Introduction

Nowadays, fresh-cut fruits are becoming increasingly popular as a result of their convenience and nutritional benefits. However, during minimal processing operations (peeling, cutting, and shredding), the natural protective epidermal barrier of fruits is removed, and the exposure of the cytoplasm may provide a better source of nutrients for the growth of microorganisms (Yu et al. 2018). Considering the fact that fresh-cut fruits are sold as ready-to-eat and will thus not be subjected to further microbial killing steps, the correct storage and hygienic production are critical to ensuring their safety. It is a common practice in industry to utilize a chlorinated water wash to eliminate microorganisms in fresh-cut produce (Ukuku et al. 2017). However, washing with chlorine and chlorine-based derivatives is becoming progressively more challenging—the main considerations include potential health problems associated with the use of chlorine and its by-products, as well as the negative impact of chlorine on the environment (CDC 2016). Therefore, new alternative treatments have been explored in recent years to establish effective and environmentally friendly processing techniques that would ensure the safety of fresh-cut products.

Cold plasma, which is an ionized gas that is produced by applying energy to a gas or gas mixture, consists of active particles such as electrons, ions, free radicals, and atoms (Laroussi 2005). Thus far, exposure to cold plasma represents a relatively novel approach for microbial inactivation of fresh produce. In the process, reactive oxygen species (ROS) serve as the major germicidal components, with the capacity to damage proteins and DNA of microbial cells. Reportedly, appropriate doses of cold plasma treatment could effectively control the growth of microorganisms and ensure the quality of fresh-cut fruits (Ramazzina et al. 2015; Tappi et al. 2014, 2016). However, since the surfaces of fresh produce are characterized by significant irregularity, the processing uniformity of cold plasma technology has not yet reached the ideal levels. In recent years, it has been demonstrated that plasma-activated water (PAW), which is generated by reacting the active compositions in cold plasma with water, possesses outstanding antimicrobial properties (Zhang et al. 2013). Moreover, the active species in PAW are stable over extended period of time and thus exhibit long-lasting antimicrobial properties. Furthermore, PAW has an improved fluidity when compared with plasma, thereby enhancing the uniformity and efficiency of the decontamination process. In fact, previous reports have shown that PAW can be used to effectively control the microbial growth without affecting the quality of postharvest Chinese bayberries (Ma et al. 2016), strawberries (Ma et al. 2015), grapes (Guo et al. 2017), button mushrooms (Xu et al. 2016), fresh-cut celery, radicchio (Berardinelli et al. 2016), and fresh-cut lettuce (Andrasch et al. 2017; Schnabel et al. 2017).

During decontamination treatments, important biochemical reactions take place in the tissue of fresh-cut products. Such reactions may have an impact on, for example, the structure of phenolic compounds, which in turn cause changes in their bioactivity (Pérez-Gregorio et al. 2011). Theoretically, the high oxidative action of PAW could accelerate the oxidation of bioactive compounds (e.g., ascorbic acid and polyphenols) contained in fruits, which could, in turn, diminish the antioxidant properties of fresh-cut products. However, Xu et al. (2016) reported that PAW treatment had a positive effect on the ascorbic acid content of button mushrooms during postharvest storage. As for cold plasma, the controversial debate regarding its effects on antioxidant activity, which can differ depending on the food matrix involved (Grzegorzewski et al. 2011; Ramazzina et al. 2015, 2016; Surowsky et al. 2015), remains ongoing. Hitherto, the effects of PAW on the activities and contents of antioxidant components in fresh-cut fruits remain largely unexplored.

In the present work, we evaluated the hypothesis that PAW treatment can be used to not only control the growth of microorganisms, but also to maintain the antioxidant activity of fresh-cut pears. This hypothesis was tested by examining the variation in microbial load, contents of antioxidant compounds (ascorbic acid and total phenolic content), and antioxidant activity of PAW-treated fresh-cut pears during storage at 4 °C for 12 days.

Materials and Methods

Material and Reagents

Huangguan pears (Pyrus bretschneideri Rehd cv. Huangguan) were obtained from the Dalian New Smart Supermarket and were chosen based on uniform size, color, hardness, and absence of any physical and fungal infections. Folin–Ciocalteu reagent, gallic acid, 2,2-diphenyl-1-pycrylhydrazyl (DPPH), 2,2′-azino-bis (3-ethylbenzothiazolin-6-sulfnic acid) diammonium salt (ABTS), and 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All other chemical reagents used were of analytical grade.

