The study was conducted in the laboratory of the Post harvest Technology Division of BARI, Gazipur. Jute leaf were collected when the leaf age was 40 days.
Drying methods: The drying of Jute leaf was conducted in using the two methods of Mechanical dryer and Freeze dryer.
Mechanical dryer: To conduct drying experiments the jute leaf were separated from plant by hands and samples were taken for determination of initial moisture content. Fresh jute leaf at constant loading density (0.5 kg/ft2) were placed in trays in the drier and drying commenced in the drier at a constant air velocity (0.6 m/sec) and at a specific air dry bulb temperature (45, 55 and 65 °C). Cabinet dryer, Model OV-165 (Gallen Kamp Company) was used for dehydration of jute leaf. The dryer consists of a chamber in which trays of products could be placed. Air was blown by a fan pass through a heater and then across the trays of products were dried. The velocity of air was recorded (0.6 m/sec) by an Anemometer. Weight loss was used as a measure of the extent of drying.
Freeze drying: Freeze drying was the mild process for drying products. It is based on the physical phenomenon of sublimation, i.e. is the direct conversion form solid to gaseous state . The frozen product was dried under vacuum without thawing .Freeze drying was done by using CHRIST Alpla 1-4 LED model machine .There were two main process one is main drying and another is final drying .In main drying time was 48 hour, temperature -560 C and vacuum was 0.018 milibar .In final drying time was 48 hour, temperature was -760 C and vacuum was 0.001 milibar.
Treatments
T1 = cabinet drying at 450C
T2 = cabinet drying at 550C
T3 = cabinet drying at 650C
T4 = freeze drying
Measurement of ascorbic acid, total soluble solids (TSS), PH and titratable acidity: Ascorbic acid measurement, 10g sample was homogenized in 50 ml of 3% cold metaphosphoric acid (HPO3) using a blender for 2 min and filtered through Whatman filter paper No. 2. The clear supernatant was collected for assaying ascorbic acid by 2, 6-dichlorophenolindophenol titration following the method of Ranganna (1986). Ten milliliters of aliquot was titrated with 0.1% 2, 6- dichlorophenolindophenol solution until the filtrate, changed to pink colour persisted for at least 15 seconds and the titration volume of 2, 6- cichlorophenolindophenol was recorded. Prior to titration 2, 6- dichlorophenolindophenol solution was calibrated by ascorbic acid standard solution. Ascorbic acid content was calculated according to the titration volume of 2, 6- cichlorophenolindophenol and results were expressed as mg 100gm-1 fresh weight. Again, 10g sample was homogenized in 50 ml of distilled water for 2 min using a kitchen blender and filtered through Whatman filter paper No. 2. The supernatant was collected in order to measure total soluble solids using a digital refractometer (Model NR151) and expressed as percentage, pH using a glass electrode pH meter (Delta 320, Mettler, Shanghai) and titratable acidity expressed as citric acid (%) was determined by titration with 0.1 mol L-1 NaOH to pH 8.1 according to the method by Ranganna (1986).
Measurement of β-carotene: The estimation of β-carotene was done by the extraction of 3g product sample with acetone (Fisher Scientific Ltd., Uk) and petroleum ether. It was further purified with acetone, metabolic KOH and distilled water. The resulting solution was filtered with anhydrous sodium sulphate and read on a spectrophotometer (T-80, PG Instrument Ltd., UK) at 451nm against petroleum ether as a blank. A standard graph was plotted using synthetic crystalline B-carotene (Fluka, Germany) dissolved in petroleum ether and its optical density measured at 451 nm (Alasalvar et al. 2005).
Measurement Color: Color measurement was done by the method of Hunt (1991). Fresh and dry jute leaf color was measured and compared using a Hunter colorimeter model “Lab scan XE” (Hunter Associates Laboratory, Reston, VA) using universal software, based on three color coordinates namely L, a*, and b*. The instrument is calibrated using a standard white (L = 90.70, a* = -1.08. b* = 0.65) and blank reference tile under illuminated conditions such as “C” illumination and via angle 2°. The color values given by L, a, b is generally expressed as total color of the sample. “L” represents the lightness index, “a” represents red-green, whereas “b” represents yellow-blue color components.
Data analysis: The experiment was carried out Completely Randomized Design (CRD) and all treatments were replicated three times. The data were analyzed for ANOVA in completely randomized design (CRD) under computerized statistical methods of M-stat and Duncan’s Multiple Range Test (DMRT) was used to compare the means.