Scholarly article on topic 'Effects of Co-60 gamma-irradiation and refrigerated storage on the quality of Shatang mandarin'

Effects of Co-60 gamma-irradiation and refrigerated storage on the quality of Shatang mandarin Academic research paper on "Biological sciences"

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Abstract of research paper on Biological sciences, author of scientific article — Ke Zhang, Yueye Deng, Haohao Fu, Qunfang Weng

Abstract The effectiveness of Co-60 gamma irradiation in controlling citrus red mite (Panonychus citri McGregor) had been proved in our earlier work. However, whether it could be used as an alternative method to replace the current way of quarantine treatment against citrus red mites depends on the performances of effective doses on citrus fruits. This study was conducted to explore the effects of Co-60 gamma irradiation on the nutrient composition of citrus (Shatang mandarin); selected fruits were divided into different groups and each group was irradiated at 0.0, 0.2, 0.3, 0.4, 0.5, and 0.6, respectively. And then the treated fruits were stored at 4°C and the nutrient composition was studied in the following days. The results showed that the shelf-life could be extended when fruits were irradiated in the dose range of 0.2–0.4kGy, while most unirradiated citrus decayed by 15 d. It also turned out that the citrus irradiated at 0.5 and 0.6kGy were fully decayed within 45 d of refrigerated storage. The content of total soluble solids (TSS), total sugar, ascorbic acid (AA), and titratable acidity had no significant differences compare to those of the control during the 15 d storage period. Nevertheless, the activities of peroxidase (POD) and superoxide dismutase (SOD) decreased after 15 d; the improvement of storage quality and shelf life may be explained by the change of the protective enzyme activity. In conclusion, the results of citrus fruit treated with irradiation at a certain dose indicated the potential use of Co-60 gamma irradiation as a safe quarantine treatment.

Academic research paper on topic "Effects of Co-60 gamma-irradiation and refrigerated storage on the quality of Shatang mandarin"

Accepted Manuscript

Title: Effects of Co-60 gamma-irradiation and refrigerated storage on quality of Shatang mandarin

Author: Ke Zhang Yueye Deng Haohao Fu Qunfang Weng

PII : DOI:

Reference:

S2213-4530(14)00003-2

http ://dx.doi.org/doi :10.1016/j.fshw.2014.01.002

FSHW 30

To appear in:

Received date: Revised date: Accepted date:

21-10-2013

7-1-2014

17-1-2014

Please cite this article as: K. Zhang, Y. Deng, H. Fu, Q. Weng, Effects of Co-60 gammairradiation and refrigerated storage on quality of Shatang mandarin, Food Science and Human Wellness (2014), http://dx.doi.org/10.10167j.fshw.2014.01.002

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1 Effects of Co-60 gamma-irradiation and refrigerated storage

2 on quality of Shatang mandarin

3 Ke Zhang , Yueye Deng , Haohao Fu , Qunfang Weng*

4 1 Key Laboratory of Pesticide and Chemical Biology, Ministry of

5 Education,Guangzhou 510642, China; Emails: zhangke2110@163.com (K. Z.);

6 dengyueye@163.com (Y. D.);280421@163.com (H. F.).

7 2 The authors who equally contribute this research.

8 * Author to whom correspondence should be addressed; E-Mail: huabao@scau.edu.cn

9 (Q. W);

10 Tel.: +86-20-8528-0308; Fax: +86-20-8528-0292.

11 Received: /Accepted: Published:

1 Abstract: The effectiveness of Co-60 gamma irradiation in controlling citrus red

2 mite (Panonychus citri McGrego) had been proved in our earlier work, however,

3 whether it could be used as an alternative method to replace the current way of

4 quarantine treatment against citrus red mites depends on the performances of

5 those effective doses on citrus fruits. This study was conducted to explore the

6 effects of Co-60 gamma irradiation on nutrient composition of citrus (Shatang

7 mandarin), selected fruits were divided into different groups and each group were

8 irradiated at 0.0 kGy, 0.2 kGy, 0.3 kGy, 0.4 kGy, 0.5 kGy, 0.6 kGy, respectively,

