Effects of Pulsed Electric Fields on ... - Wiley Online Library

Journal of Food Processing and Preservation ISSN 1745-4549

EFFECTS OF PULSED ELECTRIC FIELDS ON PHYSICOCHEMICAL PROPERTIES AND MICROBIAL INACTIVATION OF CARROT JUICE BOB XIANG1, SRIJANANI SUNDARARAJAN1, KEVIN MIS SOLVAL1, LUIS ESPINOZA-RODEZNO1, KAYANUSH ARYANA2 and SUBRAMANIAM SATHIVEL1,3,4 1

Department of Food Science, Louisiana State University Agricultural Center, Baton Rouge, LA School of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge, LA 3 Department of Biological and Agricultural Engineering, Louisiana State University Agricultural Center, Baton Rouge, LA 70803-4300 2

4

Corresponding author. TEL: (225)-578-0614, FAX: 225-578-5300; EMAIL: [email protected] Received for Publication July 5, 2012 Accepted for Publication February 12, 2013 doi:10.1111/jfpp.12115

ABSTRACT Carrot juice samples treated with pulsed electric fields (PEF) of electric field intensity of 25 kV/cm and treatment time of 144.6 ms (PEF1) and 433.8 ms (PEF2) and thermal pasteurization (TP) treatment of 90C for 1 min were evaluated for physicochemical properties and microbial inactivation. The PEF1 and PEF2 treated carrot juice retained higher amounts of ascorbic acid, a-carotene, b-carotene and lutein than the TP-treated carrot juice. The PEF-treated carrot juices were not significantly (P > 0.05) different in pH and °Brix compared to the control, while the TP-treated juices were significantly different in °Brix and total acidity compared to the control. The PEF2 was comparable with the TP for inactivation of aerobic bacteria and mold in carrot juice.

PRACTICAL APPLICATIONS Pulsed electric field (PEF) can be used as a nonthermal pasteurization method to inactivate microorganisms and enzymes in liquid food and it preserves the physiochemical and sensory properties, and nutritional value of liquid food. Our study showed that PEF treatment was more efficient in preserving some physicochemical and quality properties of carrot juice compared to conventional heat pasteurization.

INTRODUCTION Carrot juice is an excellent source of a-carotene, b-carotene and ascorbic acid, and a good source of dietary fiber and minerals (Senti and Rizek 1975; Marx et al. 2000). The pH of carrot juice is approximately 6.0 results in a high risk of bacterial growth. Thus, carrot juice requires a thermal pasteurization (TP) treatment (90 to 121C) for inactivation of microbial growth (Chen et al. 1995; Park et al. 2002). TP treatments prevent spoilage due to microbial growth in fruit juice; however, they may cause undesirable biochemical and nutritious changes (Park et al. 2002; Alwazeer et al. 2003; Zerdin et al. 2003; Aguilar-Rosas et al. 2007). Heat treatment may also cause color change, separation of particles and a change in flavor of juice products (Qin et al. 1995). If heat treatment is not performed rapidly or at a reasonably low temperature, the juice will begin to separate due to the 1556

destruction of pectin (Goodman et al. 2002). The formation of sediment and change in flavor of carrot juice are the major reasons for some consumers rejecting TP-treated carrot juice (Beveridge 2002; Alklint et al. 2004). Therefore, an alternative processing method, such as a nonthermal method, is needed for carrot juice processing. An increase in the demand for minimally processed fresh products has raised interest in the development of new techniques for food processing such as pulsed electric fields (PEF; Vega-Mercado et al. 1997; Aguiló-Aguayo et al. 2008). PEF treatment applies intense electric pulses which develop pores in the cell membranes of microorganisms either by enlargement of existing pores or by creation of new ones. These pores may be permanent or temporary, depending on the condition of PEF treatment. This process is known as electroporation, which increases membrane permeability, allowing loss of cell contents. The electroporation leads to

Journal of Food Processing and Preservation 38 (2014) 1556–1564 © 2013 Wiley Periodicals, Inc.

