Comparative study of physical, mechanical and milling properties of rice in ratooning and main crops.

With increasing ratoon rice harvested area, in addition to agronomical research, development of engineering study such as physical and mechanical of grain is a need to optimize paddy milling process for reducing rice losses in postharvest operations. In this research, physical, mechanical and milling characteristics of three rice varieties (namely, hashemi, alikazemi and tarom) in two crop types (main and ratoon rice crops) was studied in a factorial experiment (32‎أ) based on randomized complete block design with five replications. Results revealed that effect of variety on the all physical properties (with exception of porosity), grain bending rapture force and head rice yield (p0.01), on milling recovery and elongation ratio of the grain after cooking (p0.05) was significant. The crop type significantly affects length, thickness and sphericity, bending rapture force of the grains, head rice yield, whiteness degree, amylase content and gelatinization temperature (p0.01) and on the slenderness ratio (p0.05). The interaction effects of variety‎أ crop type was significant on true density and milling recovery (p0.01) and angle of repose (p0.05). Among the measured physical properties, only thickness and sphericity of brown rice in main crop (1.68 mm and 0.388, respectively) was lower than the corresponding values in ratoon rice (1.71 mm and 0.398, respectively). The maximum grain bending rupture force in (22.75 N) and head rice yield (79.75 %) was in Hashemi and the highest milling recovery (66.08 %) and degree of whiteness (43.96 %) was obtained in Tarom variety. Grain bending rupture force and head rice yield in ratoon crop (23.68 N and 80.27%, respectively) was significantly higher than main crop (20.09 N and 76.01%, respectively). Also, the rice whiteness degree in the main crop (46.45 %) was significantly higher than that of the ratoon rice (40.14%). In regarding to quality properties, the elongation ratio after cooking inTarom variety (1.64) was obtained lower than the corresponding value in Hashemi (1.70) and Alikazemi (1.71). The amylase content and rice gelatinization temperature in main crop (21.97 % and 4.27, respectively) was lower than those of ratoon crop (22.93 % and 5.50, respectively). Keywords: Rice, physical properties, mechanical properties, milling quality