Extended the lifespan of cherry tomatoes by coated with mucilage from dragon fruit -

Extended the lifespan of cherry tomatoes by coated with mucilage from dragon fruit

Cherry tomatoes are climacteric fruits that have a limited shelf life. In this study, combining mucilage from dragon fruits and UV-C irradiation was carried out. Cherry tomatoes were subjected to UV-C irradiation and edible coating, both as a stand-alone and hurdle treatment. The edible coating was prepared from the mucilage of white dragon fruits. Quality parameters including color, weight loss, total soluble solids, titratable acidity, ascorbic acid, antioxidant analysis and microbial analysis were measured throughout 21 days of storage at 4 ◦C. Results showed that the hurdle treatment extended shelf life by 21 days, reduced weight loss, color changes and inhibited microbes better than stand-alone treatments. Furthermore, fruits treated with the combination of UV-C and edible coating also contained higher total polyphenol content, total flavonoid content and ascorbic acid.

Materials and Methods

Extraction of plant mucilage fresh white dragon fruits purchased from a local supermarket was washed under running water to remove dirt and debris. Then, the skin was peeled, and the pulp was removed. The hydration method was used to extract the mucilage from the pulp. Firstly, the separated fruit pulp was weighed and then forced through a sieve with a pestle to remove the seeds. The volume of seedless pulp was measured with a measuring cylinder. Ethanol was added with the ratio of 2:3 (extract: alcohol) to precipitate the mucilage for 24 h (hour) at 4 ◦C and collected by filtration and then oven-dried for 24 h at 40 ◦C.

Treatment of Cherry Tomatoes

Cherry tomatoes were washed using distilled water and dried at room temperature. From a pool of cherry tomatoes, 25% of them were used as control (group A); 25% were dipped in a solution consisting of pure mucilage diluted threefold with distilled water for 30 s (group B); 25% of cherry tomatoes in group C were irradiated by fluorescent germicidal lamps, 15 cm away from the surfaces of the lamp for 8 min (minutes). Another 25% of cherry tomatoes (group D) were also irradiated by fluorescent germicidal lamps under the same condition as above and subsequently dipped in a solution comprising pure mucilage diluted with distilled water (1:3) for 30s. The coated fruits were air-dried (28 ◦C) for 30 min. All the fruits were stored at 4 ◦C at 95% relative humidity. The fruits were observed and evaluated on days 0, 3, 7, 14, and 21 based on their color, weight loss, total phenolic contents, total flavonoid content, ascorbic acid (DCPIP) soluble solids, titratable acidity, and microbial analysis.

Antioxidant Analysis

Preparation of extract samples for antioxidant analysis. To purify the sample, equal parts of tomato juice were added to 80% methanol (5 mL:5 mL). The mixture was placed in a shaking incubator at 250 rpm for 30 min at room temperature followed by centrifugation at 5 ◦C and 6500 rpm for 15 min. The precipitate was discarded, and the remaining supernatant was filtered using a steel sieve with an approximate diameter of 2 mm to obtain juice and subsequently stored in sterile glass bottles. The supernatant was used for the analysis of antioxidant activity.

The total polyphenol content was determined using Folin–Ciocalteu assay.

The flavonoid content of cherry tomato juice samples was determined using a colorimetric method and results were reported as milligrams of catechin equivalent (CE) per 100 mL juice extract.

The ascorbic acid content in samples was determined based on the 2,6-dichlorophenolindophenol (DCPIP) visual titration method. Cherry tomato juice was diluted with 3% metaphosphoric acid and filtered.

Total soluble solids (TSS) were determined using a digital refractometer at 25 ± 1 ◦C, and results were expressed in a standard ◦Brix unit.

To determine titratable acidity, diluted cherry tomato juice was titrated with standardized 0.1 N sodium hydroxide to a definite faint pink endpoint using phenolphthalein as an indicator. The volume of sodium hydroxide used for titration was converted to grams of citric acid per 100 mL of juice. 

Weight loss tomato samples were weighed using a Mettler Toledo weighing balance on days 3, 7, 14, and 21. The difference between initial and final fruit weight was considered as total weight loss during each storage interval and calculated as percentages on a freshweight basis by the standard AOAC (2010) method.

Microbial inactivation analysis: Microbial count of juice samples was determined using Petrifilm plates for aerobic bacteria, coliform, yeast, and mold and was calculated as colony-forming units and the results were expressed as log (CFU/mL).

 Results

The color parameters for different samples were evaluated and the results showed that the values significantly increase from Day 3 to Day 21 for all samples. The control sample showed the highest result of color change on Day 21 from Day 0, while the hurdle treatment showed the least color change on Day 21 (11.61 ± 0.95 ∆E) from day 0. ∆E is calculated based on the L* (lightness; 0 = black, 100 = white), a* (+a = redness, −a = greenness), and b* (+b = yellowness, −b = blueness). The increasing data showed the result of the ripening process where fruit color will be redder and darker.