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an osmotic imbalance and ultimately cell death (Tsong 1991; Ade-Omowaye et al. 2000). PEF can be advantageously used as a nonthermal pasteurization method to inactivate microorganisms and enzymes in food (Ho et al. 1997; Hoover 1997; Giese 1998; Mermelstein 1999; Yeom et al. 2000a) because it preserves the physiochemical and sensory properties, and nutritional value of food (Mertens and Knorr 1992; Sizer and Balasubramaniam 1999; Aguilar-Rosas et al. 2007). PEF processing in both laboratory and pilot plant scale has been successfully applied to various liquid food products (Qin et al. 1995; Evrendilek et al. 2000, 2001; Yeom et al. 2000a,b; Aguilar-Rosas et al. 2007). Jia et al. (1999) and Min and Zhang (2002) have evaluated the effect of PEF on physicochemical factors in apple juice, orange juice or the mixture of carrot and orange juices. However, there is not much literature available on evaluating the effect of PEF and TP treatments on the physicochemical properties and microbial activation of carrot juice. The objective of this study was to compare physicochemical properties and microbial inactivation of carrot juice treated by PEF and TP. The °Brix, pH, total acidity, ascorbic acid, a-carotene, b-carotene, lutein, viscosity, color, browning index and microbial inactivation of carrot juice were evaluated and compared.

MATERIALS AND METHODS Extraction of Carrot Juice Three batches of fresh carrots were purchased from a local produce market in Baton Rouge, LA and stored at 4C overnight. Carrots were washed with distilled water and the juice was extracted using a laboratory scale juice processor (Juice Extractor-Model 9816, Great Cookware, Inc., Fairbury, IL). The extracted juice was then filtered using cheese cloth (Nonwovens, Simpsonville, SC). The filtered juice was sonicated (low intensity ultrasound) using a laboratory scale ultrasonic processor (Model WU-04711-70, Cole-Parmer, Inc., Vernon Hills, IL) for 45 s. To avoid overheating, the carrot juice was placed inside an ice bath during ultrasonication. The sonicated carrot juice was centrifuged at 15,000 ¥ g for 15 min (J2-HC centrifuge, Beckman Instrument, Inc., Fullerton, CA). The centrifuged juice was filtered through filter paper Whatman No. 1 and the filtrate was collected for PEF or TP treatments.

PEF and TP Treatments of Carrot Juice PEF treatments were conducted on a bench-scale continuous system (OSU-4K, The Ohio State University, Columbus, OH) as shown in Fig. 1. Four co-field treatment chambers with a diameter of 0.23 cm and gap distance of 0.29 cm were serially connected. Two cooling coils were placed

MICROBIAL INACTIVATION OF CARROT JUICE

FIG. 1. DIAGRAM OF PULSED ELECTRIC FIELD TREATMENT APPARATUS

before and after each pair of chambers, and submerged in a circulating refrigerated bath to control the sample temperature. The temperature was monitored using type T thermocouples connected to a data acquisition system, which recorded the inlet and outlet temperature of each pair of chambers every 0.1 s during PEF treatment. The temperature of the juice sample was kept at 25C. Carrot juice was processed with two different PEF treatments: PEF1 (pulse width of 3.0 ms, treatment time of 144.6 ms) and PEF2 (pulse width of 9.0 ms, treatment time of 433.8 ms) at electric field intensity of 25 kV/cm. In the study, a bipolar pulse was applied with a pulse frequency of 1000 Hz. The flow rate of carrot juice was set at 60 mL/min with a variable speed pump (model 75 210-25, Cole Palmer, Vernon Hills, IL). Pulse waveform, voltage and electric field intensity in the treatment chambers were recorded with a digital oscilloscope (THS720, Tektronix, Inc., Beaverton, OR). Carrot juices were collected after each PEF treatment in sterilized glass bottles. PEF experiments were performed in triplicate. The PEF treatment conditions for the study of the effects of electric field intensity and pulse width on the quality properties and microbial activation of the carrot juice are shown in Table 1. PEF treatment time (t) was calculated based on the number of pulses received in the treatment chambers (Np), which was obtained from residence time in one chamber (Tr), and the volume of one chamber (Vc).

Vc = π × r 2 × H Vc = 3.142 × (0.115)2 × 0.29 = 0.0120503 mL

(1)

where r is the radius of the treatment chamber (0.115 cm), H is the height of the chamber (0.29 cm).


