PDF《American Journal of Applied》

6 (7): 1341-1346,

2009 1342 optical, thermal and nutritional properties[7] . The rehydration ratio can be considered as a measure of the injuries caused by the processing and drying to the material. It is generally accepted that the rehydration capacity is dependent on the degree of cellular and structural disruption. According to Jayaraman et al.[8] , during the drying process, irreversible cellular rupture and dislocation, results in loss of integrity and hence, in a dense structure of collapsed, greatly shrunken capillaries with reduced hydrophilic properties, which are reflected by the inability to imbibe sufficient water to fully rehydrate. Proximate analysis used to determine the proximate principles of any substance, as contrasted with an ultimate analysis. The proximate analysis of food refers to the analysis of the total content of a food component, not taking account of the individual compounds making up that food component. The macro components are generally analyzed for their proximate amounts[9] . The main objectives of this study was to produce the dragon fruit stem in powder form using freeze drying process and to measure the proximate analysis parameter such as moisture content, water activity, crude protein, crude fat, crude fiber, ash, glucose and ascorbic acid between mature stem and premature stem and also comparing the proximate analysis of dragon fruit stem between freeze drying method and drying oven method. MATERIALS AND METHODS Sample preparation: The dragon stem was cleaned and drained off before used. Nearly

200 g of stem was cut into small pieces using Warring blender (Warring Commercial, Laboratory Blender) for

2 min and was homogenized using homogenizer (Heidolph DIAX 900) with ratio 1:3 w/v and filtered using Muslin Cloth. The filtrate was transferred into freeze dryer bottle in and kept at -18°C for four days or at -80°C for two days in freezer before freeze-drying at -45°C, pressure 86*10?3 M bar for

4 days. Then the frozen sample was freeze dried using freeze dryer (LABCONCO, Freeze Dry System/ FREEZONE 4.5). The freeze dried sample was kept into a tight container before further analysis. Moisture, ash, fat, protein content, water activity and fiber in the sample were determined using method by AOAC International[10] . Moisture content: The crucible was placed inside drying oven for 105°C for

2 h. After that, the crucible was placed in the desiccators for allowing cooling. The beaker was weighed and

2 g of the powder was placed in the beaker. The sample was drying in drying oven (Memmert 600, Germany) for

3 h at temperature 105°C. Then the dried sample was weighed for percent of dry weight and percent of moisture content in sample. Ash: The preparation of ash was same as preparing crucible in moisture content. Two gram of sample was put into crucible and weight was recorded and placed in muffle oven (Furnace Nabertherm, Germany) at 550°C for

8 h. Fat: The fat content was determined directly extracting the dried ground pitaya fruit with petroleum ether in an intermittent Soxhlet extractor (Soxhlet Extractor Darmstadt, Germany) and extracted the sample for

4 h. The residue in round bottom flask after solvent removal represents the fat content of the sample. The residue was weight and checks the Reflective Index (RI) using reflextrometer (Merck KGaA,

64271 Darmstadt, Germany). Crude protein: Nitrogen was determined using micro-Kjeldahl method. About

2 g of dried sample was transferred into digestion tube by adding

2 tablets of catalyst and

20 mL of sulfuric acid to digestion in