Impact of ozone on quality of strawberry during cold storage

Abstract

The postharvest physiology of strawberry was investigated to study the effects of ozone on weight loss rate, ascorbic acid, respiration rate, peroxdase (POD) activity, catalase (CAT) activity, and malondialdehyde (MDA) content during cold storage. The strawberries were treated with ozone of 0 ppm (control), 2 ppm, 4 ppm, and 8 ppm, respectively. The results indicated that the treatment of 4 ppm ozone could inhibited the decrease of ascorbic acid, POD activity, and CAT activity, and reduced weight loss rate and MDA content. The treatment delayed the senescence of strawberry, with a significantly lower respiration rate. Thus, the best concentration of ozone was 4 ppm, and ozone treatment could be a good candidate for maintaining postharvest quality of strawberry and provide a longer storage life.

Keywords

strawberry / ozone / cold storage / quality

Introduction

Strawberry is one of the most popular fruits due to its special taste and medical function (Li et al., 2006; Yang and Kang, 2006; Cong, 2008). However, it is highly perishable and susceptible to microbiological decay during storage (Wills and Kim, 1995; Kim et al., 2010). They have a very short postharvest life, and losses reach 40%. Control of postharvest decay in strawberry fruit has been mostly dependent on the use of fungicides and low temperatures (Washington et al., 1992; Bao and Liu, 2004; Qian et al., 2006). However, because of the negative effects of fungicides on the environment and human health, and the development of fungicide resistance by pathogens (Cao et al., 2010; Gabler et al., 2010), there is an urgent need to seek alternatives.

Ozone is a highly reactive form of oxygen where three molecules are bonded together. Generated electrically on-site where needed, it has a potent antimicrobial activity and other characteristics. Interest in ozone applications for agriculture and food processing has increased in recent years (EPRI Expert Panel, 1997). Ozone has a long history as a water disinfectant in common use for this purpose in many parts of the world. Many aspects of ozone use have been reviewed: (1) water disinfection applications (Nickols and Varas, 1992; White, 1999); (2) food safety and sanitation (Graham et al.. 1997; Kim et al., 1999), (3) chemistry (Razumovski and Zaikov, 1984), and (4) responses of horticultural products to ozone (Forney, 2003). The purpose for this article was to describe experiments of the evaluated ozone applications in packinghouses.

To the best of our knowledge, there are few data regarding the effect of ozone on the postharvest quality aspects of strawberry. Therefore, the objective of this study is to identify the effects of ozone on weight loss, ascorbic acid, respiration rate, POD activity, CAT activity, and MDA content of strawberry.

Materials and methods

Plant materials

Strawberries were obtained from an orchard in Baoding City, Hebei Province, China, and transported to the laboratory. They were graded for uniform size and colour and the absence of physical damage, and then randomly divided into groups for experiments.

Ozone application and experimental design

Strawberries were treated with 0 (conmtrol), 2, 4, and 8 ppm ozone for 30 min at 0ºC everyday. The strawberries were placed in plastic bags and ozone was injected, then the bags were immediately sealed and the samples were stored for 30 min at 0ºC. Ozone concentrations were monitored and controlled using ozone analyzers (Model 1180, Dasibi Environmental Corp. Glendale, CA) and a data logger equipped with electronic relays (Model 21X, Campbell Scientific Inc. Logan, UT). Programmed set points in the data logger were used to turn the generators on and off to maintain the desired ozone concentrations. After treatment, all experimental units were stored at 0±1ºC and 90%±5% RH. Ozone was generated by electric discharge from pure oxygen (model SGA01 Pacific Ozone Technology Inc., Brentwood, CA, USA) to avoid the generation of impurities. All treatments were replicated three times as an experimental unit for each parameter studied. Following ozone treatments fruit were stored for 0, 5, 10, 15, and 20 days. At the end of each storage period, measurements were made of weight loss rate, ascorbic acid, respiration rate, POD activity, CAT activity, and MDA content.

Weight loss rate determination

Using an analytical balance (Sartorius BP 210 S, Goettingen, Germany), we determined the weight of 10 strawberries. The weight was registered. The weight loss rate was calculated using the following formula:

Weight loss rate= (The weight of fruit before storage – the weight of fruit during storage)/The weight of fruit before storage × 100%.

