Introduction: One of the common rice in Asia and Africa is parboiled (semi-cooked) rice. In parboiled rice, three processes will be added in comparison to white rice which includes: soaking, steaming and drying. The parboiled rice has a different color than white rice, which is probably the parboiled rice brown color is due to the diffusion of shell color in the grain's endosperm. Changes in the color of parboiled rice increase with increasing soaking temperature, increasing the duration of steaming and also increasing the hydration pressure of the grain. Increasing the temperature of the water during the soaking stage increases the absorption of shell color and absorption of colored materials, including the enrichment of folic acid, which has a negative effect on the whiteness of rice grain. The important process of parboiled rice production is steaming and drying which change color and transfer residual moisture of rice. One of the drying methods that is being used in food and agriculture industry is Air Jet Impingement Drying (AJID) by rapid drying rate and high performance .In air jet impingement dryer, hot air passes in up and down of product with high pressure as heat and mass transfer increases as drying time is decreased. In this research the effect of drying parboiled rice using air jet impingement dryer, drying kinetics with whiteness index and color difference was evaluated.
Materials and methods: Samples were steamed after soaking for 20 and 25 minutes and at temperatures of 90, 100 and 110 with air velocity of 15 and 20 were dried. The quantitative indicators CIELAB color system (L, a and b) were measured. The Air jet dryer contained a 3 kW air fan by air speed of 1-28 m/s, a 13.5 kW heater by air temperature of 250 , an inverter for control and measuring of air velocity and K type thermocouple. For control and measuring of air velocity two plenum chamber (a plate for direction of air to nozzle) used by a control valve, temperature circuit control, an aluminum drying chamber and two nozzles. Fan transfer air to heater then transfer to plenum chamber then to nozzle at final the hot air directly contact on product.
Results and discussion: The results showed that with increase of air velocity from 15 m/s to 20 m/s in 90 air temperature average value of whiteness index was decrease from 61.952 to 57/088. With steaming time of 25 minutes, air velocity 15 m/s and the temperature 100 minimum whiteness index was 48.537. The maximum average value of whiteness index and the minimum average value of color differences were 63.884 and 29.432, respectively was obtained with steaming time 20 minutes, air velocity 15 m/s and the temperature 90 . By increase of temperature and air velocity in steaming time 25 minutes were drying rate increases, in this condition rice drying rate more effected by change temperature. The results showed that change of temperature had significant effect on color in comparison to steaming time and air velocity. With decreases of parameters: air velocity, steaming time and air temperature parboiled rice samples with higher whiteness index was obtained. The results of statistical analysis showed that dryer air temperature have more effects than steaming time and air velocity parameters on the color changes of parboiled rice. Also drying rate decreased by moisture reduction and Increasing of both temperature and air velocity were significant effective in color index. The results of regression analysis showed that drying rate significantly decreases with decrease of moisture.