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.     

INACTIVATION OF LISTERIA INNOCUA AND PSEUDOMONAS FLUORESCENS BY PULSED ELECTRIC FIELDS IN SKIM MILK: ENERGY REQUIREMENTS

Pulsed electric field energy applied over a short duration of time was effective in the inactivation of Listeria innocua and Pseudomonas fluorescens inoculated into 0.2% skim milk. Additionally, the energy consumption was reasonable for industry applications compared with the alternative of thermal pasteurization. The energy densities required to achieve three log reductions of the microorganisms were 120, 212 and 270 kJ/L for L. innocua corresponding to input voltages of 30, 35 and 40 kV, and 88, 105 and 128 kJ/L for P. fluorescens under the same input conditions. Treatment times were, respectively, 145 µs and 290 µs, and exponentially decaying wave pulses with time duration of 3 µs were selected. For L. innocua, the inactivation of viable cells was significantly different (P < 0.05) between energy inputs of 120, 212 and 270 kJ/L. Meanwhile, the inactivation of P. fluorescens exhibited significant differences (P < 0.05) between energy inputs of 88 and 128 kJ/L, but not between inputs of 105 and 128 kJ/L. These results consistently indicated that microbial inactivation in skim milk increased as the energy intensity and the treatment time increased.     

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.     

INACTIVATION OF ESCHERICHIA COLI AND BACILLUS SUBTILIS SUSPENDED IN PEA SOUP USING PULSED ELECTRIC FIELDS

Pea soup inoculated with 10⁷ CFU/mL of Escherichia coli, Bacillus subtilis, or a mixture of the two organisms was treated with pulsed electric fields (PEF) of selected intensities of 25, 28, 30, and 33 kV/cm. The pulsing rate was adjusted to 2.9, 4.3, or 6.7 Hz to achieve 10 or 15 pulses per pass while using either 0.5 L/min or 0.75 L/min flow rate in a continuous treatment chamber. Two passes were selected to reach 20 or 30 pulses under selected conditions. Inactivation of Escherichia coli and Bacillus subtilis suspended in pea soup increased with increases in intensity of the electric field, number of pulses, and pulsing rate. A reduction of 6.5D was obtained at 33 kV/cm, 0.5 L/min, 4.3 Hz, and 30 pulses with E. coli and 5.3D for B. subtilis, each microorganism alone. Meanwhile, reductions of up to 4.8D were observed when the pea soup containing a mixture of the microorganisms was treated with PEF of 30 kV/cm at 6.7 Hz and 0.75 L/min. In general, pea soup inoculated with a mixture of the microorganisms and exposed to PEF greater than 30 kV/cm and bulk temperatures over 53C exhibited inactivation greater than 2.0D while less than 1.6D was observed when the temperature was below 53C.     

INACTIVATION of E. COLI FOR FOOD PASTEURIZATION BY HIGH-STRENGTH PULSED ELECTRIC FIELDS

Pulsed electric fields of very high field strength and short duration are effective in the inactivation of E. coli. Nine log reduction in E. coli viability was achieved using a stepwise pulsed electric field treatment where E. coli suspensions were treated repeatedly in batches. It was demonstrated that high-strength pulsed electric field treatment is adequate for pasteurization of liquid foods.
A 40,000 volt pulse generator was constructed to supply high voltage electric pulses to a treatment chamber with two parallel plate stainless steel electrodes where fluid food was contained. the gap between electrodes was 0.51 cm and the chamber volume was 14 ml. Pulse electric field strength ranged from 35 to 70 kV/cm. Pulse width was selected at 2 μs. Number of pulses per treatment varied from 1 to 80.
E. coli were suspended in a simulated milk ultra-filtrate (SMUF) and treated with pulsed electric fields in a batch mode. the suspension fluid was maintained at constant temperatures of 7, 20, or 33C. Maximum temperature change occurring during each pulse was 0.3C measured by a fiber optics temperature probe. E. coli viability before and after treatment were assayed by counting colony forming units (cfu).     

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.     

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) .     

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

The aim was to describe the inactivation kinetics of Escherichia coli O157:H7 suspended in apple juice after pulsed electric fields (PEF) and a subsequent storage under refrigeration. Escherichia coli O157:H7 showed a great PEF resistance in apple juice, when survivors were evaluated immediately after PEF. However, PEF-treated cells exhibited a great sensitivity to a subsequent holding in apple juice for 3 days. For instance, although a PEF treatment of 80 pulses at 35.0 kV/cm inactivated less than 0.5 log₁₀ cell cycles, the maintenance of the samples up to 3 days at 4C caused an inactivation of 5.0 log₁₀ cycles. An equation based on the Weibullian-like distribution accurately described the kinetics of cell inactivation.

