Effect of Low-Temperature Plasma on Microorganism ... | IEEE Xplore

IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 39, NO. 7, JULY 2011

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Effect of Low-Temperature Plasma on Microorganism Inactivation and Quality of Freshly Squeezed Orange Juice Xing-Min Shi, Guan-Jun Zhang, Member, IEEE, Xi-Li Wu, Ya-Xi Li, Yue Ma, and Xian-Jun Shao

Abstract—Dielectric barrier discharge is used to generate lowtemperature plasma (LTP) for the treatment of freshly squeezed orange juice, which was inoculated with and without three kinds of microorganisms, respectively. Four experiments were designed and conducted: 1) When freshly squeezed orange juice samples inoculated with either Staphylococcus aureus, Escherichia coli, or Candida albicans were treated with LTP for 12, 8, and 25 s, respectively, the numbers of each microorganism decreased more than 5 logs; 2) when orange juices without the aforementioned microorganism inoculation were treated with LTP for inactivating original microorganisms inside and then stored at 4 ◦ C refrigeration, the total plate count and the proliferation rate of original microorganism were both reduced significantly (counting per each 4-d during storage); 3) when orange juice samples without microorganism inoculation were treated with LTP, the LTP treatment had insignificant effect on the values of vitamin C, total acid, turbidity, ◦ Brix, and pH of orange juice; 4) when orange juice samples were inoculated with S. aureus, E. coli, or C. albicans, respectively, and their pH values were slightly decreased by adding HCl (similar to that after LTP treatment), there was no obvious inactivation effects due to the reduction of pH values. It was proposed that microorganism inactivation was mainly due to reactive species and charged particles instead of slight pH reduction, and LTP treatment was able to effectively inactivate microbes and maintain the quality of orange juice. Index Terms—Inactivation, low-temperature plasma (LTP), orange juice, quality.

I. I NTRODUCTION

S

TERILIZATION technology is one of the most important technologies in food processing, which is tightly related

Manuscript received December 28, 2010; revised March 18, 2011; accepted April 5, 2011. Date of publication May 12, 2011; date of current version July 8, 2011. This work was supported in part by the China Foundation for the Author of National Excellent Doctoral Dissertation under Grant 200338, by Shaanxi Province Science and Technology Program under Grant 2010K1604, by Intersection Subject Program of Xi’an Jiaotong University under Grant 2009xjtujc18, and by the Guanghua Foundation for Medicine Innovation Research under Grant 0203419. X.-M. Shi is with the Key Laboratory of Environment and Genes Related to Diseases of the Ministry of Education, Xi’an Jiaotong University College of Medicine, Xi’an 710061, China (e-mail: [email protected]). G.-J. Zhang is with the State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University (XJTU), Xi’an 710049, China (e-mail: [email protected]). X.-L. Wu is with the Second Affiliated Hospital, School of Medicine, Xi’an Jiaotong University, Xi’an 710004, China (e-mail: [email protected]). Y.-X. Li, Y. Ma, and X.-J. Shao are with the State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2011.2142012