The percentage of weight loss for all samples was not significant during the first seven days but increased significantly after storage at 4 ◦C. After 21 days, the control sample lost more weight (1.26 ± 0.14%), while the hurdle treatment sample has the least weight loss (0.87 ± 0.05%). The weight loss showed an increasing trend during a prolonged storage period in both treated and untreated samples. Mucilage as an edible coating can delay the migration of fruits because the coating can reduce respiration and transpiration, resulting in the lowest weight loss percentage. The coating acts as a physical barrier that helps to reduce moisture loss, solute movement, and gaseous exchange (O₂ and CO₂) due to the formation of a film/coating on top of the skin. Thus, combining two treatments as a hurdle treatment was the most effective in reducing the percentage weight loss of cherry tomatoes during storage.

The results of titratable acidity for different types of treatments on cherry tomatoes shows an increasing trend, the titratable acidity in all the treated samples was higher than the control at the end of the 21-day storage period. This can be explained by the fact that the coating may have slowed the respiration rate of the fruits; thus, the rate of utilization of respiratory substrate was very minimal. Thus, this combination treatment resulted in the highest results at the end of the storage period.

Total soluble solid content is an important quality indicator to measure the sweetness of cherry tomatoes. It shows an increasing trend for TSS (◦Brix) from day 0 to 7, and it decreased after that. This could result from increasing sugars during the storage period through the degradation of polysaccharides.

The effect of mucilage coating and UV-C on total phenolic content for all samples. The results show a general increase from Day 0 to Day 21. The hurdle treatment sample’s highest antioxidant activity was recorded on Day 21 with 0.132 ± 0.003 (mg GAE/100 mL) and the least by the mucilage-treated sample on Day 3 with 0.04 ± 0.003 (mg GAE/100 mL). The effect of mucilage coating and UV-C on total flavonoid content and the results show an increasing trend from Day 0 to 21, but a significant reduction was observed for all samples on Day 21. The hurdle treatment sample showed the highest antioxidant activity on Day 14 with 13.179 ± 0.002 (mg CE/100 mL). This sample also showed the highest activity on Day 21, even though there was a reduction for all samples.

The application of edible coatings to fresh fruit has been associated with an accumulation of phenolic compounds and ascorbic acid, causing an increase in the fruit’s antioxidant capacity. In this study, hurdle treatment was the most effective way to increase the antioxidant activity for total phenolic and flavonoid content during postharvest storage.

The effect of mucilage coating and UV-C on the ascorbic acid content of the samples increased throughout the experiment. Hurdle treatment showed the highest ascorbic acid content (1.07 ± 0.06 mg/100 mL) on Day 14. The stability of ascorbic acid in fruits is usually influenced by high titratable acidity. The mucilage-coated sample showed a gradual increase in ascorbic acids during the storage period. This suggests that that treatment did not prevent the synthesis of ascorbic acid content during the ripening process.

The effect of mucilage coating and UV-C on microbial analysis showed a significant increase from Day 0 to 21. The hurdle treatment sample showed no microbial activity on Day 0 and 7 and had the least microbial activity compared to other samples on Day 14 and 21. The control sample showed the highest activity on Day 21 with 0.133 ± 0.010 (log CFU/mL). Mucilage will create a layer to prevent bacteria or foreign substances from entering the sample’s tissue.  UV-C light will kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA, leaving them unable to perform their functions. Furthermore, UV-C can also reduce microbial activity by killing bacteria through the germicidal effect. In this study, hurdle treatment effectively delays the ripening process and extends cherry tomatoes’ storage life.

Conclusions

This study showed that the hurdle treatment was the best in extending shelf life and inhibiting microorganisms due to the potent effect of UV-C irradiation on microorganisms that cause fruit decay together with the antimicrobial and antioxidant effect of the mucilage coating. The hurdle treatment also effectively reduced weight loss due to the mucilage barrier, which limits water transpiration and respiration. Color was slightly altered by the hurdle treatment, therefore maintaining the visual appearance of the fruit even after 21 days in storage. Furthermore, fruits treated with a combination of UV-C and edible coating also contained higher ascorbic acid content and antioxidant capacity. These results demonstrate that the combination of UV-C and edible coating as a hurdle treatment could be an innovative method to preserve shelf life and quality of fruits.

Reference:

Razali, Z., Somasundram, C., Nurulain, S.Z., Kunasekaran, W. and Alias, M.R., 2021. Postharvest Quality of Cherry Tomatoes Coated with Mucilage from Dragon Fruit and Irradiated with UV-C. Polymers, 13(17), p.2919.