Some Physical and Mechanical Properties of Dargazi Pear

Considering the importance of appearance quality and effect of some factors such as area, perimeter, diameters, uniformity and defects on fruits, in grading, acceptability and final price, lead to measurement of some physical and mechanical properties of fruits. Pear fruit, belongs to Rosaceae family and Pomoideae sub-family, is one of the most important susceptible fruits in the world. Most of the pear fruits are originated of Pyrus Communis species. Iran, has degree thirteen of producing pear in the world. Dargazi pears, in Iran, are planted in north of Khorasan. By knowing the properties (physical and mechanical) of pear fruit, we can control probable damages that they undertake during harvesting, handling, packing, cooling, cleaning, sorting, storage, transporting and processing. Also, this knowledge will be useful in designing of harvesting machines and postharvest technologies. Until now, there has been no research in measuring physical and mechanical properties of Dargazi pear. So, the aim of this study was investigation of some physical properties including geometrical (length, mean diameter, radius of curvature), gravitical (mass, volume, ture density, bulk density) and mechanical properties (degree of elasticity, degree of plasticity, strain energy, resilience, hysteresis and hysteresis loss) of “Dargazi” pear. Pear fruits were hand-harvested from orchard, transported to laboratory and kept at temperature of 23˚C for further measurements. To determine mechanical properties, the Universal Test Machine (model STM 5) equipped with a 50 N load-cell (model DBBP-500, BONGSHINE, precision 2 kg) was used. The tests were performed on 75 pear samples divided to five groups for applying five force-level (40, 50, 60, 70 and 80 N) in form of uniaxial loading-unloading tests under constant speed of 10 mm/min. Samples were loaded between two solid steel plates (bottom plate was fixed). Three replications were considered for each force level. For measuring physical properties: mass, volume, dimensions; balance with 0.01 g precision, submerging method and digital caliper were used. To determine radius of curvature at three regions (crown, chick, calyx), image processing using a Canon camera model Powershot G10 and ImageJ software were used. To maintain uniform picture acquisition environmental conditions, samples were put in an isolated square wooden box, equipped by the camera at the top, three fluorescent lamps around the camera, and a hatch for putting samples in the box. The camera was connected to laptop using USB port to control image capturing. Images were obtained and processed in RGB color space. Drawing a circle from at least 3 points on the boundary line of interested region (curvatures at crown, chick and calyx) was used to determine radius of the circle and consequently, the radii of curvature at interested regions. Analysis was performed using SPSS software. Mechanical properties were analyzed in form of completely randomized design. ANOVA, Duncan test and regression were used for understanding and explaining the relation between force and mechanical parameters. The results showed with increasing loading force, the values of loading energy (strain energy capacity), unloading energy (resilience), hysteresis (absorbed energy) and hysteresis loss will be increased. Also, with increasing degree of elasticity and radius of curvature at loading region (in otherwise, concentrated stress will be reduced because of minimized loading area), hysteresis and strain energy will be reduced, respectively. Resilience and force had high linear correlation with R2 = 0.992. Degree of elasticity and hysteresis can be used as indices for showing pear fruits susceptibility. Physical properties were analyzed using mean, maximum, minimum and standard deviation. The results of physical properties including mean length, mean diameter, mass, volume, true and bulk density, porosity and packaging coefficient are 84.162 mm, 73.729 mm, 188.8742 g, 185.4037 cm3, 1.01899 g/cm3 and 0.42759 g/cm3, 57.92009 and 0.42088, respectively. Also, the biggest and smallest radius of curvature were obtained at chick and calyx, 77.701 and 24.633 mm, respectively. The CV value of physical properties was less than 10 percent that shows the fruits uniformity. Dargazi pear has higher mean length, porosity and bulk density but smaller mean volume, mass and true density in comparison with Asian pear. Also, Dargazi pear has higher mean volume, porosity and true density but smaller mean length, mass and bulk density in comparison with Santa Maria pear. The applied image processing technique to determine the radius of curvature, has no limitation in measurement ranges, it’s possible to measure the biggest and smallest radii of curvature, also is nondestructive because of its non-contacting usage, so minimize possible damages.