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.     

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.     

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 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.     

REDUCTION OF SACCHAROMYCES CEREVISIAE, ESCHERICHIA COLI AND LISTERIA INNOCUA IN APPLE JUICE BY ULTRAVIOLET LIGHT

Apple juice was inoculated separately with Saccharomyces cerevisiae, Listeria innocua (ATCC 51742) or Escherichia coli (ATCC 11775) for treatment in a double tube ultraviolet (UV) disinfection system. The apple juice was treated at six flow rates (0.073–0.548 L/min) for selected fluences (75–450 kJ/m²). The juice was also inoculated with a mixture of these three microorganisms and UV light treated from 0.548 to 0.735 L/min for 30 min. The microbial reduction was described with a first order kinetic model. Average D_uvvalues of 23.1–40.5, 8.2–20.6 and 6.0–17.7 min were obtained for S. cerevisiae, L. innocua and E. coli, respectively. A linear model was used to describe the relationship between log D_uvversus flow rate for S. cerevisiae and L. innocua. However, a third order polynomial model was more adequate for describing the E. coli D_uvvalues versus flow rate. Less than 10 (no growth), 190 and 200 cfu/mL of S. cerevisiae, L. inocua and E. coli, respectively, were observed in UV‐treated apple juice inoculated with a mixture of microorganisms.     

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.     

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 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.     

HIGH PRESSURE INACTIVATION OF SACCHAROMYCES CEREVISIAE,ENDOGENOUS MICROFLORA AND PECTINMETHYLESTERASE IN ORANGE JUICE1

Decimal reduction times (D-values) for Saccharomyces cerevisiae ascospores inoculated into pasteurized orang juice ranged from 4 to 76 s at pressures between 500 and 350 MPa. At the same pressures, D-values of S. cerevisiae vegetative cells ranged from 1 to 38 s while those for the native microflora in nonpasteurized Hamlin orange juice were between 3 and 74 s. Corresponding z-values were 123, 106 and 103 MPa for ascospores, vegetative cells and native microflora, respectively. Native microorganisms that survived high pressure treatments included yeasts, gram-positive and gram-negative bacilli. Pectinmethylesterase activity in nonpasteurized Hamlin orange juice was reduced to 5% of initial activity after 30 s at 900 MPa.     

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.     

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.     

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.     

PULSED ELECTRIC FIELD INACTIVATION of MICROORGANISMS and PRESERVATION of QUALITY of CRANBERRY JUICE

Cranberry juice was treated either by high voltage pulsed electric field (PEF) at 20 kV/cm and 40 kV/cm for 50 and 150°s, or by thermal treatment at 90C for 90s. Higher field strength and longer treatment time reduced more viable microbial cells. the overall volatile profile of the juice was not affected by PEF treatment but it was affected by thermal treatment. No Significant difference in color was observed between the control and PEF treated samples. the application of 40kV/cm for 150 1ts resulted in no growth of molds and yeasts during storage at 22 and 4C, and no growth of aerobic bacteria during storage at 4C. PEF is an alternative process to thermal pasteurization for cranberry juice.     

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.     

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.     

THE USE OF PULSED ELECTRIC FIELDS IN PRODUCING JUICE FROM PAPRIKA (CAPSICUM ANNUUM L.)

Investigations were conducted on the impact of pulsed electric field (PEF) treatment on yield and some quality parameters of juice from paprika and results were compared to the results of juice obtained from enzymatically treated or untreated paprika mash. Paprika was comminuted mechanically after which the mash was subjected to either PEF (electric field strength = 1.7 kV/cm, pulse number = 30 and specific energy input per pulse = 0.5 kJ/kg) or a pectolytic enzyme preparation treatment. The extent of cell disintegration after the different pretreatments was monitored through cell disintegration index (Z_p) measurement prior to juice extraction through pressure (10 MPa for 4 min). Some quality parameters (pH, soluble solids (°Brix), total dry matter, color, total carotenoids as β-carotene and vitamin C) of the resultant juice samples were determined. PEF and enzyme treatments resulted in about 10% and 9% increase in juice yield, respectively. Juice from PEF treated paprika compared well in quality with enzyme treated or the untreated. And in some cases such as in color (redness, a value) of the juice, PEF treated had values (+ 18) higher than enzyme treated having + 15.5 and also in the amount of β-carotene extracted into the juice PEF treated juice had more than 60% as compared to about 44% from enzyme treated juice. The pH value for enzyme treated juice was lower than the others.     

POTENTIAL USE OF 18 TESLA STATIC AND PULSED MAGNETIC FIELDS ON ESCHERICHIA COLI AND SACCHAROMYCES CEREVISIAE

Escherichia coli and Saccharomyces cerevisiae were subjected to 18T static and pulsed magnetic fields at different temperatures in either phosphate buffer, McIlvaine buffer, or peptonated water. Colony forming units were recorded after incubation in nutrient and Violet Red Bile agar for E. coli and potato dextrose agar for S. cerevisiae. No inactivation or cell injury was detected due to the influence of the magnetic field whether static or pulsed. As expected, at higher temperatures both inactivation and cell injury were increased. Although further research is needed, magnetic field technology at 18T or lower does not appear as a feasible option for processing foods.