Ozone treatment on cucumber and tomato during simulated retail storage

The effect of storage temperature and ozone treatment on the post-harvest quality of cucumber and tomato was investigated. Cucumber and tomato were stored together with or without gaseous ozone treatment at 20 8 C and 14 8 C for 16 days. Firmness, color, weight loss, DA index and decay percentage of samples were evaluated during storage period. The results showed that the combination of ozone treatment and cold storage could maintain the quality of these horticultural products and decreased the decay incidence. Additionally, this combination also reduced the weight loss of samples during storage. Furthermore, ozone treatment maintained the green skin color of cucumber. No sign of chilling injury occurred during storage at 14 8 C. Commodities stored with approximately 0.1ppm gaseous ozone at 14 8 C retained the firmness compared to other treatments until the end of the experiment. This study suggests a promising use of gaseous ozone treatment in storage chamber where ethylene-producing and ethylene-sensitive vegetables are stored together.


INTRODUCTION
Fruits and vegetables are essential foods for daily healthy diet.The postharvest ripening results in changing the quality of products including nutrition and appearance (Liu, 2014).Thus, maintaining quality of fruits and vegetables to meet consumers' demand is necessary.
Most of horticultural produce are not consumed immediately after harvest.In supply chain, ethylene-sensitive and ethylene-producing commodities are usually stored together.The high ethylene concentration in the storage room accelerates the ripening and senescence that cause postharvest loss of fresh fruits and vegetables (Liu, 2014).Therefore, to extend the shelf-life of ethylene-sensitive vegetables, removing the ethylene from storage room is necessary (Skog and Chu, 2001).
In order to prolong the postharvest life of horticultural products during retail storage, the development of new approaches is in demand.Cold storage is one of the most common applications in postharvest technology (Zhang et al., 2015).
Ozone is widely applied in postharvest management of horticultural commodities for two purposes including ethylene removal and sanitizing, particularly treatment pre-storage or during storage (Paulo et al., 2002).The efficacy of ozone in extending shelf-life and reducing microorganism of fresh products such as apple, pear, broccoli, cucumber and mushroom (Skog and Chu, 2001), as well as fresh melon (Nguyen et al., 2018) was reported.
Cucumber is one of the most important vegetables consumed as fresh or cooked, however, cucumber is highly sensitive to ethylene (Skog and Chu, 2001).Tomato is a climacteric and medium ethylene producing product (Watkins and Nock, 2012).In retail storage, cucumber and tomato are often stored with ethylene producing commodities that makes them deteriorate rapidly.Thus, this work was aimed to evaluate the effect of gaseous ozone treatment on tomato and cucumber during 16 days of storage at 20 8C and 14 8C.

Materials
Cucumber (Cucumis sativus L.) and tomato (Solanum lycopersicum L.) were bought from a wholesale fruit and vegetable market.Samples were transported to the laboratory.Vegetables with uniformity of size and shape, and free from external damage were used for the experiment.Cucumber at green stage and tomato at red stage 5 of ripening according to tomato ripeness chart (Postharvest Technology Center, UC Davis) were selected.Samples were kept at 20 8C or 14 8C before ozone treatment.

Methods
Experimental design.Samples were divided randomly into 4 groups, each group containing 15 cucumbers and 15 tomatoes.The weight of each piece was 81 ± 3 g for cucumber and 122 ± 3 g for tomato.Experiment was carried out with 4 storage conditions.Two groups were stored together at 20 8C or 14 8C, RH 95% for 16 days with gaseous ozone treatment at approximately 0.1 ppm.The other two groups were kept at the same temperature without ozone treatment and served as control.
Measurements.The measurements were performed according to Baranyai et al. (2020), Nguyen et al. (2020aNguyen et al. ( , 2020b) ) and Zsom et al. (2020).Stiffness, surface color, weight loss, decay percentage and DA index were conducted at the initial time (day 0) and on the 4 th , 8 th , 12 th and 16 th day.
-Stiffness.Firmness of samples was estimated using the acoustic vibration method and expressed as the parameter Stiffness (S, g 2/3 s À2 ).Stiffness of the samples was determined at 3 points on the exterior circumference of each fruit, using a tabletop acoustic firmness instrument of type AWETA AFS DTF V0.0.0.105 (AWETA, Nootdorp, The Netherlands) (Nguyen et al., 2020b).-Surface color.Skin color of samples was measured with a portable Minolta Chroma Meter CR-400 (Minolta Corporation, Osaka, Japan).Standard CIE L p , a p and b p color characteristics were determined at 3 points of each fruit.The hue angle value was calculated as arctangent of b p /a p (Nguyen et al., 2020b).-Weight loss.Weight loss was determined by weighing the samples at the beginning of the experiment and each interval with a balance (Sartorius, Germany).Results were expressed as percentage loss of initial readings according to Nguyen et al. (2020a).-Decay percentage.Decay was evaluated as mold growth occurred on stem or surface of tomato and on the skin of cucumber.Decay percentage was calculated as the number of decayed samples divided by initial number of samples within a group multiplied by 100 (Baranyai et al., 2020).-DA index.DA (or ΔA) index ® of cucumber was measured by a FRM01-F type Vis/NIR DAmeter ® (Sint eleia s.r.l., Italy) at three points on cucumber skin to evaluate the changes of surface color related chlorophyll content during storage.The value of DA-index ® varying from 0 to 5 is proportional to the amount of active chlorophyll existing in the sample (Zsom et al., 2020).
Statistical analysis.Data were collected and visualized on charts using Microsoft ® Excel ® (version 16.45).All data were analyzed using analysis of variance (ANOVA) via SPSS version 11.0.1 (SPSS Inc, USA).Significant differences were determined using P < 0.05.The results were reported as means with standard deviations.