Ascorbic acid determination

The ascorbic acid was measured by 2, 6-dichlorophenolindophenol titration (GB/T 6195-1986, Code of National Standard of China). Briefly, tissue (50 g) from six fruits was immediately homogenized in 50 mL of 0.02 g/mL oxalic acid solution and then centrifuged at 15000×g at 4ºC for 15 min. Afterwards, 10 mL of supernatant was titrated to a permanent pink colour by 0.1% 2, 6-dichlorophenolindophenol titration. Ascorbic acid concentration was calculated according to the titration volume of 2, 6-dichlorophenolindophenol and expressed as mg/100g fresh weight.

Respiration rate determination

Respiration rate was measured by sealing each jar, and sampling the headspace (0.5 mL) using gas-tight glass syringes (Precision Sampling Corp., Baton Rouge, LA). Gas samples were analyzed using a gas chromatograph with a thermal conductivity detector (Fisher Gas Partitioner, model 1200, Fisher Scientific, Springfield, NJ).

Peroxidase (POD) activity

Peroxidase (POD) activity was determined by measuring the increase in absorption at 470 nm according to Kalir et al. (1984) with modification. The reaction was carried out at 25ºC for 20 min in a 3 mL reaction mixture containing 0.02 mL tissue extract, 50 mmol/L sodium phosphate buffer (pH 7.0), and guaiacol 20 mmol/L. The reaction was initiated by the addition of 20 μL H₂O₂. One unit of POD was defined as the amount of enzyme that caused a 1.0 increase of A₄₇₀ per min under the assay condition. Each experiment was repeated three times.

Catalase (CAT) activity

Catalase (CAT) activity was determined by monitoring the decomposition of H₂O₂ at 240 nm following the method of Aebi (1984). The reaction mixture contained 1.7 mL 50 mmol/L sodium phosphate buffer (pH 7.0), 1 mL double distilled water and 0.2 mL tissue extract. The reaction was initiated by adding 0.1 mL 100 mmol/L H₂O₂. One unit of catalase was defined as the amount of enzyme which caused a 0.01 decrease of A₂₄₀ per min at 25ºC.

Malondialdehyde (MDA) content

MDA (malondialdehyde) content was determined with thiobarbituric acid reaction (Zhao et al., 1994). In brief, 0.25 g of tissue was homogenized in 5 mL of 0.1% (w/v) trichloroacetic acid. The homogenate was spun at 10000×g for 5 min. To a 1-mL aliquot of the supernatant, 4 mL of 20% (w/v) trichloroacetic acid containing 0.5% (w/v) thiobarbituric acid was added. The mixture was heated at 95ºC for 15 min and cooled immediately, and the absorption of the supernatant was read at 450, 532, and 600 nm, respectively.

MDA content (μmol/gFW) = 6.45 (OD₅₃₂–OD₆₀₀) -0.56 OD₄₅₀.

Statistical analysis

The SPSS 11.5 for Windows (Chicago, Illinois, USA) was used for the analysis of data. All results were treated for multiple comparisons by analysis of variance with least significant difference (LSD) between means determined at 5% level.

Results and discussion

Effect of ozone treatment on the weight loss rate

Figure 1 shows the weight loss of ozone treated strawberries measured at the end of the storage periods for samples stored for 20 days at low temperature. The weight loss increased with the storage periods for strawberry samples. On the 10th day of storage, the weight loss of control reached 5.60%; however, the weight losses in 2 ppm, 4 ppm, and 8 ppm ozone treatments were 3.41%, 2.14%, and 2.84%, respectively, which were lower (P<0.01) than those in the control and the weight loss in 4 ppm ozone treatment was lower (P<0.05) than 2 ppm or 8 ppm ozone treatments. However, there was no marked effect between 2 ppm ozone treatment and 4 ppm ozone treatment on weight loss. According to Kays (1991), water loss of more than 4%-6% (of the total fresh weight) resulted in visible wilting or wrinkling of the surface of most commodities. These results suggested that the optimal concentration of ozone treatment was 4 ppm.

