange juice POD inhibition was greater when the electric field strength, the treatment time, the pulse frequency and the pulse width increased. Monopolar pulses were more effective than bipolar pulses. Orange juice POD activity decreased with electric energy density input. The Weibull distribution function adequately described orange juice POD inactivation as a function of the majority of HIPEF parameters. Moreover, reduction of POD activity related to the electric field strength could be well described by the Fermi model. Copyright © 2005 Society of Chemical Industry