Tr (s ) = Vc F Tr = 0.0120503 mL 1 mL s = 0.0120503 s

(2)

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Parameters

PEF1

PEF2

TP

Electric field intensity (kV/cm) Pulse number (n) Pulse width (ms) Pulse wave form Flow rate (mL/min) Temperature (C) Treatment time (min or ms)

25 300 3 square-wave 60 21 144.6 ms

25 300 9 square-wave 60 21 433.8 ms

Control

0 0

0 0

/ /

TABLE 1. CONDITIONS FOR THE STUDY OF EFFECTS ON THE QUALITY PROPERTIES OF CARROT JUICES OF PEF AND TP TREATMENTS

/ / 60 21

0 90 1 min

PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse.

where Vc is the volume of one treatment chamber (mL) and F is the flow rate (mL/s) and the flow rate is 60 mL/min.

N p = Tr × f N p = 0.0120503 s × 1, 000 pps treat

= (3)

= 12.0503 pulses where f is the pulse rate (pulse per second [pps]) and it is 1,000 pulses/s (Hz)

t = N p × N c × Pw for a pulse width of 3 μs t = 12.0503 × 4 × 3 = 144.6 μs

(4)

where Nc is the number of treatment chambers (Nc = 4) and Pw is the pulse width (ms; pulse widths of 3 ms or 9 ms were used in the study). The control for PEF treatment was centrifuged carrot juice that was passed through the PEF equipment without application of any electric field or pulses with a flow rate of 60 mL/min. A 400 mL centrifuged carrot juice was placed in a 500 mL stainless steel cup and kept in a water bath at 95C for 12 min to reach the juice temperature at 90C and then the juice was held at 90C for 1 min. Then, the thermally pasteurized carrot juice was immediately cooled to room temperature in a cold water bath.

Total Soluble Solid (°Brix), pH, Total Titratable Acidity and Ascorbic Acid of Carrot Juices The total soluble solids (°Brix) of the carrot juice samples were measured using a digital handheld refractometer (AR200, Reichert, Inc., Depew, NY). The pH of the juice was measured using a bench top pH meter (SB70P, Symphony, VMR, Inc., Beverly, MA). Total acidity of carrot juice was determined by titration, with a solution of 0.1 N NaOH up to pH = 8.1. Total acidity was calculated using the following equation, and results were expressed as g per 100 mL with reference to citric acid (Barbosa-Canovas et al. 2003; Rodrigo et al. 2003; Rivas et al. 2006). 1558

⎛ g ⎞ total acid ⎜ ⎝ 100 mL ⎟⎠ NaOH solution (mL ) × 0.1 N NaOH × 100 Carrot juice (mL )

(5)

The concentration of ascorbic acid in the carrot juice was determined according to the AOAC 967.21 standard procedure (AOAC 1999). Ninety mL of 3% metaphosphoric acid were added to the carrot juice (10 mL) and then filtered through Whatman filter paper (No. 4). An aliquot of 5 mL of filtrate was titrated with 2, 6-dichlorophenol indophenol indicator. The concentration of ascorbic acid of the carrot juice was reported as mg 100/mL.

a-Carotene, b-Carotene and Lutein of PEF1, PEF2 and TP-Treated Carrot Juices a-Carotene, b-carotene and lutein in the carrot juice were determined using an Agilent 1100 high-performance liquid chromatography (HPLC) system as described by Zhang et al. (2004) and Yue and Xu (2008). The HPLC system consists of a binary pump with a vacuum degasser and thermostated column compartment, an auto-sampler and a diode array detector (Agilent Technologies, Palo Alto, CA). Carotenoids were quantified using a reversed phase C18 Luna column (Phenomenex, Lorance, CA, USA, 150 ¥ 4.6 mm; particle size of 5 mm), preceded by a guard column (Phenomenex, 4 ¥ 3.0 mm) of the same stationary phase. The following HPLC gradients and conditions were used for the experiment: solvent A was 100% acetone and solvent B was 100% methanol. Flow rate was set to 0.7 mL/min and the column and the guard column were thermostatically controlled at 25C. The gradient elution B was changed from 100 to 30% in 10 min. Then the gradient elution B was changed from 30 to 10% in 5 min, 10% to 0 in 5 min, and B was changed from 0 to 100% in 10 min. The column was washed with 100% B for 5 min and equilibrated. Total analysis time per carrot juice sample was 30 min. HPLC chromatograms were detected using a photodiode array UV


