The experiments were carried out at the laboratories of the Departments of Horticulture, Plant Pathology and Biochemistry of Bangladesh Agricultural University, Mymensingh during the period from 15 May to 15 September, 2016. The Litchi cultivar, namely ‘Bombai’ was chosen as experimental materials for this investigation.
Experimental materials Litchi cultivar ‘Bombai’ is the most important commercial cultivar of West Bengal, India. It is also cultivated commercially in Bangladesh. Fruits are mostly heart shaped and each fruit has another tiny underdeveloped fruit attached to the fruit as in case of the Chinese cultivar ‘Hom Suiuchi’. Ripening commences from the last week of May. The color at maturity is indicated as yellow-green background with red tubercles. Average weight of fruit varies from 15-20 g. Fruit pulp is greyish white, soft, juicy, sweet to sour in taste and has pleasant flavor. The seeds are bigger in shape, 2.28 cm long, 1.58 cm in diameter, average weight 3.4 g, and elongated, smooth, shiny and light chocolate in color. The ratio of rind, pulp and seed by weight is 12.11:70.08:16.8. Ghosh et al. (1987) reported that the average fruit weight of Litchi is 190 g which consisted of 17.9% skin, 19.9% seed, 62.2% pulp on the basis of fruit weight. They also mentioned that the fruit pulp contained 17.7% total soluble solids, 0.42% acidity and 11.0% sugars.
Experimental treatments and design In total 8 treatments were implemented in this experiment. The experiment consisted of two factors. Factor A: Temperature, T1: Ambient temperature, T2: 4 ºC temperature, Factor B: thickness of Polypropylene bag (PP bags), P1: Control (unwrapped), P2: 50 µm PP bag, P3: 75 µm PP bag, P4: 100 µm PP bag. The two factor experiment was laid out in completely randomized design (CRD) with three replications having eight fruits. 36 cm × 24 cm size PP bags were used in this experiment.
Application of postharvest treatments A total of 192 fruits of more or less similar shape and size and free of visible disease symptoms were randomly selected from the harvested fruits. Among eight fruits in each replication of each treatment four fruits were marked to investigate color, pericarp browning, total weight loss, disease incidence, disease severity, isolation and identification of causal pathogens, shelf life and the remaining four fruits were kept unmarked conditions for destructive sampling to examine moisture content, dry matter content, TSS, pulp to peel ratio, vitamin C and pulp pH.
Methods of studying parameters Among eight fruits in each replication four fruits were used for destructive sampling at three days intervals to investigate several parameters including moisture content, dry matter content, total soluble solids, pulp to peel ratio, vitamin C and pulp pH. The remaining four fruits were used to investigate pericarp browning, total weight loss and shelf life. The chemical parameters were estimated using the method of Ranganna (1979). Pericarp Browning was determined using numerical rating scale of 1-5, where, 1 = no browning, 2 = 1-<25% browning, 3 = 25- <50% browning, 4 = 50-<75% browning and 5 = 75-100% browning. Pulp to peel ratio was determined at 3rd, 6th and the 9th day of storage and electric balance was used for determining weight. Weight loss of fruit was estimated during storage by using the following formula:
Weight loss (%) = IW-FW / IW x 100
Where, IW = Initial weight of fruits (g), FW = Final weight of fruits (g). Percent moisture content was calculated according to the following formula: Moisture content (%) = IW-FW / IW x 100
Where, IW = Initial weight of pulp (g). FW= Final weight of oven dried pulp (g). Dry matter (%) = (100%-% moisture content)
Total soluble solid (TSS) content of Litchi pulp was estimated by using Abbe's Refractometer. A drop of Litchi juice squeezed from the fruit pulp was placed on the prism of the Refractometer. Then TSS was obtained from direct reading of the instrument. Temperature corrections were made by using temperature correction chart that accompanied the instrument. The pH meter (Hanna) was standardized by using buffer solution of pH 7 and pH 4 when correction for temperature was also taken into consideration. On completion of calibration the electrode was washed twice with distilled water, rinsed with Litchi juice and dipped into the juice. The pH was recorded.
Disease severity refers to the percentage of diseased portion of infected fruit. The infected fruits of each replication of each treatment (varieties) were observed to determine percentage of infected fruit area and was measured based on eye estimation. The mean values regarding infected fruit area were calculated, presented and discussed. Shelf life of Litchi fruits as influenced by different varieties was calculated by counting the days required to ripe fully as to retaining optimum marketing and eating qualities.
Statistical analysis: For the experiment, the collected data were statistically analyzed by Analysis of Variance (ANOVA) test. The means of different parameters were compared by least significant difference (LSD) as described by Gomez and Gomez (1984). For percentage data arcsine transformations were carried out to satisfy the assumption of ANOVA.