VACUUM PRESSURE INFLUENCE ON WATER UPTAKE AND QUALITY OF RICE GRAIN DURING SOAKING

Abstract

The soaking time can be reduced by minimizing the effect of air which is trapped in the caryopsis and between it and the husk. This air resists water movement into the kernel. This paper explores the use of a vacuum and then breaking the vacuum when the paddy rice is soaked to reduce volume of the air, for paddy rice (Giza 181). This would reduce the soaking time in cold water and leads to improvement in the quality of parboiled rice. The simple technique is used to be suitable for poor villages in Africa and Asia where there is no electricity and used rice boiled in their own homes. By using the a pump in four levels of absolute vacuum deareation as 400, 600, 800 and 1000 mbar at 30°C the moisture content of paddy rice was determined before soaking and after different times 1, 2, 4, 6 and 12 hours from the beginning of soaking. The results showed that the samples moisture absorption affected by the levels of vacuum, and the sample evacuated at 400 mbar absorbed a few amount of water more than the sample at 1000 mbar. The increasing in moisture gain due to vacuum levels was evaluated. Becker's diffusion equation was used to predict some soaking behavior of paddy; the moisture gain in the grain was noted experimentally and plotted against the square root of the absorption time under different vacuum conditions. Quality tests show that the process of vacuum deareation before soaking improved the parboiled rice quality substantially, breakage varied from 12 to 14 % for raw paddy while for parboiled paddy it varied from 1.6 to 2.1%. Also white bellies decreased from 8.5% to 5.4%, and hardness was increased from 7.8 kg to 9.3 kg when vacuum levels changes from 1000 mbar to 400 mbar.