9 then the treated fruits were stored at temperature of 4 L and the nutrient

10 composition were studied in the following days. The results showed that the

11 shelf-life could be extended when fruits were irradiated at a dose range of 0.2-0.4

12 kGy, while most unirradiated citrus decayed by 15 days. It also turned out that the

13 citrus irradiated at 0.5 and 0.6 kGy were fully decayed within 45 days of

14 refrigerated storage. Total soluble solids (TSS), total sugar, ascorbic acid (AA),

15 and titrable acidity had no significant differences against control during the 15

16 days storage period. Nevertheless, the activities of peroxidase (POD) and

17 superoxide dismutase (SOD) decreased after 15 days; the improvement of storage

18 quality and shelf life may be explained by the change of the protective enzyme

19 activity. As a conclusion, the results of citrus fruit under irradiation at a certain

20 dose indicating the potential use of Co-60 gamma irradiation as a safe quarantine

21 treatment.

22 Keywords: Shatang mandarin; irradiation; cold storage; nutrient composition

24 1. Introduction

25 Shatang mandarin (Citrus reticulate Blanco) is a kind of characteristic fruit in

26 Southeast China, mainly grows in Guangdong Province, South China. It is one of the

27 best commercial fruits of Guangdong Province. Due to the fruit tastes as sweet as

sugar, it's also called Mitangju. It is cultivated on a large area which accounts for about 80% of citrus planting area in Guangdong Province, due to its reasonably higher yield, better quality, greater taste and flavor than those of the other citrus fruits.

Gamma radiation has been used as a post-harvest food preservation process for many years[1]. Citrus is a seasonal product, which was consumed fresh or processed. Gamma irradiation emerged recently as a possible alternative technology for the citrus post-harvest processing, in order to fulfill the requirements of international phytosanitary trade laws. The goal of quarantine or phytosanitary treatments is to prevent the invasion and propagate of regulated pests [2]. Phytosanitary treatments, allow for the destruction or removal of pests or for making the pest reproductively sterile, to achieve the goal of control pest. There are various types of disinfestation treatments including cold, hot water immersion, heated air, methyl bromide fumigation and irradiation [3]. In addition, some of these techniques could help to minimize the loss of quality in terms of flavor, color and nutritional value [4]. Studies have showed that 'Clemenules' is a mandarin cultivar highly tolerant to X-ray irradiation and the commercial quality of the fruit was not adversely affected by postharvest quarantine applications that effectively controlled the Medfly [5]. Miller found that grapefruit irradiated at 0.3 kGy resulted in minimal injury to the fruit [6]. Furthermore, studies have demonstrated that 'Rio Red' grapefruit exposed to irradiation doses of up to 0.5 kGy did not affect soluble solids, titratable acidity, appearance, and organoleptic quality compared to untreated fruit[7]. Ascorbic acid is water-soluble vitamin, which is present in fresh fruit, especially citrus fruit and vegetable. In addition, ascorbic acid shows antioxidative effects and under certain conditions can protect against oxidativlely induced DNA damage [8]. Khalil reported that citrus fruits with doses of 0.25 and 0.5 kGy alone packed in cellophane bags and stored at room temperature for 42 days, acidity and ascorbic acid values were higher for the oranges irradiated at 0.5 kGy, weight loss decreased and total soluble solid (TSS) increased during storage period[9]. Girennavar studied the influence of E-beam irradiation on bioactive compounds of grapefruits, found that the acidity decreased slightly with an increasing E-beam dose, whereas the total soluble solids increased

1 and irradiation did not affect the vitamin C content at doses up to 1.0 kGy[ 10].

2 Irradiation for postharvest disinfestation has been investigated for various fruits

3 and vegetables and shows great promise in that it sterilizes insects at doses that are

4 low enough not to be detrimental to most fruits and vegetables [11' 12]. The United

5 States Department of Agriculture-Animal and Plant Health Inspection Services

6 (USDA-APHIS), together with other international regulatory bodies, such as the

7 International Atomic Energy Agency (IAEA) and the International Plant Protection

8 Convention (IPPC), have issued guidelines for irradiation treatments to meet export

9 and quarantine restrictions [13].