PRACTICAL APPLICATIONS

The storage time influences the pulsed electric fields (PEF) inactivation of Escherichia coli O157:H7 cells suspended in apple juice. The potential of Weibullian-like distributions to describe survival curves with deviations in their linearity has allowed us to obtain an equation that accurately describes the complete PEF survival profile of E. coli in apple juice, when survivors were evaluated immediately after PEF and also after a subsequent storage under refrigeration. These results underline the possibility of applying PEF to pasteurize acidic foods by taking into account the postprocessing effect of the acidity of the product.     

EFFECT OF HIGH-INTENSITY PULSED ELECTRIC FIELDS ON SURVIVAL OF ESCHERICHIA COLI K-12 SUSPENDED IN MEAT INJECTION SOLUTIONS

Survival of Escherichia coli K12 suspended in solutions used for enhancing meat products after high‐intensity pulsed electric field (PEF) treatments was examined. Solutions were prepared to provide 0.3% salt, 0.3% phosphate and/or 0, 1, 2 or 3% sodium lactate in a finished product enhanced to 110% of initial weight. Therefore, enhancement solutions contained water, 3% NaCl, 3% phosphate and/or 0, 10, 20 or 30% sodium lactate. Samples containing 0.1% NaCl were run as controls. Single PEF strengths used for PEF treatments were 12.5, 7.0, 6.0, 5.0 and 3.5 kV/cm. For 12.5, 7.0 and 6.0 kV/cm, two levels of pulse controller resistance were used, ∝ and 200 ohm; for 5 and 3.5 kV/cm, only 200 ohm was tested. Above 7 kV/cm, arcing occurred which limited the application of this technology in these solutions. Electrical field strength at 7 kV/cm with a pulse controller resistance of 200 ohm resulted in a reduction of about 2 log cfu/mL when cells were suspended in the enhancement solution containing no lactate. Lower electrical field strengths (≥6 kV/cm) were generally ineffective.     

INACTIVATION of SACCHAROMYCES CEREVISIAE IN APPLE JUICE BY SQUARE-WAVE and EXPONENTIAL-DECAY PULSED ELECTRIC FIELDS

With equivalent electrical energy input, inactivation of microorganisms by pulsed electric fields depends on pulse waveform. Exponential-decay and square-wave pulsed electric fields were selected to treat Saccharomyces cerevisiae suspended in apple juice. A parallel-plate static treatment chamber with 25 ml volume and 0.95 cm electrode gap was used. Peak electric field and pulse electric energy input were 12 kV/cm and 260 Joules per pulse for both waveforms. Both waveforms were found effective in the microbial inactivation. However, inactivation of S. cerevisiae treated with square-wave pulses was greater than yeast treated with exponential-decay pulses. For the purpose of food pasteurization, square-wave pulsed electric fields may result in significant energy savings compared to exponential-decay pulses.     

EFFICACY OF PULSED ELECTRIC FIELDS FOR LISTERIA MONOCYTOGENES INACTIVATION AND CONTROL IN HORCHATA

Although Listeria monocytogenes is readily destroyed by thermal treatment, the factor that makes it particularly difficult to control in nonpasteurized foods is its ability to grow at refrigeration temperatures. In heat‐sensitive products, nonthermal technologies such as pulsed electric fields (PEF) as part of hurdle technology could minimize the presence of foodborne pathogens. The influence of PEF‐treatment conditions, inoculum size and substrate conditions on the inactivation and recovery of L. monocytogenes in a traditional low‐acid, vegetable beverage was investigated. The combined effect of PEF, low temperature (5C) and low inoculum level contributed to slow down the recovery of sublethally injured cells. However, at 12 or 16C, this elongation of the lag phases after PEF treatment observed for low inoculum levels of cells was not achieved. Therefore, to prevent the development of L. monocytogenes in low‐acid products by PEF, it may be necessary to combine it with low refrigeration temperatures during distribution and storage, as well as to achieve a very low initial contamination by pathogens in the raw ingredients.     

高压脉冲电场杀菌机理及影响因素研究新进展及相关问题探讨

高压脉冲电场（PEF）杀菌技术有很好的处理效果,但存在着处理装置成本高、作用影响因素复杂、影响原因不明确等问题,要促进其向产业化应用的发展,必须明确影响机制,促进该技术应用的规范化,保证产品安全性.本文对高压脉冲电场杀菌技术的作用机理、杀菌作用影响因素及相关问题进行了综述与探讨.     

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 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 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.     

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.