Plasma Device and PAW Generation

Figure 1 shows a schematic diagram of the custom-made microplasma array device (Zhou et al. 2015, 2016). The plasma device is mainly composed of 6 ×6 microplasma jet units, which fixed at the bottom of a quartz cup. In each microplasma jet unit, two hollow fibers (inner diameters of 200 μm and 1500 μm, respectively) are used to generate the microplasma. One copper electrode immersed in the water is connected to the ground. Air was streamed into the 36 microplasma jet units at a flow rate of 1.0 standard L/min. PAW is produced by dielectric barrier discharge in the distilled water media. The power supply (TCP-2000K, Nanjing Suman Electronic, Nanjing, China) provided bipolar AC output with the peak voltage (Vp) of 0–20 kV at an AC frequency of 9.0 kHz. In this study, distilled water was activated using atmospheric microplasma arrays at Vp = 6 kV, 8 kV, and 10 kV. The corresponding treatments were referred to as 6-kV PAW, 8-kV PAW, and 10-kV PAW, respectively.

Fig. 1
figure 1

Cross-sectional image of the microplasma array device (a) and top view of the plasma device (b)

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Processing and Treatments of Fresh-Cut Pears

Pears were sanitized using a 200 μL L–1 sodium hypochlorite (NaClO) solution, washed with deionized water, and dried naturally. Subsequently, pears were peeled, cored, and cut into 2-cm-thick cubes using a sharp stainless-steel knife. All experimental apparatuses were sanitized with sodium hypochlorite and rinsed with deionized water prior to use. The fresh-cut pears were randomly divided into five groups: one control group and four treatment groups. For the control group, fresh-cut pears were immersed in sterile distilled water (duration = 5 min). For the four treatment groups, the fresh-cut pears were immersed in NaClO solution (200 μL L–1), 6-kV PAW, 8-kV PAW, and 10-kV PAW solutions (duration = 5 min), respectively. The ratio of washing solutions and samples was 1:3 in each case. After each treatment, the samples were put in plastic foam tray and taken into a biosafety cabinet to let them dry. Subsequently, the fresh-cut pears were wrapped in polyethylene cling film and taken into a refrigeration house (4 ± 1 °C), where they were stored for up to 12 days. The microbial load, contents, of antioxidant compounds, and antioxidant activity of the samples were evaluated every 2 days.

Microbial Analysis

The growth of aerobic bacteria, yeast, and mold in the fresh-cut pear samples was followed during the experiment according to the Chinese standard GB/T (GB 4789.2-2016; GB 4789.15-2016). In each case, a sample of fresh-cut pear (25 g) was put into a sterile Stomacher bag and combined with peptone (225 mL, 0.1%) following homogenization (BagMixer-400W, Interscience, France) for 1 min. In order to examine the growth of bacteria, mold, and yeast, 1 mL from each sample was aseptically pipetted into the appropriate culture media (plate count agar for bacteria and potato dextrose agar with chloramphenicol for yeasts and molds). Plates were incubated at 36 ± 1 °C for 48 ± 2 h (bacteria) or 28 ± 1 °C for 3–5 days (yeasts and molds). Triplicate plates were performed for each dilution. The results were expressed as log10 CFU/g–1.

Titratable Acidity and Soluble Solids Content

Titratable acidity (TA) was quantified by the titration method using NaOH (0.1 mol/L) and the results were calculated as a percentage of malic acid (mL NaOH × 0.1 N/mass of sample titrated × 0.067 × 100). Soluble solids content (SSC) was determined at 20 °C using a refractometer (WYA-Z, Shanghai Precision & Scientific Instrument Co., Ltd., China). A drop of juice obtained from fresh-cut pears was placed on the refractometer glass prism to measure the content of soluble solids.

