Experimental site: The experiment was carried out both at the laboratories of Horticulture, and Food Science and Preservation, Hajee Mohammad Danesh Science and Technology University, Dinajpur. The experiment was conducted during May to June, 2015.
Plant materials: Two commercial cultivars of litchi: ‘Bombai’ and ‘Madraji’ were chosen. Fruits of uniform size, shape, weight as well as diseases and injury-free were harvested from the well managed orchard and immediately transferred to the laboratory. Fruits along with approximately 15 cm of branches were used.
Design of the experiment: The experiment was arranged in the completely randomized design. Ten fruits of each cultivar were selected randomly and stored in the following six storage conditions: (i) T1 : Open condition at ambient temperature; (ii) T2 : Open condition at 5ºC; (iii) T3 : Bamboo baskets lined with litchi leaves and kept at ambient temperature; (iv) T4 : Bamboo baskets lined with litchi leaves at 5ºC; (v) T5 : LDPE (low density polyethylene) bags (with 4 small perforations) and kept at ambient temperature and (vi) T6 : LDPE bags (with 4 small perforations) and kept at 5ºC. Note that in Bangladesh, litchi fruits are generally packed in bamboo baskets lined with litchi leaves. For storage at 5ºC, fruits were kept in a refrigerator (Hitachi, Japan) maintaining 5ºC and 80 ± 5% relative humidity (RH) throughout the experimental period. For ambient condition, fruits were stored at the laboratory and the daily data on temperature and relative humidity during the experimental period were obtained using a combined thermometer and hygrometer. The average daily air temperature of 32ºC was recorded and 69% relative humidity was observed during the period. In all storage conditions, fruits were stored for 3, 6 and 9 days.
Pericarp colour: Litchi fruit colour was recorded using a Chromameter (Konica Minolta CM 250d, Japan) calibrated against a standard white plate. Chromatic analysis was carried out following the CIE (Commission International de l’Eclairage) system of 1976. Values of L*, a* and b* were measured to describe the three dimensional colour space and interpreted as follows: L* indicating lightness, a* value indicating redness and a positive b* value yellowness (Hutchings, 1994). The pericarp colour was expressed in terms of L, Chroma (C*) and Hue angle (Ho ) as the changes of pericarp colour during storage of the fruit was better described using C* and Ho than a* and b*. The hue angle (Hº), hue = arctangent (b*/a*), represented red-purple (0o ), yellow (90o ), bluish-green (180o ) and blue (270o) (McGuire, 1992). The chroma (C*), obtained from (a*2 + b*2 )1/2, corresponded to the intensity or colour saturation, in which low values represented dull colour while high values represent vivid colour. The pericarp colour was measured in the central region of individual fruit from the beginning till the end of the storage period. The data of each measurement are the average of duplicated measurements at two opposite points on the equator of each fruit.
Percentage weight loss: Fruits were individually weighed before the storage treatment and also weighed after the said storage period. Fruit weight loss was calculated as the percent reduction in weight. However, individual fruit weight was measured using an electronic balance (G & G, T100, Germany) and recorded in grams. Postharvest decay analysis: The incidence of postharvest disease was recorded by scoring in percentage of incidence on a visual scale of 1 to 5, which described the severity of postharvest fungal decay as 1 = no disease, 2 = 25%, 3 = 50% and 4 = 75% of fruit surface affected by fungal diseases and 5 = entire fruit surface was brown (Sivakumar and Korsten, 2006). Total soluble solids: Fruits were peeled, and the pericarp, aril tissues (flesh) and stone (seed) were separated. Aril tissue was then gently squeezed and a few drops of juice were used to determine total soluble solids (TSS) using a digital refractometer (Hanna Instruments, Romania). Ascorbic acid content: Ascorbic acid content (mg 100 g-1 fresh weight) was determined using the spectrophotometric procedure. Five grams of fresh tissue were homogenized in a 100 mL oxalic acid-EDTA cold solution. The absorbance at 760 nm was measured with a UV/ VIS spectrophotometer (PG Instrument Ltd. Model T60, UK). Ascorbic acid concentration was quantified using a standard curve of L-ascorbic acid and expressed as mg 100 g-1 of the fresh weight.
Total phenol content: The total phenolic compounds were quantified using Folin–Ciocalteu reagent (FC) and the colorimetric method of Singleton and Rossi (1965). Extraction was done according to Velioglu et al. (1998) using 1 g fresh flesh. The tissues were disrupted into the extraction medium using a homogenizer (VELP Scientifica, Italy). The absorbance was measured at 765 nm using a UV/VIS spectrophotometer (PG Instrument Ltd. Model T60, UK). Then the phenol content was estimated from a standard curve of gallic acid and results were expressed as mg of gallic acid equivalents (GAE) 100 g-1 of the fresh fruit.
Statistical analyses: One factor analysis of variance (ANOVA) was conducted for all variables using the Statgraphics Plus Version 2.1 statistical program (STSC, Inc., 1987). The means were compared using Fisher’s Least Significant Difference (LSD) while the Student t-test was used to compare pairs of means. All treatment means were regarded as significant with P≤ 0.05.