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Effects of ascorbic acid and high oxygen modified atmosphere packaging during storage of fresh-cut eggplants Xihong Li, Yuqian Jiang, Weili Li, Yao Tang and Juan Yun Food Science and Technology International 2014 20: 99 DOI: 10.1177/1082013212472351 The online version of this article can be found at: http://fst.sagepub.com/content/20/2/99

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Article

Effects of ascorbic acid and high oxygen modified atmosphere packaging during storage of fresh-cut eggplants Xihong Li, Yuqian Jiang, Weili Li, Yao Tang and Juan Yun

Abstract Ascorbic acid dip and high O2 modified atmosphere packaging were used to alleviate browning and quality loss of fresh-cut eggplants. Fresh-cut eggplants were dipped in water or 0.5% ascorbic acid solution for 2 min before being packed in polyethylene film bags filled with air or high O2. The physiochemical and sensorial attributes of cut eggplants were evaluated during 12 days for storage at 4  C. Results demonstrated that high O2 modified atmosphere packaging and ascorbic acid dip improved the preservation of fresh-cut eggplants compared with the control. High O2 showed an ability to reduce the browning and inhibit polyphenol oxidase and peroxidase activities. Higher total phenolic content and lower malondialdehyde content were also observed in ascorbic acid treated samples during storage. Moreover, the combination of ascorbic acid and high O2 was more effective than single treatments. The surface color was protected by ascorbic acid and high O2 packaging, and higher sensory scores were observed after 12 days of storage. Keywords Browning, polyphenol oxidase, peroxidase, total phenolic, sensory properties Date received: 27 August 2012; accepted: 3 December 2012

INTRODUCTION The eggplant (Solanum melongena L.) is an important non-climacteric vegetable, with around 30 million tons of worldwide production every year (Massolo et al., 2011). Like most of fresh-cut products which make up one of the most rapid growing products in food industry, a demand for fresh-cut eggplants has increased owning to the convenience. However, freshcut products are generally more perishable than whole tissues because they have been subjected to physical stress or damage (Odriozola-Serrano et al., 2009). In fresh-cut eggplant tissues, the enzymatic reactions resulted from wound-induced destruction of cellular compartmentation and consequent oxidation of phenolic compounds (Kader, 2002), and consequent tissue

Food Science and Technology International 20(2) 99–108 ! The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1082013212472351 fst.sagepub.com

browning occurring in seconds (Rocha and Morais, 2001), lead a deterioration in surface appearance. Chemical dip and modified atmospheres packaging (MAP) are largely used to extend the shelf life of a wide range of fresh-cut products (Oms-Oliu et al., 2010; Soliva-Fortuny and Martı´ n-Belloso, 2003). Ascorbic acid has been widely used in processing of fruit and vegetables as antioxidant to reduce discoloration (Son et al., 2001). Ascorbic acid was chosen as chemical for dip because it is harmless and is a required nutrient for humans, lack of which in dietary ascorbate results in the clinical syndrome scurvy (Steven and Mark, 1998). However, the chemical is quickly consumed by fresh-cut produce and provides only temporary protection (Sapers, 1993).

Key Laboratory of Food Nutrition and Safety, Tianjin University of Science and Technology, Ministry of Education, Tianjin, China Corresponding author: Yuqian Jiang, Tianjin Economic and Technological Development Area (TEDA), No. 29, 13th Avenue, Tianjin 300457, China. Email: [email protected]

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Food Science and Technology International 20(2) Several studies have recommended O2 and CO2 combinations (Beaudry, 1999) or exclusion of oxygen for the storage of fresh-cut fruit and vegetables. But in recent years, the application of high O2 atmospheres has been suggested as an improvement over the low O2 or high CO2 concentrations atmosphere packaging (Wszelaki and Mitcham, 2000). High O2 atmosphere is an effective technology (Day, 1996a) which can retain freshness, keep natural characteristics and retard microbe of fresh-cut carrot, cabbages, bayberries, strawberries and blueberries (Amanatidou et al., 2000; Hu et al., 2003; Limbo and Piergiovanni, 2006; Zheng et al., 2008). And high O2 has been found effective at preventing anaerobic fermentation reactions, offflavors, moisture losses and reducing microbial growth (Day, 1996b, 2002; Jacxsens et al., 2001). Effects of high oxygen partial pressures in combination with ascorbic acid on enzymatic browning and quality loss of minimally processed potatoes have been reported (Limbo and Piergiovanni, 2006, 2007). Considering the short-lived protection of ascorbic acid and varied effects of super atmospheric O2 on different commodity (Farber et al., 2003), the objective of this work was to determine the effects of high O2 MAP and ascorbic acid dip treatment on the preservative quality of fresh-cut eggplants, both individually and in combination. As tissue color attributes consumer’s buying decisions and brown surface is seen as spoiled, the treatments were applied to explore an effective approach to maintain the quality of fresh-cut eggplants.