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detector. The peaks of a-carotene, b-carotene and lutein were determined by matching their retention times and UV-vis absorption spectrum with known standards as described by Zhang et al. (2004) and Lätti et al. (2007). The concentrations of a-carotene and b-carotene and lutein were calculated using standard calibration curves.

Viscosity Measurements of Carrot Juices Viscosity of the carrot juice samples was determined using an AR 2000ex rheometer (TA Instruments, Inc., New Castle, DE) fitted with parallel plate geometry (acrylic plates with 40 mm diameter and gap of 400 mm). Viscosity was measured at a shear rate of 10/s for 3 min. Temperature was maintained at 25C and the delay time was set at 10 s for the viscosity measurements.

bottles were filled with 99 mL of 1% peptone water and then autoclaved at 121C for 45 min. Carrot juice was serially diluted to 10-1 and 10-2. One mL from each dilution was plated on the aerobic, coliform or mold/yeast Petrifilms. The coliform and aerobic Petrifilms were incubated at 37C for 24 h and 48 h, respectively. The mold/yeast Petrifilms were incubated at room temperature (20–25C) for 48 h to 72 h. After incubation, the colonies were counted and reported as cfu/mL using the equation below (Eq. 6). Both aerobic colonies and coliform appeared red on Petrifilms, but a colony was not counted as a true coliform unless there was gas formation.

cfu mL (carrot juice ) =

1 # of colonies × aliquot plated (1 mL ) × dilution factor (6)

Color and Browning Index of Carrot Juices Color of carrot juice samples was determined using a HunterLab Labscan XE colorimeter (HunterLab, Reston, VA) and reported as L*, a*, b*, chroma (C*) and hue angle (h*) values. Chroma (c*) and hue angle (h*) were calculated ⎡ b* ⎤ as [a*2 + b*2]1/2 and tan −1 ⎢ ⎥ , respectively. The browning ⎣ a* ⎦ index of carrot juice was evaluated using a spectrophotometric method described by Roig et al. (1999). Browning index of the juice was measured at the wavelength of 420 nm using a spectrophotometer (Genesys 20 spectrophotometer, Thermo Scientific Instruments, LLC, Madison, WI) at ambient temperature (25 ⫾ 1C) with a 1 cm path length cell.

Microbial Analysis Carrot juices treated by PEF, TP and control were analyzed in triplicate for aerobic bacteria by using 3M Petrifilm aerobic count Plate (3M Company, St. Paul, MN), for coliforms by using 3M Petrifilm E.coli/coliform count plate (3M Company) and for yeast/mold using 3M Petrifilm yeast and mold count plates (3M Company). Serial dilution TABLE 2. EFFECTS OF PEF AND TP TREATMENTS ON °BRIX, pH AND BROWNING INDEX OF CARROT JUICES

Statistical Analysis Mean values from the three replicate analyses were reported. The statistical significance of observed differences among treatment means was evaluated by analysis of variance and Tukey’s studentized test at the significant level of P < 0.05 using SAS software version 9.2 (SAS Version 9.1.3 for Windows, SAS Institute, Inc., Cary, NC).

RESULTS AND DISCUSSION Total Soluble Solid (°Brix), pH, Titratable Acidity and Ascorbic Acid of Carrot Juices °Brix is often used for grading the quality of fruit juices (McAllister 1980). The PEF control (Table 2) while the thermal pasteurized juice had an increase in the amount of total soluble solids (°Brix). Rivas et al. (2006) reported that the PEF-treated mixed orange and carrot juice had significant changes in °Brix value. The amounts of soluble solids present influenced the flow properties of the juice, hence it was suggested that the TP-treated juice would have higher viscosity due to the increases in the °Brix value. Schilling

Parameters

PEF1

PEF2

TP

Control

°Brix pH Browning index

9.77 ⫾ 0.07b 6.25 ⫾ 0.06a 3.38 ⫾ 0.05b

9.67 ⫾ 0.06b 6.26 ⫾ 0.04a 3.28 ⫾ 0.08b

10.33 ⫾ 0.06a 6.27 ⫾ 0.04a 3.35 ⫾ 0.07b

9.70 ⫾ 0.10b 6.25 ⫾ 0.05a 3.92 ⫾ 0.06a

Values are means ⫾ SD of triplicate determination. a–b Any two means in the same row followed by the same letter are not significantly (P > 0.05) different by Student’s t-test. PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse.