10 The benefits of using irradiation are that the cold treatment allows quality to be

11 maintained, leaves no residue on the product, and reduces the use of fumigants and

12 pallet loads can be treated at a time. Low dose gamma irradiation (1.0 kGy or less)

13 has been shown to control insect pests with little quality loss to fresh produce [14];

14 however, energy is imparted into metabolically alive tissues of the commodities, so

15 undesirable damage could be occurred [15]. In 2006, the USDA-APHIS approved

16 generic treatments of 0.15 kGy for fruit flies and 0.40 kGy for all insects except pupa

17 and adult Lepidoptera [16]. However, there are few reports on irradiation quarantine

18 treatments on controlling pest mites. Similarly, the content of health-promoting

19 compounds in citrus fruit may be altered by postharvest treatments such as irradiation.

20 For instance, recent studies showed that irradiation of citrus fruit significantly reduced

21 the total ascorbic acid (TAA) content when irradiation doses were high[10' 17].

22 However, information is still scarce on the effect of Co60-y irradiation on nutritional

23 quality of many citrus cultivars.

24 The objectives of this research were to evaluate the dose response of quality

25 factors of Shatang mandarin to irradiation at 0.2-0.6 kGy. Meanwhile, we found the

26 irradiation treatment extend the shelf life and delay fruit senescence of the citrus fruit.

27 2. Materials and methods

28 2.1. Fruits and Reagents

Shatang mandarin samples were obtained from a certain supermarket, the citrus fruits were harvested in a commercial orchard in Shaoguan city, Guangdong Province. The uniform size, maturity and fresh citrus were selected.

Bovine albumin was purchased from Shanghai Fanke Biological Technology Co., Ltd, Guaiacol was purchased from Tianjin Kemiou Chemical reagent Co., Ltd. SOD enzyme kit was purchased from Nanjing Jiancheng Bioengineering Institute, and other chemicals and solvents were purchased from Guangzhou Chemical reagent Factory. Water was treated in a water purification system, Unique + UV + UF, Research Scientific Instruments Co., Ltd.

2.2. Exposure to Co60yIrradiation

The experimental material was irradiated by Co60y-rays at the Furui high-energy Technology Co. Ltd., a Canadian company Nordion Co60y radiation source, Nansha District, Guangzhou Guangdong Province, China. The samples were divided into six groups to be exposed to different radiation doses (0, 0.2, 0.3, 0.4, 0.5 and 0.6 kGy, and the dose rate was 4 Gy/min, using Fricke dosimeter calibration) with 100 units per group. The groups 2-6 were placed into polyethylene plastic bags and irradiated with 0.2, 0.3, 0.4, 0.5 and 0.6 kGy, respectively. Group 1 was the control sample. The control and the treated samples were then packed in plastic bags and stored at 4 L for 45 days.

2.3. Analysis of major individual quality after Co60y radiation

2.3.1.Analysis of weight loss and decay rates

Each treatment contained 10 fruits and their weight loss were tested. The percentage of weight loss was calculated by following formula: [(Fresh weight -Weight at storage interval)/Fresh weight] x100. Similarly, the decay rate was calculated by (decayed fruits number/total tested fruits number) x100.

2.3.2. Total sugar

Total sugar was determined by saccharimeter (LB32T, Guangzhou Mingrui Electronic Technology Co., Ltd.) , measuring refractive index of fruit juices, the concentration of sugar was calculated.

2.3.3. Titrable acidity

1.0 mL fresh juice was added into bidistilled water of 10 mL, titrable acidity was determined according to the AOAC methods[18].

2.3.4. Ascorbic acid

Ascorbic acid was determined by direct iodine titration. Each 25 ml of the herbal fresh juice was transferred into a 250 ml Erlenmeyer flask. Twenty-five milliliter of 2 N sulfuric acid was added, mixed, diluted with 50 ml of water and 3 ml of starch T.S. was added as an indicator. The solution was directly titrated with 0.1 N iodine previously standardized with primary standard arsenic trioxide. A blank titration was performed prior titration of each sample (n=5). Each ml of 0.1 N iodine is equivalent to 8.806 mg ascorbic acid[19].

