Comparative study on color,viscosity and related enzymes of tomato juice treated by high-intensity pulsed electric fields or heat

The effects of high intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1,500 μs using bipolar 4-μs pulses at 100 Hz) on color parameters and viscosity, as well as peroxidase (POD), pectin methylesterase (PME) and polygalacturonase (PG), were evaluated during 77 days of storage at 4 °C and compared to thermal treatments at 90 °C for 1 min or 30 s for unprocessed tomato juice. HIPEF-treated tomato juice showed higher values of lightness than the thermally processed and the untreated juice throughout storage time (P < 0.05). Viscosity of HIPEF-treated tomato juice was also greater than both thermally treated and untreated for the first 35 days of storage. POD of HIPEF-treated tomato juice was inactivated by 97% whereas in the case of the thermally treated, 90 and 79% inactivation was achieved after 1 min and 30 s, respectively. The highest PME inactivation in tomato juice was obtained by PEF (82%) and heat treatment at 90 °C for 1 min (96%). PG of PEF-treated tomato juice was inactivated by 12% whereas thermal treatments at 90 °C for 1 min or 30 s achieved 44 and 22%, respectively. Despite the low rates of PG inactivation obtained, the pattern followed in the residual activity along the storage time was similar in the tomato juice treated by HIPEF than the thermally processed.     

Changes on flavor compounds throughout cold storage of watermelon juice processed by high-intensity pulsed electric fields or heat

The application of HIPEF processing (35 kV/cm for 1727 μs using bipolar pulses of 4-μs at 188 Hz) on watermelon juice was evaluated as an alternative to conventional heat treatments (90 °C for 30 s or 90 s) in order to achieve better preservation of watermelon aroma compounds for 56 days of storage at 4 °C. HIPEF processing not only induced a rise (roughly 20%) in the concentrations of hexanal, (E)-2-nonenal, nonanal, 6-methyl-5-hepten-2-one and geranylacetone but also achieved less reductions on the retention of volatiles than the thermal treatment at 90 °C for 60 s. In contrast, the content of (Z)-6-nonenal, 1-nonanol and (Z)-3-nonen-1-ol in the untreated and processed juices remained unchanged after processing. Despite the decrease in overall flavor compounds observed during storage irrespective of the treatment applied, HIPEF-treated juices showed better flavor retention than heat-treated samples for at least 21 days of storage. Moreover, changes in aldehydes and ketones during storage of treated watermelon juices were well fitted by a model based on the Weibull distribution function. Therefore, the application of HIPEF may be appropriate to preserve the initial volatile profile of watermelon juices during storage.     

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.     

Changes in quality attributes throughout storage of strawberry juice processed by high-intensity pulsed electric fields or heat treatments

The effects of high-intensity pulsed electric field (HIPEF) processing (35 kV/cm for 1700 μs applying 4-μs pulses at 100 Hz in bipolar mode) on color, viscosity and PME and PG activities in strawberry juice were studied and compared to those of heat treatments (90 °C for 60 s or 30 s) through 63 days of storage. L^∗ and viscosity values of the HIPEF-processed juices were higher than those found in the thermally treated. In addition, HIPEF-treated juice exhibited lower 5-(hydroxymethyl)-2-furfural (HMF) concentration and browning index than heat-treated juices throughout storage. On the other hand, HIPEF-treated juice maintained low residual pectin methylesterase (PME) activity (13.1%) for 63 days, whereas in the case of the thermally treated, 22.2 and 48.8% was retained after 60 s and 30 s, respectively. Strawberry juice treated by HIPEF achieved lower residual polygalacturonase (PG) activity (73.3%) than those of heat-processed at 90 °C for 60 s (76.2%) or 30 s (96.8%). Thus, HIPEF could be a feasible alternative to thermal processing to minimize browning and viscosity loss in strawberry juice during storage.     

Impact of high-intensity pulsed electric field variables affecting peroxidase and lipoxygenase activities of watermelon juice

The effects of pulse frequency (50-250 Hz), pulse width (1.0-7.0 μs) and polarity (monopolar or bipolar) of high-intensity pulsed electric field (HIPEF) treatments (35 kV/cm and 1000 μs total treatment time) on peroxidase (POD) and lipoxygenase (LOX) activities were evaluated using a response surface methodology. Second-order expressions were accurate enough to fit experimental results. HIPEF bipolar treatments resulted to be more effective than monopolar treatments in reducing POD and LOX activities. Watermelon juice LOX was more resistant to HIPEF than POD within the range of assayed conditions. HIPEF treatments set at 50 Hz for 1.0 μs pulse width could attain minimum residual POD activity values up to 10%, whereas the highest POD activity reduction was reached by combining high frequencies and pulse widths. Thus, POD could be totally inactivated by applying 7.0-μs bipolar pulses at frequencies higher than 114 Hz. In addition, the effect of frequency on the LOX activity was highly affected by the pulse width of the treatment. Thus, treatments conducted at 220 Hz with bipolar pulses of 1.0 μs led to the lowest residual LOX activity (50%).     