Weight loss
Figure 1 showed the results of weight loss during 16 days of storage.The control samples at 20 8C were kept only till the 12 th day due to decay.The weight loss of vegetables increased at different rates over the storage period.Cold temperature decreased the weight loss of all samples.Vegetables stored at 14 8C had lower values in weight loss compared to samples stored at 20 8C.
The control sample kept at 20 8C without ozone treatment had the highest values in weight loss for both cucumber and tomato.Gaseous ozone treatment was found to decrease the weight loss for cucumber.
Progress in Agricultural Engineering Sciences 17 (2021) S1, 45-52 The combination of storage at 14 8C and gaseous ozone treatment was effective in reducing the weight loss of vegetables.The reason could be that cold temperature caused lower respiration rate and decelerated metabolism than higher one.Moreover, ozone treatment also had an effect in removing the ethylene in the storage chamber.This could delay the senescence of vegetables.There was no visible chilling injury on the skin of gaseous ozone treated vegetables over storage.

Stiffness
Changes of firmness were shown in Fig. 2. As can be observed, the firmness of vegetables decreased during 16 days of storage.Samples kept at 20 8C without ozone were softer than others.Vegetables treated with gaseous ozone at 14 8C had the highest values in firmness compared to other groups.Control samples at 20 8C were stored until 12 th day due to decay.
Softening of horticultural products is due to biochemical processes during ripening (Ali et al., 2010).Cold storage or ozone treatment alone had less strong effect in delaying the ripening in comparison with ozone treatment at 14 8C.The combination of cold temperature and ozone could delay the softening of commodities.

Surface color
Figure 3 showed the effect of treatments on skin color of vegetables.A significant change was observed in hue angle over storage period.Control samples at 20 8C were stored until 12 th day due to decay.There was a dramatic decline in hue angle value for control cucumbers at 20 8C.
The skin color of cucumber turned from green to yellow rapidly over storage at 20 8C, while the cucumbers treated with gaseous ozone at 14 8C were still green until the end of measurement.It was probably that cucumber is highly sensitive to ethylene, thus, the senescence of cucumber occurred promptly when the ethylene in the chamber increased (Al-Juhaimi et al., 2012).Ozone treatment slowed the color change of cucumber skin during the storage.It could be explained by the effectiveness of ozone in removing ethylene in the chamber, thus ethylene did not exert its action in ripening (Skog and Chu, 2001).
The hue color of tomato decreased dramatically during storage.The control tomato at 20 8C had the lowest value in hue angle.There was no significant difference in hue angle value between cold storage alone and cold storage with ozone treatment in the case of tomato.

ΔA index
The results of ΔA index of cucumber are shown in Fig. 4. The ΔA index of all samples decreased over storage but at different rates.Control samples at 20 8C were stored until 12 th day due to decay.
The decline of chlorophyll content relates to ripening during storage (Bron et al., 2004).The control cucumber at 20 8C had the lowest value in ΔA index.The results indicated a significant loss in chlorophyll content because the control cucumber at 20 8C was at senescence stage.Cold storage with presence of ozone could delay the chlorophyll loss over storage.

Decay percentage
Figure 5 showed that there occurred les decay in the ozone treated groups than in the control groups.The fungal growth often occurred on the surface of cucumbers and on the stem of tomatoes.The result indicated that ozone had an effect in decreasing the rot percentage over storage due to the effectiveness of ozone treatment.Application of ozone in storage chamber could remove ethylene (Skog and Chu, 2001) and inhibit the growth of microbial pathogens on the fruit surface and in the chamber (Crisosto et al., 1993;Guzel-Seydim et al., 2004).Control samples at 20 8C were stored until 12 th day due to decay.The early sign of microbial development appeared on the 4 th day for control vegetables at 20 8C and 14 8C, whereas the decay occurred on the 8 th day for the ozone treated group at 20 8C.At the same time samples treated with gaseous ozone at 14 8C started decay at 12 th of storage.The susceptibility of cucumber and tomato also depended on temperature.When produce reached advancing ripening stage, the samples had more decay.This study found that the combination of gaseous ozone treatment and cold storage was effective in inhibiting the fungal growth for up to 12 days.The result of this work was in agreement with reports for peach and table grapes (Palou et al., 2002) and for date fruits (Habibi Najafi et al., 2009).

CONCLUSION
The presented work showed that gaseous ozone treatment during cold storage could maintain the quality of cucumber and tomato.Application of approximately 0.1 ppm gaseous ozone retained the firmness, decreased the weight loss of vegetables over the storage period.Moreover, ozone slowed the yellowing of cucumber throughout 16 days of storage at 14 8C.
Additionally, decay percentage of vegetables also declined when they were stored in the presence of gaseous ozone.The results of this study found that gaseous ozone has potential application in postharvest treatment, particularly in distribution chain, when the ethylenesensitive horticultural products are stored together with ethylene-producing commodities.
Further study about optimization of storage conditions for produces in retail chain should be carried out.

Fig. 3 .
Fig. 3. Effect of storage conditions on surface color of vegetables