MATERIALS AND METHODS Plant materials and sample preparation Round purple eggplants (Solanum melongena var. esculentum) were purchased from a local supermarket at Tianjin city, northeastern of China. After pre-cooled at 4  C for 24 h, eggplants were selected for uniform size, color and maturity degree (dark purple, and a slightly under-ripe maturity degree) without any injury or disease. The eggplants were peeled and cut into cubes (2  2  2 cm3) about 5 g each, using a sharp stainless steel knife, before washed by running water. Four different treatments were applied: (I) water dip þ air MAP (the control), (II) ascorbic acid (analytically pure; Jiangtian Chemical Technology Co. Ltd., Tianjin, China) solution dip þ air MAP (T-1), (III) water dip þ high O2 MAP (T-2) and (IV) ascorbic acid solution dip þ high O2 MAP (T-3). Eggplant cubes of the control and T-2 were dipped into sterilized distilled water while those of T-1 and T-3 were dipped into 0.5% ascorbic acid solution, respectively, both for 2 min at 4  C (Limbo and Piergiovanni, 2007). Then these cubes were gently dried with sterile filter papers.

The entire processing was conducted in a sterile environment. The pretreated eggplant cubes, 150 g each, were packaged in pouches (30 cm  20 cm) of polyethylene (0.06 mm thickness, and gas transmission rates for O2 and CO2: 220983.8 and 94475.9 cm3/m2dMPa, respectively, water vapor transmission: 167.4 cm3/ m2dMPa) with ambient air (for the control and T-1) or with pure O2 (for T-2 and T-3) flushed with a SF-200 packaging machine (Puzhong Instrument Ltd., Hangzhou, China). The O2 levels inside the pouches were constantly monitored with a Checkpoint II O2/ CO2 meter (PBI-Dansensor A/S, Danish, Ringsted, Denmark). The pouches were hermetically sealed when the atmosphere reached the desired O2 level (higher than 90 kPa) and stored at 4  C for 12 days and related parameters were evaluated every 3 days. Gas composition Gas composition inside the packages was measured by a Checkpoint II O2/CO2 meter (PBI-Dansensor A/S, Danish, Ringsted, Denmark). Triplicate samples from different bags of same treatment group were prepared and tested and different bags at each test date. Color measurement The color of the cut-surface was determined using a CM-3600d chromameter (Konica Minolta, Japan). Color parameters were recorded as L*, a* and b*, where L* ¼ 0 (black) to L* ¼ 100 (white), a* (greenness) to þ a* (redness), and b* (blueness) to þ b* (yellowness). The hue angle (h*) (h* ¼ arctan (b*/a*)) was also calculated and L* and h* values were used as indicators of the cut-surface browning intensity (Du et al., 2009; Oms-Oliu et al., 2008a, 2008b). The lower L* and h* values represented an increase in the degree of browning discoloration. Measurements were carried out with 3 sites on the surface of 3 cubes from each treatment (Du et al., 2009). PPO, POD activities and total phenolic content PPO activity was determined as described by Du et al. (2009). Briefly, samples (5 g) were ground in ice-cold bath containing sodium phosphate buffer (0.2 mol/L, pH 7.0, 2% polyvinylpolypyrrolidone (PVPP), stored at 4  C) at the ratio of 2.0 mL: 1.0 g of eggplant samples. The homogenates were centrifuged at 12,000  g for 10 min using a centrifuge 5804R (Eppendorf China Ltd., Beijing, China) at 4  C. The supernatants (0.2 mL) were mixed with substrate solution (2.8 mL) and the increase of absorbance was measured at 410 nm at 25  C using a 756-PC UV/VIS spectrophotometer