2.4. Analysis of protective enzyme activities

The pulp (1.00 g) of citrus tissues with 10mL 5mM phosphate buffer (pH 7.8 or pH 6.8) in a cold mortar, to grind. The seriflux was then centrifuged at 12,000xg at 4

°C for 15 min. The supernatant was used as the crude enzyme extract. Peroxidase (POD) activity was assayed by measuring the increase in absorbance at 470 nm using 4-methylcatechol as a substate prepared in a buffer solution with a pH of 6.8. The reaction was carried out in a 10 mm light path quartz cell. One unit (U) of POD was defined as the amount of enzyme that caused the increase of one absorbance unit (AU) at 470 nm in 10 min. Superoxide dismutase (SOD) activity was assayed by SOD enzyme kit. SOD activity was assessed by measuring the dismutation of superoxide radical generated by xanthine oxidase and hypoxanthine, colormetric analysis.

2.5. Statistical analysis

Statistical analysis was conducted for each of the measured traits by analysis of variance(ANOVA) and the means were separated by Duncan Multiple Range test using the SPSS software, version 18.0 (SPSS, Inc.). In addition, a linear discriminant analysis (LDA) was used to assess the influence of either different storage times or irradiation doses on proximate composition profiles as well as in major individual quality (weight loss, decay rate, total sugar, titrable acidity, ascorbic acid and

protective enzyme activities). All statistical tests were performed at a 5% significance level. All the assays were carried out in triplicate. The results were expressed as mean values with standard deviation (SD).

3. Results and Discussion

3.1 Physical Properties

There were little influence on the index of Shatang mandarin when treated at the dose of 0.2-0.4 kGy. The shelf life of Shatang mandarin could be prolonged by dose of 0.2-0.4 kGy, and only slight changes of weight losses were observed during the whole experimental process. There was no significant difference between irradiated citrus and the control in weight loss after 7 days, also, weight loss rates of groups treated at 0.2-0.4 kGy, which was no more than 0.01%, were significantly lower than the control after 15, 30 and 45 days by irradiation, respectively(Table 1.), while groups treated by 0.5 and 0.6 kGy were almost decayed, indicating that the irradiation treatment at 0.2-0.4 kGy could prolong the shelf life of Shatang mandarin, and the high dose (0.5-0.6 kGy) would cause damage to fruit.

Decay rates of groups treated by 0.5 kGy and 0.6 kGy reached 21% and 35% after 7 days of treatment, respectively (Table 2.), while, the control and the citrus treated by 0.2-0.4 kGy, no rotted fruit was appeared. 15 days after irradiation, 0.5 kGy and 0.6 kGy irradiated citrus decay rates were both more than 50%, while it was only 2.00% and 1.67% in the control group and 0.4 kGy treated group, respectively. There were no decay fruit in 0.2 kGy and 0.3 kGy irradiated groups. 30 days after irradiation, the decay rates of 0.5 kGy and 0.6 kGy irradiated groups were more than 90%, while the 0.2 kGy and 0.3 kGy treated groups still kept the lowest decay rates, less than 5%, and the decay rates of 0.4kGy irradiated group was 5.3%, as compared to the control 6.7%. The decay rates reached 100% after 45 days irradiation, with 0.5 kGy and 0.6 kGy, but they were less than 10% in 0.2 kGy and 0.3 kGy irradiated groups, and the 0.4 kGy irradiated group was lower than that of

control. The irradiation dose below 0.4 kGy (including 0.4 kGy), had a preservative effect on Shatang mandarin, while high doses would cause serious damage to the Shatang mandarin.

The effect of gamma irradiation on total sugar under different doses was carried out in this study. The results showed that the total sugar content decreased with the extension of storage time (Table 3.). There were no significant differences between treated fruits and the control group in the content of total sugar after 30 days. Irradiation dose above 0.5 kGy had cause great damage to the citrus fruit, while 0.2-0.4 kGy irradiation treatment had little effect on total sugar content.