with the freshness preservation, nutrition, quality, and flavor of food. Usually, thermal processes are traditional methods for inactivation of pathogens in fruit and vegetable juices. However, thermal processes can cause some unwanted side effects to this kind of heat-sensitive product. For example, heat can destroy the nutritional, functional, and sensory characteristics of the juice [1]. This situation has been a contributing drive for the research effort into other types of nonthermal processes. In recent years, nonthermal sterilization techniques, such as low-temperature plasma (LTP), pulsed electric fields, radiation ray, high hydrostatic pressure, oscillating magnetic fields, and high-power ultrasound, have been rapidly developed [2]. Among these techniques, LTP is new and promising, and it has become a research highlight in food processing [3]. LTP has been proved effective for reducing microbial population on the surface of materials, such as glass, metals, fabrics, and agar, with wide range of microorganisms including vegetative forms, spores, fungi, viruses and parasites, etc. [4]–[7]. For example, atmospheric-pressure LTP resulted in more than 5-log reduction in Listeria monocytogenes inoculated on the surface of sliced cheese after a 120-s treatment [1]. Lowpressure cold plasma with sulfur hexafluoride (SF6 ) treatment for 10 min resulted in approximately a 5-log decrease in fungal population on the surface of hazelnuts, peanuts, and pistachio nuts [8]. Montenegro et al. employed direct current corona discharges to generate LTP to reduce the number of Escherichia coli O157:H7 in apple juice, and more than 5 log colonyforming units (CFU)/mL reduction was achieved in 40 s [9]. Other researchers also proved that LTP was effective in killing the microorganisms in solid and liquid food [10], [11]. As the components of plasma and the concentration of the same component generated from different methods (generator, working gas, discharging method, and so on) may differ and its inactivation effect on microorganisms may also vary [12], it is necessary to study the inactivation kinetics and the corresponding condition of LTP generated from different methods. In this paper, with the air as working gas, the dielectric barrier discharge (DBD) was employed to generate LTP at atmospheric pressure. Orange juice is the most consumed fruit juice in the world, accounting for 50% of fruit juice consumption [13]. Therefore, we selected it as the research object, and four experiments were designed. The purpose of this paper is to examine the effect and mechanism of LTP on three microbial strains that were inoculated in freshly squeezed orange juice and also to explore

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the effect of LTP on the shelf life and the nutritional, physical, and chemical characteristics of fresh orange juice. II. E XPERIMENTAL A RRANGEMENT A. Preparation of Microbial Suspension Two kinds of bacteria colonies, Staphylococcus aureus ATCC6538 and E. coli ATCC8039 (supplied by the Academy of Military Medical Sciences of China) were inoculated onto nutrient agar culture medium (5 g/L beef extract, 10 g/L peptone, 5 g/L sodium chloride, and 25 g/L agar) and then incubated for 24 h at 37 ◦ C, respectively. The fifth generation colonies of the two strains were, respectively, washed into sterile test tubes with 10-mL phosphate-buffered saline (PBS), and then, the test tube was shaken to obtain a homogeneous bacterial suspension. Finally, S. aureus and E. coli suspensions were diluted with PBS, and their concentrations were determined to be approximately 5.80 × 107 and 4.20 × 107 CFU/mL with standard plate-counting method, respectively. In addition, a kind of fungi colony, Candida albicans (ATCC 10231, supplied by the Academy of Military Medical Sciences of China) was inoculated onto fresh Sabouraud’s agar culture medium (40 g/L glucose, 10 g/L tryptone, and 20 g/L agar) and incubated at 37 ◦ C for 24 h. The procedure to obtain homogenous C. albicans suspension was the same as preparing S. aureus and E. coli suspensions, except that the concentration of C. albicans was determined to be approximately 5.60 × 107 CFU/mL. B. Preparation of Orange Juice Fresh and fleshy oranges were purchased from a supermarket in Xi’an, China. After being washed, the oranges were peeled off and squeezed. The freshly squeezed orange juice was filtrated with two-layer gauzes to eliminate big granules to obtain clear orange juice, which was then stored at 4 ◦ C refrigeration. For the first experiment, freshly squeezed orange juice was sterilized in high-pressure sterilizer at 115 ◦ C for 20 min and then inoculated with S. aureus, E. coli, and C. albicans, respectively. The concentrations of the three test strains in orange juice were adjusted to be 2.90 × 107 , 2.10 × 107 , and 2.80 × 107 CFU/mL, respectively. Then, the juice was shaken to ensure the even distribution of the microorganisms. After that, the LTP inactivation experiments were conducted. For the second and third experiments, freshly squeezed orange juice, without the aforementioned high-pressure sterilization and microorganism inoculation, was employed for the LTP experiment. For the fourth experiment, the same juice similar to the first experiment was employed, but without LTP treatment, and then, the HCl was added to adjust its pH value. C. Generation of LTP The experimental principle sketch of the DBD is shown in Fig. 1. Low-temperature plasma in atmospheric air was generated between two circular plate stainless steel electrodes covered with quartz glass. The upper electrode was covered