Mass Loss and Firmness

Mass loss was measured using an electronic balance (Precisa 180 A, Switzerland) and was expressed as the mass difference value in proportion to the initial fresh mass (%). Firmness was determined using the TA.XT texture analyzer (Stable Micro Systems Ltd., Godalming, UK) with a compression probe P50. The test speed was set to 1 mm s–1. The force (N) that was required to puncture the fresh-cut pear tissue to a depth of 5 mm was used to evaluate the firmness.

Ascorbic Acid Content

The ascorbic acid content of fresh-cut pears was determined according to the method reported by Chomkitichai et al. (2014), with some modifications. In each case, 5 g fresh-cut pears were homogenized in 20 mL metaphosphoric acid (3%, w/v) and centrifuged (Allegra X-30R, Beckman Coulter, USA) at 3000 g (4 °C) for 20 min. After that, 2 mL of the supernatant were mixed with 5 mL metaphosphoric acid (3%, w/v). The resulting solution was titrated using 0.1 mM 2,6-dichloroindophenol to the end point. The ascorbic acid content of fresh-cut pears was calculated from a standard curve of ascorbic acid. The results were expressed as mg of ascorbic acid per 100 g fresh-cut pears.

Total Phenolic Content

Sample extraction of 5 g fresh-cut pears was added to 20 mL methanol and the mixture was subjected to ultrasonication for 40 min. After centrifugation at 10,000g (4 °C) for 10 min, the supernatant was used to analyze the total phenolic content (TPC). TPC was determined based on the Folin–Ciocalteu procedure (Singleton and Rossi 1965), with slight modifications. In each case, 1 mL of the supernatant was added to 1.5 mL of the Folin–Ciocalteu reagent and the sample was mixed thoroughly. Subsequently, 6 mL Na2CO3 solution (10%, w/v) were added. After the mixture was incubated at room temperature for 90 min (in darkness), the absorbance was measured at 765 nm using a spectrophotometer (Hitachi U-2800 Spectrophotometer, Japan). TPC of fresh-cut pears was calculated from a standard curve of gallic acid. The result was expressed as mg of gallic acid equivalent (GAE) per 100 g of fresh-cut pears.

Antioxidant Activity

DPPH Radical Scavenging Activity

A DPPH assay was used to evaluate the antioxidant activity of fresh-cut pears, which was measured as reported by Chen et al. (2016). Briefly, 0.1 mL methanol extract, which was utilized in the determination of TPC, was mixed with 3.9 mL of 0.1 mM DPPH solution in ethanol. After the mixture was incubated at room temperature for 30 min in darkness, the absorbance was measured at 517 nm. The DPPH radical inhibition was calculated as follows:

$$ \mathrm{DPPH}\ \mathrm{radical}\ \mathrm{inhibition}\ \left(\%\right)=\frac{A_0\hbox{-} {A}_{\mathrm{t}}\ }{A_0}\times 100\% $$

where A0 is the absorbance of the blank (ethanol), and At is the absorbance of the sample.

Trolox was used as a standard and analyzed under the same conditions. The DPPH radical scavenging activity of fresh-cut pears was calculated from a standard curve of a Trolox solution and expressed as micromoles of Trolox per grams of fresh-cut pears.

ABTS Radical Scavenging Activity

The ABTS assay was conducted as reported by Chen et al. (2016). The ABTS radical solution was prepared as follows: An aqueous solution of ABTS (7 mM) was mixed with the same volume of potassium persulfate (2.45 mmol) and stirred thoroughly. After incubation at room temperature for 12–16 h in darkness, the mixture was diluted with ethanol to obtain the ABTS radical solution with an absorbance of 0.7 (734 nm). Subsequently, 0.1 mL of sample extract, which was described in the determination of TPC, was added to 3.9 mL of the ABTS radical solution. After incubation at room temperature for 5 min in darkness, the absorbance was measured at 734 nm. The ABTS radical inhibition was calculated as follows:

$$ \mathrm{ABTS}\ \mathrm{radical}\ \mathrm{inhibition}\ \left(\%\right)=\frac{A_0\hbox{-} {A}_{\mathrm{t}}\ }{A_0}\times 100\% $$

where A0 is the absorbance of the blank (ethanol), and At is the absorbance of the sample.

Trolox was used as a standard and analyzed under the same conditions. The ABTS radical scavenging activity was calculated from a standard curve of Trolox solution and expressed as micromoles of Trolox per gram of fresh-cut pears.

