2.1. Collection of Litchi Fruits Fully ripened litchi fruits cultivar “China-3” was collected from a local orchard in Dinajpur City, Bangladesh. Harvesting time was June to July. Temperature varies from 26 to 33 0C, relative humidity varies between 82% and 83%, and rainfall ranges from 264.99 to 456.7 mm during the flowering and fruiting seasons. Fruits of uniform size, shape, and color without visible defect and disease were selected.
2.2. Fruit Treatments and Storage The fruits first were immersed in cold water for three hours at 1–2 0C until the core pulp temperature was lower than 6 0C, followed by random splitting of the fruits into two groups. One fruit group was dipped for 30 s in a 5% oxalic acid solution that allows air to dry for about two minutes. In a polyethylene container, another group of fruits were taken. Then, a beaker containing 1-methylcyclopropane (1-MCP) solution was placed into the polyethylene container and sealed. The treatment was carried out for 4 h. For the preparation of 1-methylchyclopropane (1-MCP) solution, 2 g of the 1-MCP powder was mixed with 3 mL. Following treatments, four treated fruits were taken and screened into HDPE and LDPE packs and stored in a refrigerator for 30 days at 4–6 ?C with 85% relative humidity. The untreated sample was used as a control. At 10 days, fruits were removed from the refrigerator and various quality characteristics were assessed.
2.3. Analysis of Physicochemical Properties 2.3.1. Determination of Weight Loss The loss of weight was calculated with the following equation and expressed in terms of percentage
Weight loss = Intial f ruit weight be f ore storage − f inal f ruit weight a f ter storage / Intial f ruit weight be f ore storage x 100
2.3.2. Determination of Pericarp Color, pH, and Titratable Acidity Chroma Meter (CR-2000 Japan) was used for measurement of pericarp color with L*, b*, and a* values. The digital pH meter (HI 2211 pH/ORP, China) was used for pH measurement. Titratable acidity against 0.1 N sodium hydroxide was determined by the Ranganna (1997) [9] method. 2.3.3. Determination of Total Sugar Contents The Dubois et al. (1956) [10] method was used to determine the total sugar in treated and untreated fruits. 2.3.4. Determination of Vitamin C (Ascorbic Acid) Content According to the method of Ranganna (1997), the ascorbic acid content was determined. Pulp tissues (1 g) were mixed with 50 mL of meta-phosphoric acid (3% w/v) solution by using a pestle and mortar, and permitted to stand for 20 min. The mixture was then filtered through a filter paper of Whatman No: 41. Next, 1 mL of filtrate and 10 mL of 2,6-dichlorofenol dye solution was added to a falcon tube and vortex. The absorption was measured with a spectrophotometer (UV 1800 Shaanxi, China) at 518 nm and the results were shown as mg/100 g.
2.3.5. Evaluation of Total Anthocyanin Content The overall content of anthocyanin was based on the Ranganna (1997) [9] method. One-gram pulp was mixed with 10 mL of HCl ethanol and then stood in the dark place for 30 min. The extract was filtered by a filter paper Whatman No: 41 and centrifuged for 20 min with 4000× g. The absorption was then measured with a spectrophotometer (UV 1800 Shaanxi, China) at 530 nm against the blank and the result was expressed as mg/100 g of pulp tissue. For determining the total content of anthocyanin, the following equation was used.
2.3.6. Assessment of Total Phenol Content The total phenol content was measured according to the Singleton and Rossi (1965) method. Pulp tissues (1 g) and 10 mL methanol were homogenized with the use of a mortar and pestle and filtered via a filtering paper of Whatman No: 41. One mL of homogenate sample was then taken in a falcon tube, 0.5 mL of Folin–Ciocalteu reagent was added afterwards, along with 1 mL of 7.5% saturated Na2CO3 and 8 mL of distilled water added to the tube and vortex. The tube was allowed for 35 min at room temperature in a dark place followed by centrifugation at 4000× g for 10 min. Standard concentration (0 to 0.213 mg/mL) was used to establish the calibration curve. Absorption was recorded at 765 nm against a blank using a spectrophotometer (UV 1800 Shaanxi, China). The results were expressed in mg of gallic acid equivalent to 100 g of pulp tissue.
2.3.7. Estimation of Flavonoid Content In order to establish total flavonoid contents in control and processed fruits, the Kim et al. (2003) [12] method was used. A mortar and pestle were used to mix fruit pulp (1 g) with 10 mL of methanol, followed by a filtering through Whatman Filter paper No: 41. Then, 1 mL of filtrate and 4 mL of distilled water were mixed in a falcon tube, with 5% NaNO2 for 5 min at room temperature. After that, 0.3 mL of 10% AlCl3 solution was added and stood for 1 min. Then, it was thoroughly mixed with 2 mL of 1M NaOH and 2.4 mL of distilled water. The mixture was centrifuged at 4000× g for 5 min and kept for 15 min in the dark. Different concentrations (0 to 0.49 mg/mL) were used to prepare the standard curve. The absorption was measured with a spectrophotometer at 510 nm against a blank (UV 1800 Shaanxi, China) with results of mg catechin/100 g.
2.3.8. Antioxidant Activity Measured by DPPH Scavenging The approaches used to test DPPH scavenging were Madhujith and Shahidi (2006) [13]. Tissues of pulp (1 g) were extracted using a mortar and pestle with 10 mL methanol, and were filtered through a filter paper of Whatman No: 41. Afterwards, 0.1 mL of extract was taken in a falcon tube and 1.9 mL of 0.3 mM DPPH solution was added. Then, the mixture was stood for 30 min in a dark position. Trolox was used as a positive control. At each concentration (0 to 1 µmol/g), different solutions were used to generate the standard curve. The absorption was measured with a spectrophotometer (UV 1800 Shaanxi, China) at 517 nm against the blank and the findings were shown with µmol Trolox equivalent (TE)/g.
2.3.9. Antioxidant Activity Measured by Reducing Power The method of Oyaizu (1986) was used to evaluate the reducing power in the control and treated fruit. Pulp tissue (1 g) was homogenized using a pestle and a mortar with 10 mL of methanol and was filtrated through a filter paper of Whatman No: 41. The homogenate was then combined in a test tube with 2.5 mL (0.2 m, pH 6.6) phosphate buffer solution containing 2.5 mL of ferric-cyanide potassium (1% w/v). The blend was subsequently incubated at 50 0C for 20 min. After incubation, 2.5 mL of 10% trichloroacetic acid (TCA) was added to the blend and centrifuged for 10 min at 1750 g. Trolox was used as a positive control. The standard curve was generated using different concentrations (0 to 11 µmol/g). The absorbance was recorded at 700 nm against a blank with a spectrophotometer (UV 1800 Shaanxi, China) and expressed as µmol Trolox equivalent (TE)/g.