Disinfestation Protocols And Storage Properties Of Rice Using Radio Frequency Heating

Rice is one of the most important staples in the world.China is one of the most rice producing countries with nearly 33% of world productions.However,the overall damage caused by insect infestation is about 27% of the annual total rice yield during storage.Rice weevils are considered one of the most damaging and serious stored pests worldwide in rice.Up to now,chemical fumigations,such as phosphine and methyl bromide,have been the most widely and commonly applied method throughout the world for insect control,but it has an adverse effect on human health and environment.Thus,there is an urgent need to develop an effective,novel,and safe method of controlling infestations in rice during processing and storage.Radio frequency(RF)heating has been proposed as a potential physical method for postharvest disinfestations due to its volumetric and fast heating,and self-balance of moisture content.Rough,brown,and milled rice samples were chosen as the major products to analyze the RF heating uniformity under different treatment condition,then validate the thermal mortality of adult Sitophilus oryzae and evaluate the main rice quality attributes after RF treatments and for an accelerated storage,finally to simulate the continuous industrial processing and estimate the heating efficiency and throughput.The detailed research contents and results are as follows:(1)Study the RF heating uniformity in rice samples.The RF heating temperature was selected based on the thermal mortality curves of adult Sitophilus oryzae.The RF heating uniformity improvements have been conducted in RF systems depending on different treatment conditions,such as adding forced hot air surface heating during RF process,conveyor belt movement at a certain speed,mixing samples between two RF exposures,and holding samples in hot air.Then RF heating uniformity was validated in rough and brown rice under the best uniformity condition of milled rice.The test result showed that RF heating uniformity of rice samples was improved by hot air surface heating,sample movement,mixing,and holding alone in the hot air and the optimal heating uniformity was obtained by the combination of all these methods.(2)Validating the RF treatment protocols for disinfesting rough,brown,and milled rice.First the thermal mortality of adult Sitophilus oryzae was determined at 4 selected temperatures(44,46,48 and 50 °C)and holding at corresponding target temperatures for 5 min,and with 50 °C at four different holding times(0,2,4,and 6 min).The result showed that high temperatures resulted in low survival rate of adult Sitophilus oryzae in rough,brown,and milled rice.When RF heating to 50 °C and the hot air holding time increased from 0 to 6 min,the 100% mortality of adult Sitophilus oryzae was obtained.(3)Evaluating the main rice quality attributes after RF treatments and for an accelerated storage.Milled rice was treated under the optimal RF treatment protocol.The quality was evaluated immediately and after accelerated shelf life storage.Moisture content,water activity,color,fat,free fatty acid content,protein content,starch and ash content were selected as the main quality parameters.The results showed that there were no significant differences in quality parameters between RF treatments and untreated controls during storage.(4)Industrial-scale radio frequency treatments to control Sitophilus oryzae in rice.RF treatment parameters(electrode gap and conveyor belt speed)and heating uniformity in rough,brown,and milled rice were determined.Then validated the practical RF treatment protocol using the infested rice under three electrode gaps of 11.0 11.5,and 12.0 cm and estimated the heating efficiency and throughput of the industrial-scale RF treatments.The optimal RF heating uniformity was obtained with an appropriate electrode gap of 11.5 cm,conveyor belt speed set to 12.5 m/h,together with 50 °C forced hot air surface heating and holding for 6 min in hot air alone.The electrode gap ? 11.5 cm acquire complete mortality of adult Sitophilus oryzae.The average heating efficiency and throughput of the RF system for milled,brown,and rough rice were 76.1,76.3,and 75.2%,and 268.8,247.3,and 224.8 kg/h,respectively.