Statistical Analysis

All data are presented in the form of mean ± standard deviation. Analysis of the variance (ANOVA) followed by Duncan’s multiple range test with a significance level of P < 0.05 was performed using SPSS software (version 17.0, SPSS, Chicago, IL, USA).

Results and Discussion

Effects of PAW on Microbial Loads of Fresh-Cut Pears

Even minimal processing removes the outer-layer peel and epidermis of fruits, which means that the physical and chemical barriers protecting the fruits against microorganisms are lost. The cut surface and available nutrients provide ideal conditions for microbial growth, which accelerates the decay rate and shortens the shelf life of fresh-cut fruits (Jia et al. 2015). As shown in Table 1, the total counts of aerobic bacteria, yeasts, and molds in fresh-cut pears were under the detection limit (1.0 log10 CFU/g) at the beginning of storage period (day 0), regardless of the treatment employed. Similar results were also reported by Xu et al. (2013), who showed that aseptic processing conditions successfully prevent microbial transference from surface to internal tissues. By contrast, the total aerobic bacterial count in the control and NaClO-treated fresh-cut pear samples increased gradually after 2 days of storage, and reached 5.11 and 5.07 log10 CFU/g at the end of the storage, respectively. Moreover, the treatment with PAW delayed the growth of bacteria, which remained below the detection limits until day 4. After this point, the bacteria in the PAW-treated samples grew rapidly. Over the course of the entire storage period, the PAW-treated samples exhibited smaller populations of aerobic bacteria than the control samples (P < 0.05), with the 8-kV PAW-treated samples displaying the lowest aerobic bacterial counts.

Table 1 Microbial growth of fresh-cut pears during storage at 4 °C for 12 days
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Similar to the trends observed for the total aerobic bacteria, the population of yeast gradually increased in all samples, and was significantly lower in the PAW-treated fresh-cut pears when compared with the control and NaClO-treated groups during the entire storage period (P < 0.05). The washing with 8-kV PAW resulted in the lowest growth of yeast in the fresh-cut pears. In the case of molds, the growth remained below the detection limit up to day 6 for the PAW-treated fresh-cut pears. In contrast, mold counts of 1.61 and 1.32 log10 CFU/g were observed for the control and NaClO-treated samples at this time, respectively. During the 12-day storage period, the counts of mold were higher in the control and NaClO-treated samples than in the PAW-treated samples, with the 8-kV PAW treatment being the most efficient at killing mold. The yeast populations were more prevalent than the mold populations in fresh-cut pears during the 12-day storage, regardless of the treatment. Oms-Oliu et al. (2008) and Piga et al. (2000) have also reported that yeast populations were prevalent in fresh-cut pears during the time of storage, irrespective of the storage temperature and atmosphere. Usually, the consumers can perceive the spoilage of fresh-cut fruits when yeast counts were above 5 log10 CFU/g (Oms-Oliu et al. 2008). PAW treatment maintained the yeast counts in fresh-cut pears below 5 log10 CFU/g throughout the storage, whereas both the control and NaClO-treated samples exceeded this level after 10 days of storage.

In general, the total counts of aerobic bacteria, yeasts, and molds in all samples treated with PAW were significantly lower than those in the control and NaClO-treated fresh-cut pears over the 12-day storage period (P < 0.05). In previous studies, PAW treatment also reduced the microbial loads in postharvest button mushrooms (Xu et al. 2016), Chinese bayberries (Ma et al. 2016), and grapes (Guo et al. 2017) during storage. According to these studies, the antimicrobial mechanism of PAW is related to the bacterial cell wall and membrane damage. PAW treatment resulted in the accumulation of intracellular ROS, which could lead to a drop in the membrane potential, followed by a reduction in the membrane integrity, and finally to bacterial cell death. ROS in PAW have been identified as one of the most important ingredients in the inactivation process (Ma et al. 2015). The number of ROS generated in the 10-kV PAW treatment was higher than that in the 8-kV PAW treatment, and might induce oxidative stress in the fresh-cut pears, thereby accelerating their senescence and decay. Therefore, improved disinfection is not always correlated to an increase in the plasma intensity. Similarly, Xu et al. (2016) has reported that there is no dose–effect relationship between PAW treatment time and microorganism inactivation efficiency in button mushrooms. Overall, the microbial growth was best controlled in fresh-cut pears by the application of 8-kV PAW treatment, and the pears subjected to this treatment exhibited the lowest counts of aerobic bacteria, yeasts, and molds during the entire studied period.