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Li et al. (T6 New Century, Beijing Purkinje General Instrument Co., Ltd, China). The substrate solution was 0.02 mol/ L catechol dissolved in 0.05 mol/L sodium phosphate buffer (pH 7.0). A unit of enzyme activity was defined as the change of 0.001 in the absorbance value per 30 s. Enzyme extraction of POD was performed similarly to PPO extraction but using 10 mL of extraction phosphate buffer (0.05 mol/L, pH 7.8, 2% PVPP, stored at 4  C; Liu et al., 2010) and assayed at 470 nm (Venisse et al., 2001). The reaction mixture contained phosphate buffer (50 mM, pH 6.8), 1.0 mL of 0.46% H2O2, 1.0 mL of 2% guaiacol and 1.0 mL of enzymatic extract in a total volume of 5.0 mL. One unit of POD activity was defined as the change of 0.001 absorbance value per 30 s. PPO and POD assessment were conducted in triplicate and expressed as U/g fresh weight (FW) (Luo et al., 2008). The total phenolic content was measured according to the Folin-Ciocalteu (FC) procedure (Rocha and Morais, 2001) with some modifications. Samples (5 g) were crushed, homogenized and diluted to 100 mL with distilled water. After heated (100  C, 20 min) and cooled in icy water, the mixture was centrifuged at 5000  g (4  C, 15 min) using a centrifuge 5804R. The supernatant (1 mL) was mixed with 1 mL of FC reagent (Sigma Chemical Co. St Louis, MO, USA) before adding 5 mL distilled water and 3 mL of 7.5% (w/v) Na2CO3 carbonate solution (Jiangtian Chemical Technology Co. Ltd., Tianjin, China). The system reacted for 60 min at room temperature (about 25  C) before measured at 765 nm by the 756-PC UV/VIS spectrophotometer. Chlorogenic acid (Sigma Chemical Co., St Louis, MO, USA) was used as standard and the results were expressed as mg chlorogenic acid/100 g FW. Total soluble solid, membrane permeability and malondialdehyde Samples (10 g) were crushed and homogenized before measuring total soluble solid (TSS) using a PLA-a pocket refractometer (Atago Co., Ltd., Japan) at 20  C and expressed in percentage. Electric conductivity was determined to reflect membrane leakage according to the method of Zhang et al. (2005) with some modifications. Samples were sliced into discs of 0.05 cm thick and washed three times with deionized water to remove surface-adhered electrolytes. Disks (5 g) were put into deionized water (30 mL) and shaken on a rotary shaker for 30 min at 25  C. The conductivity of the solution (EC0) was measured with a DJS-1C conductivity meter (Shanghai Analytical Instrument Co., Shanghai, China). After boiling (10 min) and cooled at room temperature, the solution was readjusted to 30 mL using deionized

water. Total conductivity of the solution (ECT) was measured and the relative electric conductivity (%) ¼ (EC0 / ECT)  100 (Luo et al., 2008). Thiobarbituric acid reactive substances (TBARS) were determined and expressed as malondialdehyde (MDA) equivalents (Shahidi and Hong, 1991; Song et al., 2009). Eggplant tissues (5 g) were homogenized with 20 mL of trichloroacetic acid (TCA, 10%) and then centrifuged at 10,000  g for 10 min. The supernatant (1 mL) was mixed with 3 mL of 0.5% thiobarbituric acid (TBA) dissolved in 10% trichloroacetic acid. The reaction mixture solution was heated (95  C, 20 min) and cooled before centrifuged at 10,000  g for 10 min. The supernatant was assayed at 450, 532 and 600 nm, respectively, with the 756-PC UV/VIS spectrophotometer (T6 New Century, Beijing Purkinje General Instrument Co., Ltd, China). MDA content (nmol/g FW) ¼ [6.452  (OD532–OD600)  0.559  OD450]  Vt/(m  VS), where Vt and VS were the total volume of the extract solution and the volume of the extract solution contained in the reaction mixture solution, respectively, and m was the mass of fresh samples. All the measurements above were conducted in triplicate. Microbial analysis The total bacterial count (TBC), yeast and mold count (YMC) were measured and performed based on a direct plating technique as described by Rojas-Grau et al. (2008) with minor modifications. Eggplant samples (5 g) were removed aseptically from each package and homogenized with 45 mL of sterile 0.85% NaCl solution before blending for 60 s with a homogenizer (Kangjian Medical Apparatus Co. Ltd., Jiangsu, China). Ten-fold dilution series were made in distilled water of serial dilution tubes by taking 1.0 mL of sample solution of predecessor into 9.0 mL of sterile distilled water. Then 1.0 mL of the appropriate sample dilution was pour-plated on plate count agar (PCA, Shanghai Hufeng Chemical Industry Co. Ltd., Shanghai, China) for TBC measurement (30  C, 24–48 h). Similarly, YMC was pour-plated on potato dextrose agar plate (28  C, 24–48 h). Duplicate and the control plates were prepared for each sample, and only counts of 30–300 colony-forming units per gram (cfu/g) were recorded. Microbial measurements above were done in triplicate and the results were reported as log cfu/g. Sensory evaluation Sensory evaluation of fresh-cut eggplants were performed by a panel of 10 experienced panelists on days 0, 6 and 12. The panelists received different treated 101