It turned out that citrus fruits treated at doses of 0.2-0.4 kGy had no significant differences in weight loss, soluble solids content, total sugar content and titratable acid content compared with the control group. Fernandes[20] studied the effects of irradiation on chestnut fruits and found that the gamma irradiation doses < 3 kGy did not affect the nutritional and chemical quality of chestnut fruits, which indicated that fruits irradiated at suitable doses would have little harm on its physical properties. According to Farkas[21] , irradiation could reduced storage losses, extended shelf life and/or improved microbiological and parasitological safety of foods. In this study, the similar phenomenon was observed, showing that irradiation doses at proper level could prolong the shelf life of Shatang mandarin as well as enhance its appearance quality. We also found that the citrus fruit treated at 0.5 kGy and 0.6 kGy completely decayed in 45 days, suggesting that high dose irradiation treatment had a damaging effect on Citrus fruits, however, at which exactly dose would this happen is still unknown, further studies would be needed to carried out to conform the dosage.

No significant differences in ascorbic acid between treated fruits and the control were observed in the first 7 days yet it was increased a little bit in control group, which, then sharply decreased on the 30th day and increased again on the 45th days. Nevertheless, the ascorbic acid of irradiation treatment groups kept decreasing during the whole experimental period, and this kind of change was much faster in the control than it in irradiation treatment. There were no significant differences between treated groups and the control in ascorbic acid after 15 days and 30 days,

respectively.When tested on the 30th day, the ascorbic acid of the treatment groups was slightly higher than control but it in groups irradiated at 0.2-0.4 kGy then became lower than control after 45 days treatment. The results illustrated that irradiation treatment doses of 0.2-0.4 kGy on citrus fruits could somehow affect its ascorbic acid content; however, the difference between irradiated samples and the control was not obvious after 30 days of storage at 4 □.

The results showed that titratable acid of Shatang mandarin decreased as the storage time extended. There was no significant difference between groups treated by 0.2 kGy and 0.3 kGy in titratable acid content when tested on the 7th day and 15th day, but it significantly decreased after 30 and 45 days irradiation treatment in these two groups when it was compared with control (Figure 1.), and the content of titratable acid in groups treated at a dose at 0.3 kGy or higher than it were less than a half of control. It indicated that the irradiation effect on titratable acid content of Shatang mandarin would become serious after 30 days.

Figure 2 showed that although irradiation treatment had a certain impact on the ascorbic acid content, the ascorbic acid content of irradiation treatment group reduced much slower than that of the control. Kaewsuksaeng[22] reported that UV-B treatment induced a gradual increase in citric acid and suppressed the increase of sugar contents during storage. In addition, the ascorbic acid content with or without UV-B treatment decreased during storage, but the decrease in the control was faster than that with UV-B treatment. Others also pointed that irradiation not only affected seed formation in the fruit, but also lowed acidity [23]. From figure 1 we can conclude that titratable acid of Shatang mandarin decreased with the extension of storage time, which revealed that irradiation treatments had significant effect on titratable acid content of Shatang mandarin.

Table 1. Weight loss Rate (%) of the citrus fruits after different gamma irradiation treatments.

Sample Days in storage

7 15 30 45

Control 0.17±0.03a 0.64 ±0.09a 0.96±0.10c 1.22±0.02a

0.2 kGy 0.16±0.05a 0.35±0.01d 0.56±0.03d 0.75±0.02c

0.3 kGy 0.16±0.05a 0.36±0.03d 0.59±0.01d 0.79±0.02bc

0.4 kGy 0.16±0.02a 0.40±0.05cd 0.65±0.05d 0.83±0.004b

0.5 kGy 0.18±0.02a 0.44±0.05bc 1.26±0.09b —

0.6 kGy 0.18±0.02a 0.49±0.01b 1.57±0.07a —

Table 2. Decay Rate (%) of the citrus fruits after different gamma irradiation

treatments.