Titratable Acidity and Total Soluble Solid Content

The results from the TA and SSC measurements of fresh-cut pears are shown in Fig. 2. A slight and statistically insignificant decrease in TA was observed in the fresh-cut pears over the 12-day storage period. Also, there were no significant differences in TA among the control, NaClO-, and PAW-treated samples when examined at the same storage times (Fig. 2a). Similarly, Won et al. (2017) found no significant differences in the values of TA of the control and cold plasma-treated mandarins.

Fig. 2
figure 2

Changes in the titratable acidity (a) and soluble solids content (b) of fresh-cut pears during storage at 4 °C for 12 days

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The SSC in fresh-cut pears increased during the first 6-day of storage, after which it declined (Fig. 2b). Moreover, NaClO and PAW treatments did not affect the SSC of the fresh-cut pears. The initial increase in SSC was most likely associated with the conversion of starch into sugars. As the storage time increased, the contents of carbohydrates and pectins decreased, proteins were partially hydrolyzed, and glycosides decomposed during respiration, therefore diminishing the SSC (Sharma and Rao 2015).

Mass Loss and Firmness

Mass loss and firmness are common physical parameters that are used to assess the quality of fresh-cut fruits. As shown in Fig. 3a, water loss in fresh-cut pears increased upon prolonging the storage time. Treatment with NaClO and PAW did not significantly affect the mass loss values, with the exception of samples treated with 8-kV PAW, which displayed significantly reduced mass loss and thus delayed the shriveling and quality deterioration of fruits.

Fig. 3
figure 3

Changes in the mass loss (a) and firmness (b) of fresh-cut pears during storage at 4 °C for 12 days. a-cMeans with different alphabets are significantly (P < 0.05) different between each sample type for a particular storage time

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As shown in Fig. 3b, all fresh-cut pears appeared to gradually soften during storage, regardless of the treatment. NaClO treatment decreased the firmness, the firmness values of the NaClO-treated pears were significantly lower than those of the control during the first 8 days of storage (P < 0.05). However, the firmness of fresh-cut pears subjected to the 6-kV PAW treatment increased immediately (day 0) when compared with the control (P < 0.05). During the first 8 days of storage, the 6-kV PAW-treated samples exhibited higher values of firmness than the control group. The samples subjected to 8-kV and 10-kV PAW treatments also exhibited slightly higher values of firmness than the control group, but these differences were not significant at all storage times. In the past, Xu et al. (2016) and Ma et al. (2016) showed that the PAW treatment can delay the softening of button mushrooms and Chinese bayberries, respectively—the delay was attributed mostly to the reduced microorganism counts caused by the PAW treatment. These results are in agreement with the well-known fact that the cell wall components of fruits can be degraded by bacterial enzymes, thus leading to structural breakdown. Moreover, the degradation of internal structures, e.g., the solubilization of polyuronide, enzyme-mediated degradation of hemicellulose, and dehydration, also results in the reduction of firmness (Tappi et al. 2016; Ramazzina et al. 2015). Although the 8-kV PAW treatment achieved the largest reduction in microbial populations and lowest mass loss in fresh-cut pears in the present work, it did not afford the best results in terms of tissue firmness. Therefore, the mechanisms responsible for the influence of PAW on the firmness of fresh-cut pears should be investigated in future studies.