Fig. 1. Experimental principle sketch of DBD plasma. (1) Stainless steel plane electrodes. (2) Dielectric barrier. (3) High-voltage divider. (4) Cover glass with test samples.

with 2-mm-thick glass, and the bottom electrode was covered with glass of 1 mm thick. The discharge was driven by an intermediate-frequency high-voltage power supply. It can provide an output of maximal peak voltage of 30 kV. Applied voltage was measured by a voltage divider, the discharge current was measured by an inductance-free resistance R (100 Ω), and the charge quantity was measured by a capacitor C (0.47 μF). The applied voltage and discharge current waveforms were recorded by a TDS210 oscilloscope. In all our experiments, the frequency of the device was fixed at 60 kHz, and the gap between two glass barriers was 3 mm. The temperature of plasma generated by this supply is from 22 ◦ C to 25 ◦ C, which was measured by a thermal infrared imaging viewer (Thermal CAM P30, FLIR, USA), which is close to the room temperature. According to other literature, the components of low-temperature atmospheric plasma generated with the similar experimental setup to ours included complicated components, such as charged particles (electrons; positive and negative ions), reactive species (RS) (atomic oxygen (O), ozone (O3 ), hydroxyl (OH), NO, NO2 , etc.), and ultraviolet (UV) photons, etc. [14], [15]. To maintain the samples in asepsis surroundings, the discharge electrode system was placed in a super clean bench, and the high-voltage power supply was outside.

D. Experiment I: Effect of LTP on Inactivation of Inoculated Microorganism in Orange Juice Orange juice of 50 μL inoculated with the test strains was spread evenly on cover glass (2.4 cm × 5.0 cm, thickness of 0.15 mm) for each experiment, which was operated in atmospheric air with the room temperature of ∼ 20 ◦ C and the relative humidity of ∼60%. Cover glasses spread with orange juice were placed at the center of the underlying plate electrode and treated with LTP. Five treatment times for S. aureus and E. coli were set as 3, 5, 8, 10, and 12 s (one cover glass was placed in every treatment interval), and another five treatment times for C. albicans were set as 5, 10, 15, 20, and 25 s. The applied voltage was fixed at 20 kV, and the discharge power was 1.14 ± 0.06 W/cm2 . After being treated with plasma, all the samples remained liquid, and there is no obvious temperature change of the treated samples. Each cover glass was put into 2-mL PBS in a

SHI et al.: EFFECT OF PLASMA ON INACTIVATION AND QUALITY

sterile test centrifugal tube immediately, which was then shaken repeatedly to wash the test samples into PBS completely. After that, the centrifugal tube was centrifuged at 2000 r/min for 10 min, and the pellets were collected. S. aureus and E. coli were inoculated into nutrient agar culture medium, and C. albicans was inoculated into Sabouraud’s agar culture medium. All of them were incubated at 30 ◦ C for 12 h to make wounded test samples repaired [16], followed by incubation at 37 ◦ C for 24 h. Finally, the survival colonies were counted with the standard plate-counting method.

E. Experiment II: Effect of LTP on Shelf Life of Fresh Orange Juice Freshly squeezed orange juice of 50 μL without the aforementioned microorganism inoculation was dripped on the same cover glasses as described earlier. Then, each cover glass was placed at the center of the underlying electrode and treated with LTP. The experimental conditions such as treatment intervals, environmental factors, (temperature and relative humidity), gas spacing, applied voltage, and discharge power were the same as the conditions for S. aureus and E. coli as described in Experiment I. After being treated with LTP, orange juice was transferred from the cover glass into a beaker flask, and remained samples were washed with 450-μL sterile PBS to ensure complete transfer. The same process for each treatment period was repeated for several times until the amount reached 4 mL for accurate counting. After that, the beaker flasks with different juice samples were stored at 4 ◦ C refrigeration. Orange juice of 100 μL was taken out from each beaker flask on the 4th, 8th, 12th, and 16th day to determine the total plate count (TPC) with the standard plate-counting method.