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Food Science and Technology International 20(2)

Control T-1 T-2 T-3

Partial pressure of O 2 (kPa)

90

25

20

80 15 70 20

10

5

10

Partial pressure of CO 2 (kPa)

100

0

0 0

3

6

9

3 12 0 Storage time (day)

6

9

12

Figure 1. Changes in O2 and CO2 concentrations inside polyethylene film bags containing fresh-cut eggplants during the storage at 4  C for 12 days. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP. Bars represent means  SD (n ¼ 3, p  0.05).

samples in random order. The samples were presented to the panelists in individual partitioned compartments of a controlled environment room (Xiao et al., 2010). The sensory panel scored quality attributors including visual appearance (feeling on global surface), browning (specific color for brown surface), texture (the firmness when chewing), taste (the mouth-feel and unique flavor of eggplants) and overall acceptability (edible value). Scores for each sample were based on a 1 - to 9-point scale where 9 – like extremely, 8 – like strongly, 7 – like very well, 6 – like fairly well, 5 – like moderately, 4 – like slightly, 3 – dislike slightly, 2 – dislike moderately, 1 – dislike extremely (Mishra et al., 2012). Statistical analysis Origin 8.1 software was used for data analysis. The mean values were calculated and reported as the mean  SD (n ¼ 3). Analysis of variance one way ANOVA and Ducan’s new multiple range test, followed by least significant difference (LSD) and with a significance level p  0.05.

RESULTS AND DISCUSSIONS

1. Significant differences were found in gas concentrations of packages for cut eggplants in different treatments during the storage period. Peeling and cutting operations can increase the respiration rate as the whole fruit structure is damaged by cutting (Limbo and Piergiovanni, 2007). The O2 partial pressure of packages with T-3 samples was higher than that of T-2 samples, reaching levels of 82.0 and 78.4 kPa on the 9th day, respectively. The higher O2 depletion of T-2 possibly indicated a more rapid O2 diffusion into the cell interior of injured tissues, which increased metabolic activity. Lower O2 decrease rates were detected in high O2 MAP treatments (T-2 and T-3) compared with air MAP (the control and T1) (Figure 1(a)). This was in accordance with the research by Day (1996b). The pattern of O2 decrease corresponded to that of CO2 increase (Figure 1). The CO2 accumulation rates were relatively lower in T-2 and T-3 especially after day 6 (Figure 1(b)). According to Figure 1, ascorbic acid solution dips reduce the rates of O2 decrease and CO2 increases compared with the control. The results seemed suggest that both ascorbic acid dip and high O2 atmosphere package had significant effects on O2 depletion and CO2 accumulation, and T-3 kept suitable gas environment even on the 12th day.

Changes in gas composition Changes in gas composition (O2/CO2) of packages containing treated samples, achieved by respiration of freshcut eggplant tissues during storage, are shown in Figure

Changes in color and browning Enzymatic browning is resulted from reactions between phenolic compounds and oxidative enzymes owning to

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T-2 T-3

Control T-1

88

100 86

L*

82

92

h*

96

84

80 88 78 B

A 76

84 0

3

6

9

12

0

3

6

9

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Storage time (day)

Figure 2. Changes in the color (L* value and h* angle) of fresh-cut eggplants during storage at 4  C for 12 days. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP. Bars represent means  SD (n ¼ 3, p  0.05). MAP: modified atmosphere packaging.