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FIG. 2. EFFECTS OF PEF AND TP TREATMENTS ON TOTAL ACIDITY OF CARROT JUICE PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse. The same letter (a or b) indicate no significant difference among the values.

et al. (2008) reported that the increase in total soluble solid content was mainly associated with changes in levels of glucose and fructose in TP-treated apple juice compared to the control. Such an increase might have occurred in the TP-treated carrot juice thereby leading to the increase in °Brix value. The pH of the carrot juice after PEF and TP treatments was the same as the control. Other studies also indicated that PEF and TP did not affect the pH in carrot juice. For example, Saldana et al. (1976) reported that pH in carrot juice was not affected during thermal pasteurization. Yeom et al. (2000b) reported no change in pH of orange juice treated by PEF (35 kV/cm, 59 ms of treatment time). Rivas et al. (2006) also reported that there was no significant change (P > 0.05) in pH of orange and carrot juices treated by PEF and thermal pasteurization. A suitable ratio of sugar and acidity of the juice is a basic requirement for good juice flavor. The total acidity of the TP-treated juice was significantly (P < 0.05) higher than the control (Fig. 2). Both control and PEF-treated juices had similar total acidity values. Akinyele et al. (1990) observed an increase in the total titratable acidity for the TP-treated orange juice. Rivas et al. (2006) reported that PEF treatments increased the total acidity of the mixed orange and carrot juices and the changes were significant. However, Zárate-Rodríguez et al. (2000) did not find variations in total acidity of PEF-treated apple juice when compared with an untreated sample, probably because it was a different juice and they used different PEF conditions. Acidity in carrot juice is an important 1560

sensory attribute associated with its characteristic flavor and astringency. In the current study, PEF treatments did not affect the total acidity of the juice compared to TP treatment, so this important feature remains practically intact with the consequent advantage in overall quality of the juice product. The concentration of ascorbic acid in the carrot juice was affected differently by each treatment. PEF1- and PEF2-treated carrot juice had higher concentrations of ascorbic acid (14.59 and 14.58 mg per 100 mL) similar to the control than the TP-treated juice (4.34 mg per 100 mL) as shown in Fig. 3. Ascorbic acid is sensitive to heat (Saguy et al. 1978) and thermal processing causes loss of ascorbic acid (Nagy and Smoot 1977). The results obtained in this study were similar to those reported by Yeom et al. (2000b) for the ascorbic acid concentration in orange juice with PEF treatment (35 kV/cm and 59 ms of treatment time) and TP treatment (94.6C and 30 s of treatment time).

a-Carotene, b-Carotene and Lutein of Carrot Juices The PEF-treated carrot juice had greater levels of acarotene (PEF1-treated juice: 37.40 mg/mL; PEF2-treated juice: 37.01 mg/mL) than the TP-treated juice had a significantly lower level of a-carotene (12.99 mg/mL as shown in Fig. 4). Both PEF treated juices were closer to the control level of alpha-carotene (12.99 microgram per mL) than the TP treated juice. PEF treated juices had 6.3–7.3% less a-carotene compared to the control while the TP-treated

FIG. 3. EFFECTS OF PEF AND TP TREATMENTS ON THE CONCENTRATION OF ASCORBIC ACID IN CARROT JUICE PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse. The same letter (a or b) indicates no significant difference among the values.


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FIG. 4. EFFECTS OF PEF AND TP TREATMENTS ON Α-CAROTENE, Β-CAROTENE AND LUTEIN CONTENT OF CARROT JUICE PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse. The same letter (a or b) indicates no significant difference among the values.