F. Experiment III: Effect of LTP on Nutritional, Physical, and Chemical Characteristics of Fresh Orange Juice As the most easily oxidized vitamin in fruit juices, vitamin C was chosen as an indicator for nutritional loss of the fresh orange juice after LTP treatment. LTP treatment intervals were adjusted as 5, 10, 15, and 20 s. Orange juice of 4 mL treated with LTP at the same treatment intervals was collected. Then 2, 4-dinitrophenylhydrazine colorimetry was used to detect the concentration of vitamin C [17]. Another LTP treated 4-mL orange juice with different treatment intervals as mentioned earlier was collected into centrifugal tube, followed by centrifuge at 2000 r/min for 20 min. After that, the supernatant was collected for measuring physical and chemical indicators. The total acid was assessed by sodium hydroxide titration [18] and expressed as gram per liter L-hydroxy butanedioic acid in the sample. The optical density value was measured at 660 nm, using a spectrophotometer (UV-7504, Shanghai Xinmao Instrument Company Ltd., Shanghai, China) to indicate turbidity [19]. Abbe refractometer (2WAJ, Shanghai Changfang Optical Instrument Company Ltd., Shanghai, China) and acidimeter (ET-1, Shashi City Titrator Factory, Shashi, China) were used to detect the ◦ Brix and pH, respectively.

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G. Experiment IV: Effect of pH Value on Orange Juice With Different Inoculation Strains A separate set of experiments was performed to test the role of pH value in microorganism inactivation by LTP. Freshly squeezed orange juice of 50 mL was first sterilized in highpressure sterilizer at 115 ◦ C for 20 min and, then, was separated into three test tubes, with 3 mL in each one. After that, the three test strains (S. aureus, E. coli, and C. albicans) were inoculated into different test tubes with the concentration of 3.10 × 104 , 5.00 × 104 , and 4.20 × 104 CFU/mL, respectively. After this procedure, 0.1-M HCl was dripped into the juice to slightly adjust its pH value. After an interval of 1 min, the survival colonies of the test samples were counted with the standard plate-counting method. H. Statistical Analysis The experiments were repeated five times for each test strain and duration period. The experimental data were expressed as the style of mean value ± standard deviation (SD). The differences among means were tested for statistical significance by one-way analysis of variance and independent sample T-test. All analysis was performed with SPSS Version 13.0 for Windows. The p-value < 0.05 was defined as statistically significant. III. E XPERIMENTAL R ESULTS A. Effect of LTP on Microorganism Inactivation Inoculated in Orange Juice The experiments were repeated for five times. The original concentrations of S. aureus, E. coli, and C. albicans in orange juice were (2.90 ± 0.04) × 107 , (2.10 ± 0.03) × 107 , and (2.80 ± 0.03) × 107 CFU/mL, and corresponding to 50-μL orange juice on each cover glass, their original numbers were approximately (1.45 ± 0.04) × 106 , (1.05 ± 0.03) × 106 , and (1.40 ± 0.04) × 106 CFU before experiment. The treatment effects of LTP on the three test strains inoculated in orange juice were shown in Fig. 2. Fig. 2 showed that the survival colony numbers of the three test strains decreased gradually with the increase of plasma exposure time. Compared with the untreated samples, the survival numbers of S. aureus, E. coli, and C. albicans were all reduced significantly at each treatment interval (P < 0.05). Furthermore, the survival colony numbers of the three test strains were below the detection limit (1 CFU/50 μL) within 12, 8, and 25 s, respectively, indicating that more than 5 logs of microorganisms were inactivated. B. Effect of LTP on Shelf Life of Fresh Orange Juice The original TPC in freshly squeezed orange juice was determined to be (3.20 ± 0.08) × 102 CFU/mL. Table I showed that, with the extension of storage time at 4 ◦ C refrigeration, the TPC in untreated orange juice (control sample) gradually increased, and after 16-d storage, its TPC reached 2 × 105 CFU/mL. In orange juice treated with 3- and 5-s LTP, after 4-d storage,

