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.     

PULSED ELECTRIC FIELD TREATMENT OF LIQUID WHOLE EGG INOCULATED WITH SALMONELLA ENTERITIDIS

The effects of pulsed electric fields (PEF) on the inactivation of Salmonella Enteritidis inoculated into liquid whole egg (LWE) and on the physical properties and the shelf-life of LWE were studied. PEF processing conditions were 1.2 mL/s flow rate, 200 pps frequency, 2.12 μs pulse duration, 25 kV/cm electric field strength, and 250 μs total treatment time. The PEF processing caused up to 1 log₁₀ cfu/mL reduction in S. Enteritidis population in LWE. The PEF-treated samples were subjected to heat at 55C for 3.5 min to inactivate the remaining bacteria without denaturing the LWE. The combination of PEF and heat treatments led to a 4.3 log₁₀ cfu/mL reduction in S. Enteritidis population (P < 0.05 ) and caused no significant change in viscosity, electrical conductivity, color, pH, and Brix, relative to control samples (P > 0.05 ). The PEF + 55C treated LWE samples presented significantly longer shelf-life at 4C compared with the control and heat treated samples (P < 0.05 ).     

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.     

ENZYME INACTIVATION ON APPLE JUICE TREATED BY ULTRAPASTEURIZATION AND PULSED ELECTRIC FIELDS TECHNOLOGY

Apple juice was pasteurized by an ultra-high temperature treatment (UHT) at 115, 125 and 135C for 3 and 5 s, and compared with a high-voltage pulsed electric field treatment (PEF) at ranges between 33 and 42 kV/cm with frequencies of 150, 200, 250 and 300 pulses per second (pps). Enzyme inactivation and physicochemical properties of the treated juices were compared using a nontreated sample as control. The UHT treatment was more efficient in enzyme inactivation, reducing 95% the residual activity of polyphenoloxidase at the maximum temperature and time. However, a PEF treatment at 38.5 kV/cm and 300 pps combined with a temperature of 50C achieved a 70% reduction of residual PFO activity. In terms of quality characteristics as a function of physicochemical properties, color, pH, acidity and soluble solids were all less affected by PEF than by UHT when compared with the untreated juice.

PRACTICAL APPLICATIONS

Apple juice is a popular beverage worldwide and it is consumed nearly as much as orange juice. Consumers prefer fresh-squeezed fruit juices with high nutrient value and fresh-like sensory attributes. Enzymatic browning negatively impacts appearance, nutritive value and flavor of fruit juices. The use of ultra-high temperature processing is efficient in microbial control, as well as in enzyme inactivation. Any thermal processing may, however, decrease the overall quality of the treated juices. Pulsed electric field processing provides a potential alternative to thermal pasteurization of fruit juices.     

AQUEOUS EXTRACTION OF SOLUTES FROM FENNEL (FOENICULUM VULGARE) ASSISTED BY PULSED ELECTRIC FIELD

Aqueous nonthermal extraction was performed from thin and coarse fennel gratings of seven different sizes in order to obtain extracts, which could be used as natural food preservatives (antioxidants). A moderate pulsed electric field (PEF) with different intensities, E =  0–600 V/cm, and number of pulses, n =  0–850, was used to electropermeabilize the cell membranes and to accelerate the following extraction. The optimal parameters of PEF were found for each size of grating to maximize the yield of solutes. Extract turbidity and color were evaluated.
Additionally, the thermal extraction from coarse gratings was studied at different temperatures (60–90C). The concentration of vitamins C and E in extracts, obtained with thermal and PEF treatments, was estimated.
The PEF treatment was more effective for coarse gratings (the juice yield increased from 60 to 98%, and the obtained extracts were less turbid and less colored) than for thin gratings.
Furthermore, extracts obtained from coarse gratings treated by PEF contained more vitamins C and E than extracts obtained by thermal extraction.

PRACTICAL APPLICATION

Researches presented in this article should find their entire practical application in the field of food industries. They will permit preparation of fennel extracts, used as food preservatives (antioxidants), by PEF-assisted aqueous extraction, which preserves better antioxidant substances. Extracts should be of good quality and the energy consumption should be reasonable.     