FIG. 5. EFFECT OF TREATMENT METHODS ON APPARENT VISCOSITY OF CARROT JUICE PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse. The same letter (a or b) indicates no significant difference among the values.

juice had 67.5% less. Similarly, PEF-treated juice had a significantly greater level of b-carotene (PEF1-107.00 mg/mL; PEF2-106.27 mg/mL) than the TP-treated juice (39.44 mg/ mL). The control had a b-carotene level of 112.06 microgram/mL. The total b-carotene level of PEF-treated juice was 4.5–5.2% less than the control while that of TP-treated juice 64.8% less than the control. Similar results were observed for lutein content of the carrot juice. The TP-treated juice had significantly lower lutein (0.31 mg/mL) content compared to the PEF-treated juice (PEF11.86 mg/mL and PEF2-1.83 mg/mL), which was closer to the control (1.91 mg/mL) than the TP-treated juice as shown in Fig. 4. Both control and PEF-treated juices had similar lutein contents, while TP-treated juice had significantly lower levels of lutein. This study showed that the carotenoids in carrot juice could be preserved better by PEF processing than by TP.

Viscosity of Carrot Juices

TABLE 3. EFFECTS OF PEF AND TP TREATMENTS ON COLOR CHANGES OF CARROT JUICES

PEF and control juices had a similar viscosity (Fig. 5). Reiter et al. (2003) reported similar results, which indicated that PEF treatments did not change the viscosity of the carrot juice. The viscosity of TP-treated juice (1.54 ¥ 10-3 Pa·s) was significantly (P < 0.05) higher than that of the control (1.34 ¥ 10-3 Pa·s), which could be attributed to an increase in solubilization of pectin in the carrot juice. The TP treatment increased the total soluble solids which might also account for the increase in viscosity.

Color and Browning Index of Carrot Juices The color of the PEF-treated juice was not significantly different from the control in terms of a*, b*, chroma and hue angle values as shown in Table 3. The L* value of the PEF-

L* PEF1 PEF2 TP Control

a*

40.02 ⫾ 0.39 40.37 ⫾ 0.21b 41.61 ⫾ 0.18a 39.46 ⫾ 0.34c b

b*

33.52 ⫾ 0.21 33.49 ⫾ 0.21b 33.94 ⫾ 0.05a 33.35 ⫾ 0.23b b

Chroma

51.24 ⫾ 0.18 51.45 ⫾ 0.78a 49.93 ⫾ 0.17b 50.99 ⫾ 0.25a a

Hue angle

61.26 ⫾ 0.25 61.38 ⫾ 0.69a 60.36 ⫾ 0.16b 60.92 ⫾ 0.14ab a

61.49 ⫾ 0.18a 61.46 ⫾ 0.28a 60.57 ⫾ 0.07b 61.55 ⫾ 0.24a

Values are means ⫾ SD of triplicate determination. a–c Any means in the column followed by the same letter are not significantly (P > 0.05) different by Student’s t-test. PEF1 = PEF treatment at 25 kV/cm and treatment time of 144.6 ms; PEF2 = PEF treatment at 25 kV/cm and treatment time of 433.8 ms; TP = TP treatment at temperature 90C for 1 min; control = carrot juice that passed through the PEF equipment at the same flow rate of 60 mL/min but with no electric field intensity and no pulse.


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Treatment methods

Coliforms (cfu/mL)

Aerobic bacteria (cfu/mL)

Yeast (cfu/mL)

Molds (cfu/mL)

PEF1 PEF2 TP Control

135 ⫾ 12b 9 ⫾ 3.5c 0 197 ⫾ 4a

4 ¥ 102 ⫾ 2a 1 ¥ 102 ⫾ 0b 1 ¥ 102 ⫾ 0b 5 ¥ 102 ⫾ 1a

0 0 0 0

0 0 0 2⫾0

Values are means ⫾ SD of triplicate determinations. column are significantly different (P < 0.05).