Inactivation of tomato juice peroxidase by high-intensity pulsed electric fields as affected by process conditions

A response surface was used to establish the high-intensity pulsed electric fields (HIPEF) conditions in processing tomato juice to obtain the greatest peroxidase (POD) inactivation. Keeping constant the electric field strength at 35 kV/cm and the temperature below 35 °C, the treatments were set at pulse frequency from 50 to 250 Hz, pulse width from 1 to 7 μs and treatment time from 1000 to 2000 μs, using monopolar or bipolar mode. The effect of these parameters on POD inactivation was evaluated through a second order model that adequately fitted the experimental data (p = 0.0001), with a determination coefficient (R²) of 0.85. HIPEF treatment resulted to be more effective in bipolar than monopolar mode to reduce POD activity and the longer the treatment time, the greater the reduction on the enzyme activity. A pulse frequency of 200 Hz was enough to reach a minimum value of residual POD activity. The significant interaction term pulse frequency and treatment time was included in the model, showing that different combinations of both variables can lead to the same level of residual POD activity. The effect of pulse width was enhanced by using a bipolar mode, being feasible to maximize POD inactivation selecting pulse width higher than 5.5 μs in bipolar mode.     

Inactivation of Saccharomyces cerevisiae suspended in orange juice using high-intensity pulsed electric fields

Saccharomyces cerevisiae is often associated with the spoilage of fruit juices. The purpose of this study was to evaluate the effect of high-intensity pulsed electric field (HIPEF) treatment on the survival of S. cerevisiae suspended in orange juice. Commercial heat-sterilized orange juice was inoculated with S. cerevisiae (CECT 1319) (10(8) CFU/ml) and then treated by HIPEFs. The effects of HIPEF parameters (electric field strength, treatment time, pulse polarity, frequency, and pulse width) were evaluated and compared to those of heat pasteurization (90 degrees C/min). In all of the HIPEF experiments, the temperature was kept below 39 degrees C. S. cerevisiae cell damage induced by HIPEF treatment was observed by electron microscopy. HIPEF treatment was effective for the inactivation of S. cerevisiae in orange juice at pasteurization levels. A maximum inactivation of a 5.1-log (CFU per milliliter) reduction was achieved after exposure of S. cerevisiae to HIPEFs for 1,000 micros (4-micros pulse width) at 35 kV/cm and 200 Hz in bipolar mode. Inactivation increased as both the field strength and treatment time increased. For the same electric field strength and treatment time, inactivation decreased when the frequency and pulse width were increased. Electric pulses applied in the bipolar mode were more effective than those in the monopolar mode for destroying S. cerevisiae. HIPEF processing inactivated S. cerevisiae in orange juice, and the extent of inactivation was similar to that obtained during thermal pasteurization. HIPEF treatments caused membrane damage and had a profound effect on the intracellular organization of S. cerevisiae.     

Changes in viscosity and pectolytic enzymes of tomato and strawberry juices processed by high-intensity pulsed electric fields

The effects pulse frequency (50–250 Hz), pulse width (1.0–7.0 μs) and polarity (monopolar or bipolar) of high-intensity pulsed electric field treatments (35 kV cm⁻¹ and 1000 μs) on viscosity and the pectin methylesterase (PME) and polygalacturonase (PG) activities were evaluated using a response surface methodology. Second-order expressions were accurate enough to fit experimental results. Tomato juice apparent viscosity increased within the range of the assayed conditions, achieving the highest values at 250 Hz and 7.0 μs in bipolar mode. At the same conditions the lowest residual PME (RA_PME = 10%) and PG (RA_PG = 45%) activities were observed in the juice. Apparent viscosity of strawberry juices slightly rose when frequencies higher than 100 Hz and 1-μs monopolar pulses were applied to the juice. Treatments causing the greatest increase in strawberry juice apparent viscosity also led to the lowest RA_PME (10%) and RA_PG (75%) values. In contrast, viscosity loss was promoted under the rest of assayed HIPEF conditions despite the low RA_PME values (<20%) achieved. Moreover, RA_PG did not decrease below 75% throughout the range of studied conditions.     