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the TPC was significantly less than that of the control sample (P < 0.05). With the extension of treatment time, although the TPC increased gradually, the increase rate was lower than that of the control sample. After 4- and 8-d storage, the TPC in orange juice treated with 8-s LTP was still at 0 CFU/mL, which reached approximately 50 CFU/mL on the 16th day. After 4-, 8-, 12-, and 16-d storage, the TPC in orange juice treated with 10- and 12-s LTP all remained at 0 CFU/mL. It is clearly shown that LTP can greatly extend the shelf life of freshly squeezed orange juice. C. Effect of LTP on Nutritional, Physical, and Chemical Characteristics of Fresh Orange Juice Table II showed the nutritional, physical, and chemical characteristics of orange juice before and after LTP treatment. The vitamin C concentration in untreated orange juice was 43.71 mg/100 mL, and with the extension of LTP treatment time of 5, 10, 15, and 20 s, the concentration slightly decreased. However, there was no statistically significant difference (P > 0.05) on vitamin C concentration between concentration of control and LTP-treated samples. For 20-s exposure, vitamin C concentration in orange juice was 43.22 mg/100 mL, and its reservation rate was 98.88%. Therefore, vitamin C loss in orange juice treated with LTP was insignificant. With the extension of treatment time, total acid and turbidity values increased slightly, while the pH and ◦ Brix values were reduced slightly. However, compared with those of control samples, these values in orange juice treated by LTP with different treatment time were not changed significantly, with no statistically significant difference (P > 0.05). D. Effect of pH Value on Three Test Strains Inoculated in Orange Juice Considering the insignificant reduction of pH values due to LTP treatment, a separate set of experiments was performed to test the role of pH value in microorganism inactivation. As shown in Table III, the original pH value of the orange juice was 3.75, and by adding 0.1-M HCl, its pH value was decreased to 3.70 (identical to 20-s LTP exposure as in Table II). The survival colony numbers of S. aureus and E. coli both decreased slightly at 3.70 compared with that at 3.75, but there was no statistical significance (P > 0.05). For C. albicans, the survival colony numbers were the same at 3.70 and 3.75. IV. D ISCUSSION Fig. 2 showed that LTP had a good inactivation effect on the three test strains inoculated in orange juice. With the extension of treatment time, the decrease rate of the number of viable E. coli was largest, followed by S. aureus, and then C. albicans. The reason might be explained by their different cell wall structures. C. albicans is a single-cell fungus whose cell wall is composed of a four-stratum structure, with chitin microfiber as the inner stratum, then protein, glucoprotein, and aminoglycoside as the outer stratum. Compared with bacteria, its cell wall is more compact. For the two kinds of bacteria,

Fig. 2. Inactivation results of three test strains inoculated in orange juice. The numbers beside the dots indicate the SD. The stars (∗) beside the dots indicate that P < 0.05 versus control. The triangles (Δ) beside the dots indicate that the survival colony numbers of three test strains are below the detection limit (1 CFU/50 μL).

S. aureus (gram positive) has a thicker and stronger cell wall than E. coli (gram negative) [20]. Therefore, E. coli is the most susceptible microorganism to LTP treatment, while C. albicans is the least susceptible one. Recently, Liu et al. have used cold atmospheric-pressure air plasma microjet in water for S. aureus and achieved a 98% inactivation rate in 6 min [21], similar to our experimental results. Scholtz et al. found that, 120 s, 4–5 min, and 30 min of treatment time with LTP generated by negative corona discharge were needed to achieve complete sterilization of E. coli, Staphylococcus epidermidis, and C. albicans, respectively [22], and the susceptibility of these three strains to LTP in their experiment is accordant with our results. Table I revealed that, after being treated with LTP with short treatment time (3, 5, and 8 s), the original microorganisms in the orange juice were partly inhibited or inactivated. After the juice was treated with LTP for longer treatment time (10 and 12 s), the original microorganisms inside were all inactivated. This time/dose-dependent inactivation effect of LTP on microorganisms was also seen in Fig. 2. Vitamin C in food has two types, i.e., deoxidation and dehydrogenation, both with biological activity. Vitamin C in fresh food exists mainly in the deoxidation type. The alkaline environment, heat, oxygen, and UV can promote its oxidation, and it can turn into the dehydrogenation type. The dehydrogenation type of vitamin C can be oxidized or hydrolyzed to diketogulonic acid, leading to the loss of its biological activity [23]. Vitamin C is rich in orange juice and is easily oxidized in aqueous solution, and this was why we chose it as our nutritional indicator after the juice was treated with LTP. Total vitamin C concentrations (including deoxidation and dehydrogenation types) in the orange juice before and after LTP treatment were measured in our study. The aforementioned results (shown in Table II) indicated that, after being treated with LTP, the great majority of vitamin C in the orange juice still had biological activity (both deoxidation and dehydrogenation


