Advanced Ohmic Heating for Rice Cooking: Quality Factor Assessment

Abstract

Conventional thermal processing of foods containing particulates significantly relies on several heat transfer steps, including conduction and convection, which usually take longer cooking times for the solid-liquid mixture foods and tend to be overly conservative ensuring microbial safety, thus compromising quality. Rice is one of the world’s biggest cereal crops next to wheat and maize and is one of the most important staple foods for the world population. Asian people consume cooked rice at almost every meal. Existing methods of cooking are about 10-15% thermally efficient. The ever-increasing population will need more amount of energy and water to be spent on rice cooking. Advanced food processing technologies such as ohmic heating and microwave heating have been developed in the last few decades as alternatives to conventional processing methods. The advanced technologies could contribute to shortening processing times, energy savings, and high-quality safe food. The microwave heating has been employed to cook rice and can reduce the cooking time by more than 40% compared to the conventional cooking method. However, the energy consumption was nearly doubled. Therefore, a new concept to use ohmic heating for rice cooking has been extensively evaluated. This technology was an attractive alternative method with high energy transfer efficiency, time savings, and high quality of purpose. In this study, an alternative cooking method that offers both high energy efficiency and short cooking time was developed, and a static ohmic heater was designed and fabricated to heat treat rice-water mixtures. The energy consumption, textural characteristics of rice, and simulation of the electrical field in ohmic heater were investigated. Two types of rice were used in this study: white rice and brown rice respectively. The electrical conductivities of rice-water mixtures at various volume ratios were measured during the rice cooking process. The endpoint of rice cooking by using ohmic heating was identified. The results showed that the rice cooked by the ohmic heating method has significantly different textural properties from rice cooked by an electric rice cooker. The magnitude of texture difference was dependent on the type of rice. The electrical conductivities of white rice, and the brown rice mixture were approximately 0.03-0.08, 0.04-0.1, 0.06-0.12 S/m at volume ratios of 1:0.8, 1:1.2, 1: 1.5 and 0.025-0.16, 0.032-0.2 S/m at volume ratios of 1:1.5 and of 1:2 respectively. The research also found that ohmic heating required a cooking time of around 17-18 min. The estimated amount of energy consumed by the ohmic heating process was about 1/4 of the total energy consumed by electric rice cooker. The developed ohmic heating technique showed a great potential over the conventional electric cooker regarding the high energy efficiency, shorter cooking time, and lower water usage.