Flavour retention and related enzyme activities during storage of strawberry juices processed by high-intensity pulsed electric fields or heat

The effects of high-intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1700 μs using pulses of 4 μs at 100 Hz in bipolar mode) and thermal treatments (90 °C for 30 s or 60 s) on lipoxygenase (LOX) and β-glucosidase (β-GLUC) activities as well as on the production of volatile compounds were assessed in strawberry juice for 56 days of storage. HIPEF-treated juice kept higher residual LOX activity than heat-treated juices during the first 28 days of storage. Moreover, β-GLUC increased its initial activity just after HIPEF processing. The concentration of DMHF in HIPEF-processed strawberry juice was above those of untreated and heat-treated juices during the first 14 days of storage. On the other hand, concentrations of ethyl butanoate and 1-butanol obtained after HIPEF processing were better maintained than after thermal processing. However, thermally-treated samples showed an increase in the amount of 1-butanol beyond day 35, causing an unpleasant flavour to the product. Thus, flavour stability in HIPEF-processed strawberry juice was greater than in thermally-treated samples during storage.     

Comparative study on shelf life of orange juice processed by high intensity pulsed electric fields or heat treatment

The effects of high intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1,000 μs; bipolar 4-μs pulses at 200 Hz) on the microbial shelf life and quality-related parameters of orange juice were investigated during storage at 4 and 22 °C and compared to traditional heat pasteurization (90 °C for 1 min) and an unprocessed juice. HIPEF treatment ensured the microbiological stability of orange juice stored for 56 days under refrigeration but spoilage by naturally occurring microorganisms was detected within 30 days of storage at 22 °C. Pectin methyl esterase (PME) of HIPEF-treated orange juice was inactivated by 81.6% whereas heat pasteurization achieved a 100% inactivation. Peroxidase (POD) was destroyed more efficiently with HIPEF processing (100%) than with the thermal treatment (96%). HIPEF-treated orange juice retained better color than heat-pasteurized juice throughout storage but no differences (p<0.05) were found between treatments in pH, acidity and °Brix. Vitamin C retention was outstandingly higher in orange juice processed by HIPEF fitting recommended daily intake standards throughout 56 days storage at 4 °C, whereas heat-processed juice exhibited a poor vitamin C retention beyond 14 days storage (25.2–42.8%). The antioxidant capacity of both treated and untreated orange juice decreased slightly during storage. Heat treatments resulted in lower free-radical scavenging values but no differences (p<0.05) were found between HIPEF-processed and unprocessed orange juice.     

Physical and structural changes in liquid whole egg treated with high-intensity pulsed electric fields

Liquid whole egg (LWE) is currently pasteurized through the application of heat; however, this treatment entails deleterious effects against some of the functional and technological properties of the product. In this study, the effect of high-intensity pulsed electric fields (HIPEF) processing (field strength: 19, 32, and 37 kV/cm) was compared to the traditional heat pasteurization (66 °C for 4.5 min). Different physical and structural characteristics of LWE, subjected or not to homogenization, were evaluated and compared, having the untreated LWE as a reference. Thermal treatment caused an increase in the viscosity of LWE, especially in nonhomogenized samples. HIPEF treatments did not modify the original color of LWE, whereas thermally treated samples developed an opaque appearance. LWE treated at 19 and 32 kV/cm exhibited a similar foaming capacity as fresh untreated egg, whereas thermal processing and PEF treatments of 37 kV/cm caused a substantial decrease in the foaming capacity of untreated liquid egg. Regarding the microstructure, the lipoprotein matrix appeared to be less affected by the HIPEF than by heat treatment if compared to the control. In addition, heat pasteurization had a significant impact on both the water-soluble protein content of the LWE samples (19.5% to 23.6% decrease) and the mechanical properties of the egg gels (up to 21.3% and 14.5% increase in hardness and cohesiveness, respectively). On the other hand, these parameters were not substantially affected in the HIPEF-treated samples. Heat-induced gels obtained from HIPEF-treated samples did not exhibit remarkable differences in the water-holding capacity (WHC) with respect to heat-pasteurized samples. PRACTICAL APPLICATION: The impact of high-intensity pulsed electric fields (HIPEF) processing on technological properties of liquid-whole egg was investigated and compared to that of thermal processing. Heat treatments cause a severe impact on the foaming capacity, the water-soluble protein content, and the rheological properties of liquid egg samples, whereas HIPEF treatments better preserved the food matrix structure. Microscopic observations support these results, thus suggesting that HIPEF-processing has potential application for the preservation of liquid egg through nonthermal means.     