cellular disruption (Massolo et al., 2011). It is a common and important problem in the process and storage of fresh-cut products and the eggplant is not exceptional. The degree of browning depends on the nature, amount of phenolic compounds, oxygen, pH and the activity of relative enzyme (Carbonaro and Mattera, 2001; Goupy et al., 1995). Figure 2 shows the changes in color or browning (L* value and h* angle) of cut eggplants pretreated with water or ascorbic acid solution dip and stored under different MAPs at 4  C. As shown in the figures, the most severe color degradation was observed in the control which indicated that the rest of the treatments had effects on browning of cut eggplants. Moreover, initial L* value and h* angle were best maintained by T-3 treatment, indicating the least cut-surface browning. Since the values of L* and h* indicates the extent of browning and thus the quality of eggplant, T-3 treatment was a more applicable approach to keep cut eggplants than other treatments. In other words, the natural surface color could be maintained to a certain extent by pretreatment of ascorbic acid dip or high O2 MAP, but the combination of the two treatments (ascorbic acid dip combined high O2 MAP) was stronger than single treatments. This result was in accordance with previous studies on apples (Lu and Toivonen, 2000), peach (Zhu et al., 2009) and potatoes (Limbo and Piergiovanni, 2006).

PPO, POD activities and total phenolic content The eggplant has strong antioxidant capacity because of high phenolic compounds content (Cao et al., 1996). Phenolic compounds which are potent antioxidants also have effects on preventing human disease (Sawa et al., 1999; Triantis et al., 2005). However, this kind of antioxidant leads cut surface of fruit and vegetables to brown extremely fast. Browning is mainly catalyzed by PPO and POD. In the whole issue, PPO and phenolics are present in chloroplast and vacuoles, respectively. However, physical cutting permits PPO catalyzing phenolic compounds to quinones at the presence of oxygen, then the secondary non-enzymatic reactions result in the accumulation of melanin-like pigments which bring undesirable brown appearance (Martinez and Whitaker, 1995; Massolo et al., 2011). Besides, phenolic oxidation catalyzed by POD produces a variety of free radical species directly or indirectly (Hoover et al., 1998). As shown in Figure 3(a) and (b), the minimum PPO and POD activities were both observed in T-3 group and the change was more smoothly than other treatments during the entire storage period. The phenolic compound levels correlate with browning in eggplant (Prohens et al., 2007) and these changes are shown in Figure 3(c). T-3 presented a significantly (p < 0.05) higher level of total phenolic content than other treatments and the phenolic 103

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Food Science and Technology International 20(2)

Control T-1 T-2 T-3

(a)

6

(b)

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(b) Electric conductivity (%)

POD activity (U/g FW)

40 30 20 10 4

(c) MDA (nmol/g FW)

Total phenolic content (mg/g FW)

(c)

3 2 1 0

3 6 9 Storage time (day)

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(a) 5.5 TSS (%)

PPO activity (U/g FW)

8

5.0 4.5 4.0

35 30 25 20 1.4 1.2 1.0 0.8

12

Figure 3. Changes in PPO, POD activities and total phenolic contents of fresh-cut eggplants during storage at 4  C for 12 days. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP. Bars represent means  SD (n ¼ 3, p  0.05). PPO: polyphenol oxidase; POD: peroxidase; MAP: modified atmosphere packaging.

contents were correlated with the PPO and POD activities. T-1 and T-2 had effects on reducing enzymatic activities and maintaining phenolic contents compared with the control. It can be concluded that ascorbic acid solution dips and high O2 MAP both influenced the fresh-cut eggplant samples. Super atmospheric O2 has an effect on inhibiting PPO enzymatic reaction, or high level of colorless quinines have a feedback constraint on PPO activity (Day, 1996a). However, the exact mechanism of high O2 inhibiting enzymatic browning is not known. Dipping eggplant cubes in ascorbic acid solution could remove the surface reactants such as enzymes and phenolics and thus decrease the oxidation and reduce the browning. Moreover, ascorbic acid not only has a direct effect on PPO but also can reduce the o-benzoquinones converting back to o-diphenols (Apintanapong et al., 2007). Delaying or reducing enzymatic oxidation of phenolics is an important

0.6

0

3 6 9 Storage time (day)

12

Figure 4. Changes in TSS, relative solute leakage and MDA content of fresh-cut eggplants during storage at 4  C for 12 days. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP. Bars represent means  SD (n ¼ 3, p  0.05). TSS: total soluble solid; MDA: malondialdehyde; MAP: modified atmosphere packaging.