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TABLE I TPC IN THE O RANGE J UICE T HAT WAS T REATED AND U NTREATED W ITH LTP (CFU/ M L)

TABLE II E FFECT OF LTP ON N UTRITIONAL , P HYSICAL , AND C HEMICAL C HARACTERISTICS OF O RANGE J UICE

TABLE III E FFECT OF P H VALUE ON T HREE T EST S TRAINS I NOCULATED IN O RANGE J UICE

types) and was not oxidized to diketogulonic acid. The values of total acid, turbidity, ◦ Brix, and pH in fruit juice often indicate the color, flavor, and taste of the juice [24]. Table II revealed that LTP treatment had almost no effect on nutritional, physical, and chemical characteristics of the orange juice. Remarkably, Tang et al. reported the effects of LTP on eukaryotic microalgae in aqueous media, and their SEM observations showed that morphological change of eukaryotic microalgae cells in a medium with pH decreasing to 3.0 (by artificially adding HCl acid) was identical to those caused by 320-s plasma exposure [25]. Therefore, they considered that a decreased pH was the mechanism whereby plasma exerted deleterious effects on microalgae in aqueous environments. As shown in Table II, we also observed the slight reduction of pH value in orange juice after LTP treatment, so we designed a similar experiment. Table III revealed that such slight pH decrease by LTP (3.75 to 3.70) played a little role in microorganism inactivation in orange juice. Maybe a much larger pH value reduction due to plasma with longer time treatment would play a significant role as Tang et al. reported. Moreover, the differences of plasma generation, test microbial strains, and medium would lead to different results about the role of pH decrease in LTP inactivating microorganisms. In recent years, much attention has been paid on microorganism inactivation by LTP, but the exact inactivation mechanisms are largely unknown. During our plasma treatment, there were a lot of charged particles and RS inside the plasma volume. They were dynamically generated and recombined before reaching microbial cells. Some of them interacted with microbial cells and destroyed their outer structure. Air plasmas are excellent sources of reactive oxygen species (ROS) and reactive nitrogen species (RNS), such as atomic oxygen (O), ozone (O3 ),

hydroxyl (OH), NO, NO2 , etc. ROS and RNS were able to react with various macromolecules, such as membrane lipids, protein, nucleic acid, and so on, on the outer structure and/or inside the microorganisms, which resulted in microbial death or injury [7], [26], [27]. Moreover, H2 O in the orange juice acted as the other source of OH radicals. The formation of OH radicals was illustrated hereinafter [(1)–(3)] [26]. Therefore, we consider that charged particles and RS (ROS and RNS) in low-temperature plasma might play a dominant role during the microbial inactivation O + H2 O → 2OH

(1)

H2 O + O3 → O2 + 2OH

(2)

H2 O + e → OH + H + e.