Degradation behaviour of methamidophos and chlorpyrifos in apple juice treated with pulsed electric fields

Apple juice (13 °Brix) spiked with methamidophos and chlorpyrifos (2–3 mg/l of each compound) was treated by pulsed electric fields (PEF), and pesticide residues were quantified by gas chromatography with flame photometric detection (GC-FPD). Results showed that electric field strength (8–20 kV/cm) and pulse number (6–26 pulses) have significant effects on the degradation of methamidophos and chlorpyrifos. PEF treatment is effective for the degradation of methamidophos and chlorpyrifos residues in apple juice, and chlorpyrifos is much more labile to PEF than methamidophos. An increase in either pulse number or electric field strength could speed the degradation of methamidophos and chlorpyrifos, and the kinetics equations and related parameters quantitatively characterized the degradation behavior of the pesticides. The exponential model better fits the experimental data for all treatments than the linear model.     

Phenolic acids, flavonoids, vitamin C and antioxidant capacity of strawberry juices processed by high-intensity pulsed electric fields or heat treatments

The effect of high-intensity pulsed electric fields (HIPEF) processing (35 kV/cm for 1,700 μs in bipolar 4-μs pulses at 100 Hz) on individual phenolic compounds (phenolic acids and flavonoids), vitamin C and antioxidant capacity of strawberry juice was evaluated and compared to heat (90 °C for 60 or 30 s) and fresh juice as a reference. Although strawberry juice underwent a substantial depletion of health-related compounds with storage time irrespective of the treatment conducted, ellagic acid was enhanced. HIPEF-treated strawberry juice maintained higher amounts of phenolic acids (ellagic and p-coumaric acid) and total anthocyanins than the thermally treated juices during the storage period. Regarding the antioxidant capacity, similar DPPH and ABTS values were obtained so that differences among pasteurized juices were non significant. HIPEF processing may be a technology as effective as thermal treatments not only to achieve safe and stable juices, but also to obtain juices with a high content of antioxidant compounds.     

Comparative study on antioxidant properties of carrot juice stabilised by high‐intensity pulsed electric fields or heat treatments

BACKGROUND: The effect of high‐intensity pulsed electric field (HIPEF) processing (35 kV cm^?1 for 1500 µs using 6‐µs bipolar pulses at 200 Hz) on the antioxidant features (vitamin C, β‐carotene, total phenolic compounds and antioxidant capacity) of carrot juice as well as on peroxidase activity was investigated and compared to the observed in heat pasteurised juices (90 °C for 60 s or 30 s) having the fresh juice as a reference. RESULTS: HIPEF and heat‐treated carrot juices had higher β‐carotene and lower vitamin C contents than the untreated juices immediately after processing. The antioxidant capacity of the juices was significantly modified neither by HIPEF nor by thermal treatments. POD activity decreased drastically (≥93.3%) after processing irrespective of the treatment applied. Vitamin C and β‐carotene content decreased throughout the storage following an exponential trend (R² = 0.801–0.984) with degradation rates between 1.7 × 10^?2 and 3.5 × 10^?2 day^?1. Vitamin C and β‐carotene contents were better maintained in HIPEF‐treated than in heat‐pasteurised juices throughout the storage. Total phenolic content and the antioxidant capacity of the HIPEF‐treated juice did not substantially differ from that of the thermally treated juice for 56 days. CONCLUSION: HIPEF processing may help to achieve fresh‐like carrot juices with increased amounts of health‐related phytochemicals. Copyright © 2009 Society of Chemical Industry     

Impact of high-intensity pulsed electric fields variables on vitamin C, anthocyanins and antioxidant capacity of strawberry juice

A response surface methodology was used to determine the combined effect of HIPEF critical processing conditions on vitamin C, anthocyanins and antioxidant capacity of strawberry juice. Keeping constant the electric field strength at 35 kV/cm and the treatment time at 1000 μs, the treatments were set at frequencies from 50 to 250 Hz, pulse width from 1 to 7 μs using monopolar or bipolar mode. A second order response function covering the whole range of experimental conditions was obtained for each health-related compound. Strawberry juice antioxidant potential was affected linearly by frequency, pulse width and pulse polarity. The quadratic term of frequency and the combined effect of frequency and pulse width were also significant. HIPEF treatments conducted at 232 Hz with bipolar pulses of 1 μs led to strawberry juices with the greatest presence of health-related compounds. The evaluation of the HIPEF critical parameters influence on health-related compounds can contribute to achieve optimal processing conditions to obtain strawberry juices with high antioxidant potential.     