means to maintain quality (Duan et al., 2009). Therefore, the surface color of fresh-cut eggplants could be maintained by single treatments (T-1 and T-2), and the combined treatment (T-3) showed a stronger ability for keeping storage quality. Changes in TSS, membrane permeability and MDA Eggplant samples from T-3 had significantly higher TSS than those from T-1, T-2 and the control on the 6th day (p < 0.05), and the TSS of T-3 samples was more stable through the whole storage period than other groups (Figure 4(a)). Relative leakage rate has been used as an indicator of injury degree of fruits. There was an increase in relative leakage rates of fresh-cut eggplants stored at 4  C indicating the

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5

Control T-1 T-2 T-3

7 6

4

3 4 2

3

YMC (cfu/g)

TBC (cfu/g)

5

2 1 1 0

0 0

3

6

9

12

0

3

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Figure 5. Total microbial count (TBC and YMC) of fresh-cut eggplants treated differently during storage at 4  C for 12 days. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP. Bars represent means  SD (n ¼ 3, p  0.05). TBC: total bacterial count; YMC: yeast and mold count; MAP: modified atmosphere packaging.

increase of membrane permeability (Figure 4(b)). There were significant differences (p < 0.05) between treated and control samples. On the 12th day, the relative leakage rate was 24.8% in T-3 and 30.0% in T-1, 26.6% in T-2 while 33.8% in the control. T-3 was the most effective in reducing electrolyte with the relative leakage rate of 30.7% at the end of storage. The results suggested that tissues in T-3 might suffer less injury in cell membrane, and this might due to the effects of ascorbic acid and gas composition. As an indicator of oxidative stress, MDA is the final product of lipid peroxidation and its accumulation causes the membrane permeability rise and then makes electrical conductivity value increase and damages cell membrane system (Hodges et al., 1999). Lower MDA content indicates less lipid peroxidation. As shown in Figure 4, the MDA content increased over 12-day storage time and all treated groups delayed this increase significantly from the 6th day (p < 0.05) compared with the control. This was because the persistence of lipid peroxidation increased the membrane permeability and damaged cell membrane system (Hodges et al., 1999). In summary, T-3 significantly maintained TSS, reduced the increase of membrane permeability and MDA content of fresh-cut eggplant cubes. Ascorbic acid dip combined with high O2 MAP was the most effective in extending the quality of cut eggplant. The oxidation reaction was inhibited and the membrane integrity was maintained.

Microbial quality Microbial safety is regarded as an important factor in preservation of fresh-cut fruit and vegetables (Bico et al., 2009). Fresh-cut eggplants become more susceptible since natural protection is removed after cutting and they are easily contaminated by bacteria, yeast and mold which can cause spoilage. During the entire storage period, there was no visible mold growth observed which was considered as an indicator of spoilage suggesting products unusable (Mishra et al., 2012). Legislation in different countries has set the upper limits of microbial load as 4.7 log cfu/g at the production stage and 7.7 log cfu/g at the consumption stage for fresh-cut vegetables (Erturk and Picha, 2006; Mishra et al., 2012). In this study, the microbial counts increased from about 2 to 7 log cfu/g during the storage (Figure 5), so all samples could be considered acceptable during the entire storage period as they were within the limitation. In this study, samples were stored at relative low temperature (4  C), which could reduce microbial growth (Carbonaro and Mattera, 2001). As shown in Figure 5, total bacterial counts (TBC), yeast and mold counts (YMC) of all treated samples were lower than those of the control during the storage time period. On the 12th day, TBC of T-3 was 6.0 log cfu/g while the control had increased to 7.2 log cfu/g. T-1 and T-2 were 6.4 and 6.2 log cfu/g, respectively (Figure 5(a)). 105

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7.6  0.5 c 7.1  0.6 c 6.8  0.4 b 7.2  0.6 bcd 7.5  0.6 bc 8.8  0.2 d 8.3  0.5 d 8.6  0.5 d 8.3  0.3 e 8.7  0.6 e Appearance Browning Texture Taste Overall quality

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The data are mean  SD of scores given by 10 panelists. Mean values in a row with different letter are signiEcantly different (p < 0.05). MAP: modified atmosphere packaging.

