Study On The Effect Of Microwave On The Heating Characteristics Of Hydrated Starch Systems

In the field of food processing, microwave heating behavior with water as carrier mainly focuses on drying, blanching and sterilization, which is suitable for low moisture materials or short processing products, however, microwave processing technology aiming for curing has been promoted rarely. The main reason is that the response form of water in microwave processing is different from that in heat transfer processing, so that microwave baked bread appear texture collapse and microwave steamed buns have hard heart, therefore, carrying out the study on changes of microwave heating characteristics of hydrated starches caused by moisture is conducive to the development of controlling technology in microwave processing of traditional staple food, which would become an effective way to utilize microwave energy with low pollution and low energy consumption.In this paper, rice starch and potato starch, the representatives of cereal and potato starch, were chosen as the subjects. The proton distributions of the hydrated rice starches and potato starches with water content from 20% to 90%, and the causes of the difference between them were investigated; changes of the proton distributions under microwave irradiation during temperature rising process was analyzed; dielectric property and reflection loss(RL) at 2.45 GHz were explored. The research contents and results were listed as follows:1. Proton distributions of hydrated starch systemsThe longitudinal and transverse proton relaxation(T1 and T2) of the two starch systems were obtained by using the inversion recovery sequence IR, and the free induction decay sequence FID and the multi-pulse echo sequence CPMG. The results showed that, through the detection of longitudinal relaxation, 2 different populations were found in the two hydrated starch systems. Through the detection of transverse relaxation, 2 populations were distinguished in the hydrated rice starches, while 2-4 different populations were detected in the hydrated potato starches.2. Proton distributions of hydrated starch systems under microwave irradiationMicrowave heating could significantly change the water distributions and flow state of the hydrated starch systems. Longitudinal relaxation results showed that when temperature exceeded a certain value, with the increase of temperature, the longitudinal relaxation time of the two proton groups tended to be the same, eventually became one group.Transverse relaxation results showed that when moisture content increased to a certain degree, with the rise of temperature, the number of transverse relaxation proton groups of the hydrated potato starches decreased gradually, and the system finally showed one transverse relaxation peak.3. Dielectric properties and microwave absorbing properties of hydrated starch systemsBased on the vector network analysis technique of materials, bow coaxial probe and improved arch method were used to study the dielectric property and microwave reflection loss(RL) of the two starch systems with different hydration levels at 2.45 GHz. The results showed that at room temperature, rice starches and potato starches had similar dielectric responses, which showed a steady rise along the increase of water content. Further studies found that the microwave absorption property of water and native starch differed significantly: starch-water systems with different proportions varied nonlinearly in the microwave absorption performance, which matched with the "true single layer" theory when moisture content was low.