Antioxidant Constituents

Ascorbic Acid Content

Ascorbic acid, which is an antioxidant in fresh-cut fruits, is thermo-labile and sensitive to storage conditions and the processing methodology (Tiwari et al. 2008). Figure 4a shows that the ascorbic acid content in fresh-cut pears did not change significantly (day 0) after treatment with PAW and NaClO (day 0) and decreased rapidly during the first 4 days of storage, regardless of the treatment. The content of ascorbic acid rapidly decreased in early storage indicated that ascorbic acid had played an important role as non-enzymatic antioxidant for eliminating ROS in fresh-cut pears. The 10-kV PAW treatment delayed significantly this decrease and the samples subjected to it maintained higher ascorbic acid contents than those of the control group over the storage period from day 2 to day 6 (P < 0.05). By contrast, the reduction in ascorbic acid content of fresh-cut pears was accelerated after the 6-kV and 8-kV PAW (2 to 4 days) and NaClO treatments (2 to 6 days). On day 2, the ascorbic acid content in the control, NaClO, 6-kV, 8-kV, and 10-kV PAW-treated fresh-cut pears were reduced by 55.58%, 43.16%, 51.56%, 59.28%, and 28.52%, respectively. After 6 days of storage, no significant differences were found between the control and PAW-treated samples (P > 0.05) and the reduction in ascorbic acid content of these samples increased from 52.90 to 63.52% on day 12. Xu et al. (2016) reported that there was a positive effect of PAW on the ascorbic acid content in button mushrooms during postharvest storage. However, previous studies have shown that cold plasma treatment causes a slight reduction in the ascorbic acid content of fresh-cut cucumbers, carrots, pears, and kiwifruits (Ramazzina et al. 2015; Wang et al. 2012), most likely as a result of the direct reaction of ascorbic acid with the ROS generated by plasma. In addition, Sarangapani et al. (2017) reported that the ascorbic acid content in blueberries was enhanced by 80-kV plasma treatment and reduced by 60-kV plasma treatment. Nevertheless, the specific mechanisms responsible for the observed effects are still unclear. It is hypothesized that plasma-induced reactive species could induce oxidative stress in fresh-cut fruits. As a defense response, the fruits might produce more antioxidants to enhance their antioxidant capacity (Ma et al. 2016). This pattern of behavior is in good agreement with the fact that only the 10-kV PAW treatment increased the ascorbic acid content of fresh-cut pears. However, the over production of ROS during storage aggravated the oxidative damage, which exhausted ascorbic acid. Therefore, no significant differences in ascorbic acid content were found between the control and PAW-treated samples during the later storage.

Fig. 4
figure 4

Changes in the ascorbic acid (a) and the total phenolic (b) contents of fresh-cut pears during storage at 4 °C for 12 days. a-dMeans with different alphabets are significantly (P < 0.05) different between each sample type for a particular storage time

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Total Phenolic Content

As observed for ascorbic acid, the total phenolic content (TPC) decreased over time for both the control and PAW-treated fresh-cut pears (Fig. 4b). In general, the control samples displayed a better retention of TPC—after 12 days of storage, the TPC of the control group was reduced by 15.26%. In contrast, the values by which TPC decreased in the NaClO, 6-kV, 8-kV, and 10-kV-treated fresh-cut pears after 12 days were 22.52%, 24.18%, 24.45%, and 16.08%, respectively. In addition to these general trends, it is important to note that the TPC of fresh-cut pears increased significantly (P < 0.05) in response to the 6-kV PAW and 10-kV PAW treatments, and that during the first 6 days of storage, the values obtained were higher than those in the control samples. Later on, in the storage (8 to 12 days), there were no significant differences in TPC among the control, 6-kV PAW, and 10-kV PAW-treated samples. By contrast, the 8-kV PAW treated fresh-cut pears maintained lower contents of TPC when compared with the control over the course of the entire storage period. Sarangapani et al. (2017) has reported that the TPC of blueberries increased after 1 min of cold plasma treatment and decreased when the blueberries were subjected to longer plasma treatments. It was also indicated that the increase in TPC can be attributed to the activation of phenylalanine ammonia lyase (PAL), one of the most important enzymes in the synthesis of phenolic compounds. Cold plasma treatment can increase the PAL activity of tomatoes under disease stress (Jiang et al. 2014). At the same time, the plasma-induced ROS can also cause the oxidation of phenolic compounds (Ramazzina et al. 2015), and this effect might surpass the contribution to the TPC provided by the phenolic synthesis when higher intensity PAW is applied. Furthermore, excessive ROS was generated with prolonged storage time and inducing oxidative damage, which accelerated the degradation of phenolic compounds in fresh-cut pears. Therefore, the beneficial effect of 6-kV PAW and 10-kV PAW on TPC disappeared during the later storage (8 to 12 days). However, the mechanism to explain the effect of PAW on TPC of fresh-cut pear provided in this work is only speculative, since little work has been carried out on the effects of PAW on either the PAL activity or the pear matrix. Therefore, investigations directed at examining the influence of PAW on the phenolic metabolism and activities of related enzymes will be conducted in our future work in order to explain the present results.

