Effect of Ultrasound on Rough Rice Moisture Absorption Rate during Initial Soaking

Abstract

Discharge of untreated wastewater to the environment, especially to the natural water bodies, is common in most of the small and medium scale parboiled rice processing facilities in Sri Lanka although the regulations do not permit such discharge. Effluents are added to water during soaking in the parboiling process where raw paddy is soaked in stagnant cold water for 36 to 72 hours or 2 to 4 hours in hot water at 60-650C. Soaking followed by steaming and drying are the main steps of rice parboiling and this pre-gelatinization process reduces rice breakage during milling. Hot soaking can reduce water consumption by 91.6 – 95.8 % and soaking time by 95%. However, hot soaking needs significantly high energy and special equipment including a boiler and therefore, it is not popular among the small-scale millers. The small-scale millers are accountable for about 70% of rice milling in the country and about 55% of rice is consumed as parboiled rice. Due to high prevalence of cold-water soaking method, adverse environmental effects could occur as a result of discharging of waste water. Therefore, finding solutions to minimize the impact is essential. Recent studies have shown evidence that different types of grain soaking can be improved by applying ultrasound waves during soaking. The objective of this research was to investigate the influence of moisture absorption rate of rough rice during cold soaking when combined with sonication to reduce soaking time, water usage and effluent generation. The laboratory studies revealed that ultrasound treatment could increase the soaking rate significantly. However, this is mainly due to the temperature rising during sonication and not due to the cavitation shear forces causing high sound pressure. When the temperature of water was maintained at room temperature, the soaking rate increased only slightly. Therefore, this study proved that sonication improved soaking rate of paddy.