Effect of ozone treatment on the ascorbic acid

Variations in ascorbic acid values during storage are shown in Fig. 2. Ascorbic acid values decreased continuously during low temperature storage. On the 10th day of storage, ascorbic acid values of control only reached 46.21 mg/100g, reserving rate was 68.76% and fruit quality began dropping. Ascorbic acid in 2 ppm, 4 ppm, and 8 ppm ozone treatments were 58.32 mg/100g, 59.65 mg/100g, and 57.94 mg/100g, respectively, reserving rate was 87.32%, 88.37%, and 85.84%, with fruit still keeping in preferable quality, and the ascorbic acid in these treatments was higher (P<0.01) than that in the control. Accordingly, the 4 ppm ozone treatment was used as the most preferred concentration in all other treatments.

Effect of ozone treatment on the respiration rate

Variations in respiration rate during storage are depicted in Fig. 3. The respiration rate declined initially, and then rose steadily during low temperature storage. On the 15th day of storage, the respiration rate of control just had 43.20 mLCO₂/kg/h FW; however, it declined quickly when treated with 2 ppm, 4 ppm, and 8 ppm ozone respectively, reaching 40.10 mLCO₂/kg/h FW, 30.23 mLCO₂/kg/h FW, and 38.45 mLCO₂/kg/h FW, which were lower (P<0.05) than that in the control. Compared with control storage, ozone treatments can better restrain the respiration rate of strawberries, and then extend its shelf life. 4 ppm ozone treatment was used as the preferred concentration in all the other treatments.

Effect of ozone treatment on POD activity

Changes in POD activity of strawberries during storage are shown in Fig. 4. POD activity rose initially, and then declined steadily during low temperature storage. On the 5th day of storage, the POD activity of the control reached its peak value (50.126 ΔOD₄₇₀/min/g FW). POD activity of 2 ppm, 4 ppm, and 8 ppm ozone treatments reached their peak value (respectively 61.874 ΔOD₄₇₀/min/g FW, 53.298 ΔOD₄₇₀/min/g FW, and 65.326 ΔOD₄₇₀/min/g FW) which were higher (P<0.05) than that in the control, indicating that 4 ppm ozone treatment could better inhibit the decrease of POD activity. These results proved that different concentrations of ozone treatment could delay the manifestation of the peak and restrain the decrease of POD activity during storage anaphase.

Effect of ozone treatment on CAT activity

Changes in CAT activity of strawberries are shown in Fig. 5. The CAT activity rose initially, and then declined steadily with time during low temperature storage. The CAT activity of the control slowly increased in the first 5 days. On the 5th day of storage, it reached its peak value 2.56 (0.01ΔOD₂₄₀/min/g FW). Compared with the control, the CAT activity in ozone treatments kept a higher level during the storage life, and simultaneously delayed the peak value. In 2 ppm, 4 ppm, and 8 ppm ozone treatments, The CAT activity reached their peak value of 3.15 (0.01ΔOD₂₄₀/min/g FW), 3.49 (0.01ΔOD₂₄₀/min/g FW), and 3.47 (0.01ΔOD₂₄₀/min/gFW) respectively, lower (P<0.01) than that in the control. It is demonstrated that the different concentrations of ozone treatment could effectively delay the manifestation of the peak and inhibit the decreasing of CAT activity at subsequent storage. These results suggested that the optimal concentration of ozone treatment is 4 ppm.

Effect of ozone treatments on MDA content

MDA content of strawberries gradually increased over the 20-day storage period (Fig. 6). On the 10th day of storage, the MDA content of ozone treatments was lower (P<0.01) than that of the control, and the MDA content of 4 ppm ozone treatment was lower (P<0.05) than that of 2 ppm or 8 ppm ozone treatment; however, no significant difference between 2 ppm ozone treatment and 8 ppm ozone treatment was found. It is demonstrated that different concentrations of ozone treatment could effectively inhibit the accumulation of MDA content, with the optimal effect of 4 ppm ozone treatment among them.

Conclusions

According to the results obtained in the study, the different concentrations of ozone treatment could be used to maintain the quality of strawberry. The most effective ozone treatment was at 4 ppm which could inhibit the decrease of ascorbic acid, POD activity, and CAT activity, reducing the weight loss rate and MDA content, and simultaneously delaying the senescence of strawberry and significantly lowering the respiration rate. Thus, 4 ppm was the best concentration of ozone as a good candidate for maintaining postharvest quality of strawberry and providing a longer storage life.

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