WATER ACTIVITY AND THE INACTIVATION OF ENTEROBACTER CLOACAE INOCULATED IN CHOCOLATE LIQUOR AND A MODEL SYSTEM BY PULSED ELECTRIC FIELD TREATMENT

Effects of water activity (a_w) on the inactivation of Enterobacter cloacae inoculated in chocolate liquor and in a model system of 0.1% (w/v) peptone water and glycerol by pulsed electric field (PEF) treatment were investigated. An electric field strength of 24.5 kV/cm, a total treatment time of 320 μs, a pulse duration time of 4 βs, a pulse delay time of 15 βs, and a pulse cycle time of 15 s were selected for PEF treatment. The inactivation ofE. cloacae by PEF increased significantly as a^w increased (P < 0. 05). As a^w of chocolate liquor increased from 0.48 to 0.89, the log reduction of E. cloacae increased from 0.1 to 1.3. The measured temperature change inside the PEF treatment chamber was 0.4C when the log reduction was 1. 3. Similarly, as a^w increased from 0. 51 to 0.91 in the model system, the log reduction increased from 0.4 to 1.3. E. cloacae surviving a low a_w environment had high resistance to PEF. PEF inactivated E. cloacae in the chocolate liquor with a_w of 0.85 by 1 log at O h incubation. However, the log reduction was only 0.1 when PEF treatment was applied to E. cloacae which was incubated for 2 h in the chocolate liquor with a_w of 0.85 before PEF treatment. E. cloacae surviving the low a_w environment might have resistance not only to the low a_w but also to PEF. The resistance to low a_w environment may need to be considered when the inactivation of microorganisms by PEF is evaluated.     

INACTIVATION OF SELECTED MICROORGANISMS AND PROPERTIES OF PULSED ELECTRIC FIELD PROCESSED MILK

Raw skim milk and ultra-high-temperature (UHT) skim milk inoculated with Pseudomonas fluorescens, Lactococcus lactis and Bacillus cereus were processed by pulsed electric field (PEF) treatment. A continuous PEF bench scale system was set to deliver 35 kV/cm field strength with 64 pulses of bipolar square wave for 188 μs. The flow rate of milk was 1 mL/s. Milk temperature was raised to 52C and cooled to 22C during PEF treatment. Pasteurization at 73C for 30 s was used for comparison. Microbial inactivation and cell morphology were investigated. Analyses for protein, total solids, color, pH, particle size, density and electrical conductivity showed no significant effects (P > 0.05) in the PEF processed milk. PEF treatment accomplished a 0.3 to 3.0 log reduction of P. fluorescens, L. lactis and B. cereus in UHT milk and of total microorganisms in raw milk.     

INACTIVATION OF ESCHERICHIA COLI SUSPENDED IN LIQUID EGG USING PULSED ELECTRIC FIELDS

Liquid egg inoculated with Escherichia coli was exposed to a 26kV/cm pulsed electric field with 2 and 4 μs pulse duration, 1.25 and 2.50 Hz pulsing rates, up to 100 pulses/unit volume, and stepwise and continuous recirculation treatment schemes while maintaining a bulk temperature below 37C. The inactivation of E. coli was a function of the pulse duration and the number of pulses. The destruction of Escherichia coli in liquid egg followed a first order kinetic and the treatment was more effective when the applied pulses were of a 4 μs pulse duration. A 6D reduction was obtained for viable E. coli using both pulsing rates and treatment schemes with no protein coagulation.     

PROCESSING OF YOGURT-BASED PRODUCTS WITH PULSED ELECTRIC FIELDS: MICROBIAL, SENSORY AND PHYSICAL EVALUATIONS

Yogurt-based products similar to a dairy pudding dessert were formulated and processed by mild heat and pulsed electric fields (PEF) to investigate the effects of combined mild heat and PEF treatment on the microbial stability and quality of high viscosity foods. Commercial plain low fat yogurt was mixed with fruit jelly and corn syrup and processed by mild heat treatment at 60C for 30 s and 30 kV/cm electric field strength for 32 μs total treatment time using OSU-2C pilot plant scale PEF system. Control and processed products were aseptically packaged and stored at 4 and 22C. Mild heat combined with PEF treatment significantly decreased the total viable aerobic bacteria and total mold and yeast of yogurt-based products during storage at both 4 and 22C (P ≤ 0.05). Mild heat treatment alone without any PEF treatment did not prevent the growth of microorganisms in yogurt-based products. Sensory evaluation indicated that there was no significant difference between the control and processed products (P ≤ 0.05). Color, pH and °Brix were not significantly affected by mild heat and PEF processing conditions.     