Sample Days in storage

7 15 30 45

Control 0.00±0.00c 2.00±1.00c 6.67±1.15c 14.67±2.52b

0.2 kGy 0.00±0.00c 0.00±0.00c 3.00±1.00d 8.00±2.00d

0.3 kGy 0.00±0.00c 0.00±0.00c 3.67±1.15cd 8.33±1.15d

0.4 kGy 0.00±0.00c 1.67±0.58c 5.33±0.58cd 11.33±1.53c

0.5 kGy 21.00±3.61b 55.00±5.00b 92.33±2.52b 100.00±0.00a

0.6 kGy 35.00±5.00a 69.67±4.73a 97.67±2.52a 100.00±0.00a

Table 3. Total sugar (g/100 ml) of the citrus fruits after different gamma irradiation

treatments.

Sample Days in storage

7 15 30 45

Control 15.42±0.42a 15.11±0.38a 14.99±0.38a 14.65±0.33a

0.2 kGy 15.40±0.42a 15.08±0.38a 14.77±0.37a 14.24±0.29ab

0.3 kGy 15.40±0.42a 14.93±0.37a 14.68±0.35a 14.02±0.26ab

0.4 kGy 15.39±0.42a 14.84±0.35a 14.49±0.32a 13.69±0.29bc

0.5 kGy 0.6 kGy

15.37±0.42a 15.07±0.38a

14.66±0.34a 14.15±0.25a

14.16±0.33a 13.98±0.28a

1 Note: Date(Table 1, 2 and 3) are presented as the mean±standard deviation of

2 triplicate measurements. Days in storage (7, 15, 30, 45.) mean the days after treatment.

3 Values in the same column with different letters are significantly different (p<0.05).

5 Figure 1.Chang in content of Titrable acidity after 7, 15, 30, 45 days of different

6 gamma irradiation treated samples and control as compared to time 0 day. Values

7 with an asterisk differ significantly (p < 0.05) from the start of the experiment [CK

8 (control) ; 0.2 kGy treatment, 0.3 kGy treatment, 0.4 kGy treatment, 0.5 kGy treatment and 0.6

9 kGy treatment, for parts A=7 days after treatment, B=15 days after treatment, C=30 days after 10 treatment, and D=45 days after treatment respectively].

CK 0.2kGy 0.3kGy 0.4kGy 0.5kGy 0.6kGy

CK 0.2kGy 0.3kGy 0.4kGy 0.5kGy 0.6kGy

0.2kGy 0.3kGy 0.4kGy 0.5kGy 0.6kGy

CK 0.2kGy 0.3kGy 0.4kGy

12 Figure 2.Chang in content of Ascorbic acid of the citrus fruit after 7, 15, 30, 45

13 days of different gamma irradiation treated samples and control as compared to time 0

1 day. Values with an asterisk differ significantly (p < 0.05) from the start of the

2 experiment [CK (control) ; 0.2 kGy treatment, 0.3 kGy treatment, 0.4 kGy treatment, 0.5 kGy

3 treatment and 0.6 kGy treatment, for parts A=7 days after treatment, B=15 days after treatment,

4 C=30 days after treatment, and D=45 days after treatment respectively].

7 3.2 Enzyme Activities

8 Results of the enzymatic analyses in Figure 3 and 4 illustrated that the SOD

9 enzyme activity of citrus fruits with or without irradiation all increased greatly at the

10 first 7 days, and the control showed the highest activity than other irradiation

11 treatment groups (Figure 3). This may attibute to free radicals induced by

12 irradiation. The SOD activity of all groups dropped to a low level after 15 days, it

13 might because the free radical were cleared by SOD enzyme increased before.

14 After 30 days, SOD enzyme activity of all groups bounded back to a relatively higher

15 leve again, in which, the value of 0.4 kGy and 0.5 kGy treatment groups

1 increased greatly. 45 days after irradiation treatment, 0.5 kGy and 0.6

2 kGy irradiated citrus fruit decayed completely, and the SOD activity of other groups

3 decreased, and the SOD activity of irradiation treatment groups were lower than the

4 control, showing that irradiated fruit produced more free radicals, however, the

5 treated groups had a larger fluctuations in SOD activity than the control group during

6 the whole experimental process; overall, low dose (D0.4 kGy) irradiation could play

7 a role in delaying Citrus Reticulate Blanco fruit senescence as well as in prolonging

8 the storage shelf.