Antioxidant Activity

The antioxidant activities of fresh-cut pears, as evaluated by DPPH and ABTS assays, are presented in Fig. 5. A general decreasing trend in the DPPH radical scavenging activity was observed for the control and PAW-treated samples during the entire storage (Fig. 5a). Compared with the control samples, slight reduction in DPPH radical scavenging activity of fresh-cut pears was found when they were subject to NaClO, 6-kV, and 8-kV PAW treatments (0–4 days), whereas the scavenging activity was enhanced by the 10-kV PAW treatment during the early storage period (0–6 days). The dose effect of PAW on the DPPH radical scavenging activity of fresh-cut pears was similar to that observed for the ascorbic acid content. As shown in Fig. 5b, the ABTS radical scavenging activity of fresh-cut pears also tended to decrease during the storage period. During the early storage (0–4 days), the 6-kV and 10-kV PAW treatments improved the antioxidant activity (ABTS assay) in fresh-cut pears relative to the control. In addition, no significant differences were detected between the PAW-treated samples and the control group after 6 days of storage. Although the 8-kV PAW treatment resulted in a significant loss initially when compared with the control, it exhibited the best retention of ABTS radical scavenging activity during the entire 12-day storage period. After 12 days, the control and the NaClO, 6-kV, 8-kV, and 10-kV PAW-treated fresh-cut pears lost about 26.38%, 25.36%, 19.27%, 4.18%, and 32.17% of their original antioxidant capacity, as determined by the ABTS assay, respectively. The decrease in both ABTS and DPPH radicals scavenging activities indicated that ROS scavenging capacity in fresh-cut pears deteriorated with storage time. Ascorbic acid and polyphenol were the two main antioxidants in fresh-cut fruits. Unexpectedly, the effect of the PAW dose on the ABTS radical scavenging activity did not correlate well with its effect on the ascorbic acid content and/or TPC. The antioxidant activity of phenolic compounds is directly linked to their structure (Kolniak-Ostek et al. 2013), which may be the reason for the observed lack of correlation. Furthermore, it is possible that there exists a synergetic relationship between the phenolic compounds or between ascorbic acid and polyphenol that could play a role in affecting the ABTS or DPPH radical scavenging activity (Xie et al. 2017). Even though the 10-kV PAW treatment caused a reduction in TPC, it did not decrease the content of monomeric free phenolic compounds that possess a powerful radical scavenging activity. Although the exact mechanism is not known, the soaking of fresh-cut pears in 10-kV PAW facilitates more the better preservation of antioxidant activity (DPPH and ABTS assays) than other treatments during the early period of storage.

Fig. 5
figure 5

DPPH (a) and ABTS (b) radical scavenging activities of fresh-cut pears during storage at 4 °C for 12 days. a-cMeans with different alphabets are significantly (P < 0.05) different between each sample type for a particular storage time

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Conclusions

It is apparent that exposure to appropriate doses of PAW is beneficial to reducing the levels of microbial contamination, as well as to maintaining the quality and antioxidant properties of fresh-cut pears. Generally, the antimicrobial effectiveness and quality maintenance of PAW was better than that of NaClO. Although the 8-kV PAW treatment caused a slight initial loss of antioxidant activity when compared with the control group, it exhibited better antioxidant activity retention during the later stages of storage. Furthermore, the 8-kV PAW treatment was marginally more effective when compared with the 6-kV PAW and 10-kV PAW treatments in inhibiting the growth of bacteria, yeast, and mold, as well as reducing the mass loss during storage. Therefore, the 8-kV PAW treatment is recommended for application to the preservation of fresh-cut pears. Overall, the present results confirm the hypothesis proposed in this work. That is, PAW can control the microbial growth effectively, while at the same time maintaining the antioxidant properties of fresh-cut pears. In the future, this technology should be optimized to further enhance the inactivation efficiency of PAW and to reduce the nutritional and chemical changes that occur in fresh-cut fruits over time.