(3)

About the effect of UV emission from plasma, some researchers considered that, in atmospheric-pressure plasma, most of the produced UV radiation was reabsorbed in the plasma volume and not delivered to the treated surface [6], [7], [28], [29]. Furthermore, UV always attenuates rapidly in liquid. Vitamin C measurement results in our experiments (Table II) also indirectly confirmed the feeble role of UV in LPT inactivating microorganisms. V. C ONCLUSION The results have indicated that LTP was safe and effective in inactivating microorganisms in orange juice and can significantly extend its shelf life. Moreover, LTP treatment had almost no effect on nutritional, physical, and chemical characteristics of the orange juice. It was proved that, under

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the experimental conditions in this paper, the pH decrease of orange juice induced by LTP played an insignificant role in the inactivation of microorganisms. We predicted that charged particles and RS (ROS and RNS) in atmospheric-pressure LTP played a major role during the microbial inactivation. However, the chemical processes of interaction between LTP and test samples (microbial strains and their medium) are extremely complicated, which has been still largely unknown. Therefore, more research is needed to explore the detailed mechanisms of LTP inactivating microorganisms in liquid food. Compared with traditional thermal methods, LTP method needs lower energy consumption and shorter duration. Therefore, LTP will be a promising alternative food processing technique available to inactivate most microorganisms. More research efforts should be undertaken in order to treat larger volume of fresh juice.

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Xing-Min Shi was born in Shaanxi Province, China, in January 1974. He received the B.S. degree in public health and the M.S. and Ph.D. degrees in medical immunology from the School of Medicine, Xi’an Jiaotong University (XJTU) (formerly Xi’an Medical University), Xi’an, China, in 1999 and 2008, respectively. Since 1999, he has been with the School of Medicine, XJTU, where he was a Teacher Assistant and is currently a Lecturer. His research mainly focuses on the mechanism of low-temperature plasma on medicine. He has published more than 20 journal articles and conference papers.


Guan-Jun Zhang (M’02) was born in Shandong Province, China, in October 1970. He received the M.S. and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University (XJTU), Xi’an, China, in July 1994 and April 2001, respectively. Since July 1994, he has been a Teacher Assistant with XJTU. From October 1998 to September 1999, he was with the Tokyo Institute of Technology, Tokyo, Japan, as a Visiting Researcher, engaged in surface electroluminescence and discharge phenomena of solid insulating materials. From August 2006 to February 2007, he was with Princeton University, Princeton, NJ, as a Senior Visiting Fellow, engaged in secondary electron emission characteristic and plasma simulation. Since June 2004, he has been with the State Key Laboratory of Electrical Insulation and Power Equipment, XJTU, and the School of Electrical Engineering, XJTU, where he is currently a Professor. He was the recipient of many awards and prizes from the Chinese government. His current research interests are electrical discharge and plasma, pulsed power technology, and condition maintenance of power equipment. He has published more than 80 journal articles and conference papers.

Xi-Li Wu was born in Shandong Province, China, in February 1975. He received the B.S. degree in Chinese traditional medicine and the M.S. degree in clinical integrated traditional and western medicine from Zhangjiakou Medical College, Zhangjiakou, China, in 1999 and 2002, respectively, and the M.D. degree in internal medicine from the School of Medicine, Xi’an Jiaotong University (XJTU) (formerly Xi’an Medical University), Xi’an, China, in 2010. Since 2002, he has been with Second Affiliated Hospital, School of Medicine, XJTU, as a Physician. His research mainly focuses on the mechanism of kidney disease and traditional Chinese drug. He has published more than 30 journal articles and conference papers.

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Ya-Xi Li received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 2008. She is currently working toward the M.S. degree in electrical engineering with Xi’an Jiaotong University, Xi’an, China. Her research interests include gas discharge and nonthermal atmospheric plasma and their applications.

Yue Ma was born in Liaoning Province, China, in May 1982. He received the B.S. degree in measuring and controlling and instrument from Xi’an Jiaotong University, Xi’an, China, in 2005, where he is currently working toward the Ph.D. degree in electrical engineering. His research interests include gas discharge and nonthermal atmospheric plasma and their applications.

Xian-Jun Shao received the B.S. and M.S. degrees in electrical engineering from the Shenyang University of Technology, Shenyang, China, in 2006 and 2009, respectively. He is currently working toward the Ph.D. degree in electrical engineering at Xi’an Jiaotong University, Xi’an, China. His research interests include dielectric barrier discharge and its applications.