9 SOD catalyzes the dismutation of superoxide anions to produce hydrogen peroxide,

10 which is then removed by catalase, and the two enzymes are thought to extend food

11 freshness by protecting the integrity of membranes [24]. SOD activity in the citrus fruit

12 increased with or without irradiation treatments, but the activity of the enzymes

13 significantly lower than that in the control throughout 7 days of storage at 4D (Figure

14 3.). It may result of free radicals induced by irradiation . SOD is a primary scavenger

15 for superoxide free radicals, which plays a role in the dismutation of superoxide

16 radicals, whereas catalase (CAT), ascorbate peroxidase (APX) and glutathione

17 reductase (GR) activities would contribute, at least to some extent, to the elimination

18 of hydrogen peroxide[25].

19 Studies have been carried out a certain correlation browning of fresh fruits and

20 vegetables with tissue POD, POD through the oxidation reaction can lead to

21 deterioration of the quality of fruits and vegetables. The results of Fig. 2 showed

22 that POD activity of Shatang mandarin fruit increased at first place, then decreased as

23 the storage time extended. The POD activity of all experimental groups increased

24 after 7 days irradiation, showing a dose-dependent manner; The POD enzyme activity

25 of 0.2-0.4 kGy treatment groups were significantly lower than control after 45 days

26 irradiation, And the 0.5 kGy and 0.6 kGy irradiation groups showed the highest POD

27 activity of the whole experiment process, indicated that high dose

28 irradiation would cause damage to the citrus fruit.

29 With regard to the effects on enzyme activity, a study by Falguera et al. [26] found a

30 slightly decrease in the activity of POD in fresh apple juices from Golden, Starking

and Fuji, in UV irradiation treatments, this value slightly decreased during the experiment. Meanwhile, in the juice from King David the loss was 70.0%. In this study, a decrease of POD activity was also observed (Figure 4), Shatang mandarin, low acidity, contained ascorbic acid isn't enough to inhibit the activity of POD for long time, a large number of high active POD will lead to deterioration of the quality. The reason of gamma irradiation to extend the shelf life of the fruit might be due to the influence of the basic metabolism of the fruit, suppressing respiratory enzyme activity, inhibiting the release of CO2 and ethylene production, resulting in delayed ripening and senescence; secondly, irradiation could kill microbial spoilage of fruit, and irradiation played a role in preservation. And it suggested that the SOD and POD enzymes played important roles in the citrus fruit senescence, while the mechanism needed to be further explored.

Figure 3.Effect of Co60y irradiation on the superoxide dismutase (SOD) of the citrus treated with or without irradiation after 7, 15, 30, 45 days at different dose during storage at 4 □.

- 0.2kGy

- 0.3kGy

- 0.4kGy

- 0.5kGy

- 0.6kGy

21 28 Days in storage

Figure 4.Effect of Co60y irradiation on the peroxidase (POD) activity of the citrus treated with or without irradiation after 7, 15, 30, 45 days at different dose during storage at 4 □.

• 8,ray

r 0.4iGy

* o.etGy

- 0 6KGy

\ ^Jt X ^

I-.. •

3 -1-—--r—----r " »■-T"1—""-T-T—1 r-T--T" —™r

q r M » M m

1 Days in storage

3 The results of effects of Co-60 gamma-irradiation on the quality index and the

4 protective enzyme activities of the citrus fruit during storage. Our studies showed that

5 low dose irradiation (0.2-0.4 kGy) treatments could prolong the shelf life of the citrus

6 fruit and these doses would slightly affect the quality of citrus fruits with an

7 acceptable level. These results showed that certain Co-60 gamma-irradiation could be

8 a promising safety method to control insect pests and extend the shelf life and delay

9 senescence of other kinds of fruits and vegetables.

10 4. Conclusions

11 Our results suggested that irradiation at the dose range of 0.2-0.4 kGy and in

12 combination with refrigerated storage is an effective post-harvest technique in

13 mitigating the risk of pest and decay of quarantined fruit.

14 Acknowledgments

15 The researchers gratefully acknowledge the grants from the International Atomic

16 Energy Agency under Research Contract No. 15630.

17 Conflicts of Interest

18 The authors declare no competing financial interest.

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