abc

means with different letters in each

treated juice (40.20) showed a slight increase compared to the control (39.46). The color of the TP-treated juice was significantly (P < 0.05) different from the control. The TP-treated juice had a higher L* value (41.61) and a* value (33.94) while it had a lower b* value (49.93), chroma value (60.36) and hue value (60.57) compared to the control. PEF treatments better preserved the color of the juice compared to the TP treatment. Zhang et al. (2004) have reported that PEF treatment preserves color of the liquid food better than the thermal pasteurization treatment. Rivas et al. (2006) have reported that mixed orange and carrot juices retains color after PEF treatment. Browning index of the juice indicates nonenzymatic browning. The browning index for the PEF-treated and TP-treated juice was significantly different compared with the control as shown in Table 2. The results suggested that temperature of the TP treatment may play an important role in the reactions leading to increase in browning index during TP processing. PEF treatments could result in residual polyphenoloxidase activity in the juice, which can contribute to browning and haze formation during PEF processing. The results were in agreement with other results reported by Gao et al. (1997) and Beveridge et al. (1997) where they found that insufficient TP treatment during processing can result in residual polyphenoloxidase activity in the juice, which can contribute to browning and haze formation during storage. Yeom et al. (2000b) reported that PEF-treated orange juice had lower browning index than the TP-treated orange juice.

Microbial Inactivation of Carrot Juices TP and PEF2 were more effective in reducing microbial counts than PEF1 (Table 4). The TP treatment decreased aerobic bacteria counts and coliform counts by 80 and 100%, respectively, while PEF2 decreased aerobic bacteria counts by 80% and coliform counts by 95%. TP, PEF1 and PEF2 effectively eliminated 100% of molds. Rivas et al. (2006) reported that TP (98C 21 s) inactivated 100% of the total plate count bacteria in blended orange and carrot juice while PEF treatment (25 kV/cm, 280 ms, 2.5 ms and 60 mL/ min) inactivated more than 99% of the total plate count bacteria. Our study showed that increased treatment time and pulse width of the PEF treatment significantly reduced 1562

TABLE 4. EFFECT OF PEF AND TP TREATMENTS ON COLIFORMS, AEROBIC BACTERIA AND YEAST/MOLD LOADS OF CARROT JUICES

aerobic bacteria and coliforms counts. This indicated that aerobic bacteria and coliforms counts in the juice depended on the PEF treatment conditions. The carrot juice treated with PEF 2 had aerobic bacteria and coliforms counts lower than commercially required for pasteurized and aseptically stored juices before packaging.

CONCLUSIONS PEF, a nonthermal preservation technique, proved to be more efficient in preserving some physicochemical and quality properties of carrot juice compared to a conventional heat pasteurization. The thermal pasteurization treatment caused significant losses in the ascorbic acid, a-carotene, b-carotene and lutein content of carrot juice, while the PEF-treated juice was closer to the control. PEFtreated juice also retained physicochemical properties such as color, viscosity and total acidity better than the thermal pasteurized juice. PEF2 treatment was comparable to TP treatment for aerobic bacteria and mold inactivation in the carrot juice. The use of PEF2 treatment can produce carrot juice with low aerobic bacteria counts, zero mold counts and minimal nutritional losses. REFERENCES ADE-OMOWAYE, B.I.O., ANGERSBACH, A., ESHTIAGHI, N.M. and KNORR, D. 2000. Impact of high intensity electric field pulses on cell permeabilisation and as pre-processing step in coconut processing. Innov. Food Sci. Emerg. Technol. 1, 203–209. AGUILAR-ROSAS, S.F., BALLINAS-CASARRUBIAS, M.L., NEVAREZ-MOORILLON, G.V., MARTIN-BELLOSO, O. and ORTEGA-RIVAS, E. 2007. Thermal and pulsed electric fields pasteurization of apple juice: Effects on physicochemical properties and flavour compounds. J. Food Eng. 83, 41–46. AGUILÓ-AGUAYO, I., SOLIVA-FORTUNY, R. and MARTÍN-BELLOSO, O. 2008. Comparative study on color, viscosity and related enzymes of tomato juice treated by high-intensity pulsed electric fields or heat. Eur. Food Res. Technol. 227, 599–606. AKINYELE, I.O., KESHINRO, O.O. and AKINNAWO, O.O. 1990. Nutrient losses during and after processing of pineapples and oranges. Food Chem. 37, 181–188.


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