Inactivation of Salmonella enterica Ser. Enteritidis in tomato juice by combining of high-intensity pulsed electric fields with natural antimicrobials

ABSTRACT:  The effect of high-intensity pulsed electric field (HIPEF) treatment (35kV/cm, 4 μs pulse length in bipolar mode without exceeding 38 °C) as influenced by treatment time (200, 600, and 1000 μs) and pulse frequency (100, 150, and 200 Hz) for inactivating Salmonella enterica ser. Enteritidis inoculated in tomato juice was evaluated. Similarly, the effect of combining HIPEF treatment with citric acid (0.5%, 1.0%, 1.5%, and 2.0%[wt/vol]) or cinnamon bark oil (0.05%, 0.10%, 0.2%, and 0.3%[vol/vol]) as natural antimicrobials against S. Enteritidis in tomato juice was also studied. Higher treatment time and lower pulse frequency produced the greater microbial inactivation. Maximum inactivation of S. Enteritidis (4.184 log₁₀ units) in tomato juice by HIPEF was achieved when 1000 μs and 100 Hz of treatment time and pulse frequency, respectively, were applied. However, a greater microbial inactivation was found when S. Enteritidis was previously exposed to citric acid or cinnamon bark oil for 1 h in tomato juice. Synergistic effects were observed in HIPEF and natural antimicrobials. Nevertheless, combinations of HIPEF treatment with 2.0% of citric acid or 0.1% of cinnamon bark oil were needed for inactivating S. Enteritidis by more than 5.0 log₁₀ units (5.08 and 6.04 log₁₀ reductions, respectively). Therefore, combinations of HIPEF with organic acids or essential oils seem to be a promising method to achieve the pasteurization in these kinds of products.     

Critical electric field strengths of onion tissues treated by pulsed electric fields

The impact of pulsed electric fields (PEF) on cellular integrity and texture of Ranchero and Sabroso onions (Allium cepa L.) was investigated. Electrical properties, ion leakage rate, texture, and amount of enzymatically formed pyruvate were measured before and after PEF treatment for a range of applied field strengths and number of pulses. Critical electric field strengths or thresholds (E(c)) necessary to initiate membrane rupture were different because dissimilar properties were measured. Measurement of electrical characteristics was the most sensitive method and was used to detect the early stage of plasma membrane breakdown, while pyruvate formation by the enzyme alliinase was used to identify tonoplast membrane breakdown. Our results for 100-μs pulses indicate that breakdown of the plasma membrane occurs above E(c)= 67 V/cm for 10 pulses, but breakdown of the tonoplast membrane is above either E(c)= 200 V/cm for 10 pulses or 133 V/cm for 100 pulses. This disparity in field strength suggests there may be 2 critical electrical field strengths: a lower field strength for plasma membrane breakdown and a higher field strength for tonoplast membrane breakdown. Both critical electric field strengths depended on the number of pulses applied. Application of a single pulse at an electric field up to 333 V/cm had no observable effect on any measured properties, while significant differences were observed for n≥10. The minimum electric field strength required to cause a measurable property change decreased with the number of pulses. The results also suggest that PEF treatment may be more efficient if a higher electric field strength is applied for a fewer pulses.     

Comparative study on shelf life of whole milk processed by high-intensity pulsed electric field or heat treatment

The effect of high-intensity pulsed electric fields (HI-PEF) processing (35.5 kV/cm for 1,000 or 300 μ with bipolar 7-μs pulses at 111 Hz; the temperature outside the chamber was always < 40° C) on microbial shelf life and quality-related parameters of whole milk were investigated and compared with traditional heat pasteurization (75° C for 15 s), and to raw milk during storage at 4° C. A HIPEF treatment of 1,000 μ ensured the microbiological stability of whole milk stored for 5 d under refrigeration. Initial acidity values, pH, and free fatty acid content were not affected by the treatments; and no proteolysis and lipolysis were observed during 1 wk of storage in milk treated by HIPEF for 1,000 μ. The whey proteins (serum albumin, β-lactoglobulin, and α-lactalbumin) in HIPEF-treated milk were retained at 75.5, 79.9, and 60%, respectively, similar to values for milk treated by traditional heat pasteurization.