7.2  0.4 b c 6.4  0.3bc 6.7  0.6 b 7.0  0.5 bc 7.0  0.4 ab 6.8  0.5 b 5.9  0.5 b 6.3  0.4ab 6.6  0.5 b 6.8  0.5 ab 5.7  0.3 a 4.8  0.3 a 5.5  0.4 a 5.8  0.4 a 6.1  0.7 a 8.3  0.3 d 8.5  0.2 d 8.5  0.5 d 8.1  0.4 de 8.3  0.5 ce 8.5  0.3 d 8.1  0.4 d 8.4  0.5 d 8.0  0.5 de 8.2  0.3 ce 8.3  0.4 d 7.8  0.6 d 8.1  0.7 d 7.6 a  0.5 cde 7.8  0.4bce

T-1 Control

7.5  0.4b c 6.8  0.5 c 7.7  0.3 d 7.3  0.4 bcd 7.7  0.6 bce

T-3 T-2 T-2

T-3

Control

T-1

Day 12 All

Overall, ascorbic acid solution dips pretreatment and high O2 MAP were found effective in maintaining quality and extending shelf-life of fresh-cut eggplants. The combined treatment decreased tissue browning and delayed deterioration of fresh-cut eggplants during a 12-day storage at 4  C. The gas composition was examined and showed a relationship with biochemistry attributors and quality of cut eggplants. Ascorbic acid and high O2 played important roles in restraining PPO and POD activities, maintaining total phenolic and soluble solid content, as well as the integrity of the cell membrane. The fresh-cut product could be microbiologically and sensorially accepted after 12 days of storage. Therefore, ascorbic acid dip and high O2 MAP associated with proper refrigerated temperature as a

Features

CONCLUSIONS

Day 6

Changes in sensory qualities of fresh-cut eggplants during 12 days storage are presented in Table 1. T-3 was more effective than the other treatments in maintaining the sensory qualities of fresh-cut eggplants. The sensory score was 7.7 at the end of storage, which was significantly higher than the other treatments which had 6.8 and 7.0 of scores for T-1 and T-2, respectively. An obvious lower score in sensory quality was observed in control and the score was 5.9 on the 12th day. Ascorbic acid dips and high O2 MAP seemed to maintain visual appearance and reduce the browning and thus preserve the sensory quality of eggplants.

Day 0

Sensory evaluation

Storage time

These results showed that all of the treatments delayed the increase in bacterial population. A similar effect was observed in reducing the growth of yeast and mold population (Figure 5(b)). YMC of T-3 was 3.8 log cfu/g while 4.7 log cfu/g in the control on the 12th day and 4.0 log cfu/g in T-1, 4.2 log cfu/g in T-2. Elevated O2 was found to inhibit the growth of both anaerobic and aerobic microorganisms since the optimal O2 level for growth (21% for aerobes, 0–2% for anaerobes) was surpassed (Farber et al., 2003). However, another study using super atmospheric O2 levels to control microorganisms showed that only O2 close to 100 kPa are truly effective (Kader and BenYehoshua, 2000; Wszelaki and Mitcham, 2000). In our study, O2 partial pressure higher than about 80 kPa could reduce the microorganisms. And 0.5% ascorbic acid solution pretreatment seemed have an effect on retarding the microorganisms. However, TBC reduction degree was not so strong on day 12 compared with the previous 9 days. This might be due to the change of gas composition and elongation of storage time.

Table 1. Sensory evaluation of fresh-cut eggplants treated with ascorbic acid and/or MAP. Fresh-cut eggplants were dipped in water or 0.5% ascorbic acid followed by regular passive MAP or high O2 MAP. After treatments, the samples were stored for 12 days at 4  C. Control ¼ water dip þ passive MAP; T-1 ¼ ascorbic acid dip þ passive MAP; T-2 ¼ water dip þ high O2 MAP; T-3 ¼ ascorbic acid þ high O2 MAP

Food Science and Technology International 20(2)

Li et al. wholesome approach would be used as a method to preserve fresh-cut eggplants. In the future, the relation between ascorbic acid concentration and O2 partial pressure and effects on storage quality will be investigated. FUNDING This study was supported by a grant from the National High Technology Research and Development Program of China (863 Program) – Detection and control of harmful factors during the whole supply and physical distribution of agricultural products (No. 2012AA101703).

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