REDUCTION OF BACTERIAL LEVELS IN FLOUR BY PULSED ELECTRIC FIELDS

Pulsed electric fields (PEF) were tested for efficacy in bacterial reduction in dark rye flour. Field strengths greater than 20kV/cm resulted in approximately 0.6-log reduction of aerobic plate counts. Bactericidal effect was related to field strength, pulse polarity, pulse width, and pulse number. Changing pulse period and altering gas atmosphere did not improve bactericidal effect. Reversing polarity pulses were more effective than synchronized bipolar pulses.     

INACTIVATION OF ESCHERICHIA COLI O157:H7 BY THE COMBINATION OF ORGANIC ACIDS AND PULSED ELECTRIC FIELD

Inactivation of Escherichia coli O157:H7 by pulsed electric field, benzoic or sorbic acid, alone or in combination was investigated. When the cells of E. coli O157:H7 were suspended in 10% glycerol and treated with a single highvoltage (12.5 kv/cm) electric pulse at 25C, the count decreased by 1.1–1.6 log₁₀ CFU/mL. Presence of benzoic and sorbic acid (1000 ppm) in the suspending medium, at pH 3.4 without electric treatment, decreased the count by 1.9–2.5 and 0.6–1.1 log, respectively. A synergistic killing effect between the high electric pulse and organic acid was observed at pH 3.4, but not at pH 6.4. When the cell suspension of E. coli O157:H7 was treated with five electric pulses in the presence of benzoic or sorbic acid at pH 3.4, the count decreased by 5.6 and 4.2 logs, respectively. Inactivation of the pathogen by combinations of electric pulse and organic acid was enhanced by an increase in temperature, field strength, and number of electric pulses, Inactivation was greater when the cells were suspended in ionic suspending media (0.1% NaCl or 5mM phosphate buffer) than in nonionic media (10% glycerol or 1% sucrose) .     

EFFECT OF CENTRIFUGAL FORCE ON THE AQUEOUS EXTRACTION OF SOLUTE FROM SUGAR BEET TISSUE PRETREATED BY A PULSED ELECTRIC FIELD

In this article, the centrifugal aqueous extraction of solute from sugar beet tissue is investigated at ambient temperature after a pulsed electric field (PEF) treatment. Two kinds of samples of fresh sugar beet were used: a sample with a determined discoid shape and gratings. Both samples were pretreated by a PEF with 250 rectangular pulses of 100 µS each. The PEF intensity was fixed at 940 V/cm for the disk samples and 670 V/cm for gratings. The pretreated samples were placed in distilled water at ambient temperature with a water‐to‐solid ratio equal to 3 and subjected to different centrifugal accelerations. The centrifugal field significantly enhanced the kinetics of extraction from the electrically pretreated tissues of sugar beet. However, the increase of centrifugal acceleration was only effective up to a certain value (5430 × g for disk samples and 600 × g for gratings). The centrifugal extraction can be assumed to proceed in two stages: a first rapid washing followed by a slow diffusion stage. A two‐exponential kinetics model taking into account these two stages was applied and correctly described the centrifugal extraction from beet samples (disks and gratings).     

INACTIVATION OF ESCHERICHIA COLI O157:H7 DURING THE STORAGE UNDER REFRIGERATION OF APPLE JUICE TREATED BY PULSED ELECTRIC FIELDS

The sensitivity of pulsed electric fields (PEF)‐treated E. coli O157:H7 cells to subsequent holding in apple juice has been evaluated. Escherichia coli O157:H7 cells in apple juice were resistant to PEF. A PEF treatment of 400 µs at any electrical field strength was not sufficient to inactivate one log₁₀cycle of cells. However, PEF injured a large proportion of E. coli O157:H7 cells that became sensitive to a subsequent storage under refrigeration in apple juice. The total lethal effect of the combined process depended on the electrical field strength and storage time. The combination of a PEF treatment at 25 kV/cm for 400 µs and a subsequent storage of the apple juice under refrigeration for 48 h allowed five log₁₀cycles of inactivation to be achieved. The combination of PEF and maintenance under refrigeration has been demonstrated to be an effective pasteurization method, by sufficiently reducing the presence of E. coli O157:H7 in apple juice in order to meet U.S. FDA recommendations.