The Effect And Application Of Ultrasonic-microwave Treatments To The Properties Of Rice Starch Gels

Gelatinization and retrogradation properties of non-waxy rice starch and waxy rice starch gels prepared by microwave (MH), ultrasonic-microwave synergistic (UMSH) and conventional heat (CH) treatments were investigated in this study. The reason was also discussed. Furthermore, the effect of ultrasonic-microwave synergistic and microwave treatments to non-waxy rice and waxy rice quality was studied. The purpose is to provide theoretical basis for the application of the new heating method in the starch and rice products processes.The work made it clear that ultrasonic-microwave synergistic and microwave heat treatments could obviously save time for the gelatinization process of rice starch suspension. The results showed that there was no significant difference in the water content of starch gels made by the three different treatments (P> 0.05). For non-waxy rice starch, the gel firmness of starch gels obtained from UMSH and MH were 90.2±12.2 g and 67.3±10.8 g, which were significantly lower than that from the conventional heat (115.7±13.1 g). Furthermore, the microwave treatment caused a higher cohesiveness. The results showed that during amyloglucosidase hydrolysis, the descending order of in vitro hydrolysis degree was as follow: conventionally heated (78.5±3.0 %) > ultrasonic-microwave synergistic-heated (66.1±2.2 %) > microwave-heated (60.8±1.7 %). The microcosmic results showed that the rice starch gels prepared by microwave and ultrasonic-microwave synergistic treatments had smaller holes and larger connection parts compared to conventionally heated one. These differences were attributed to the higher heating rate and the non-synchronous process of the starch gelatinization occurring under UMSH, MH treatments, Especially, these could result in the less damage extent of starch gels and the less starch leaching. Nevertheless, there was no significant difference in the gelatinization properties under different heat treatments for waxy rice starch gels, which indicated that the starch with low content of amylose was less sensitive to physical radiation than that with high content of amylose.The retrogradation data of the tested samples were analyzed by the DSC. The results indicated that there was no obvious influence on melting temperature and scope of crystallization under UMSH, MH and CH treatments. However, the retrogradation rate of non-waxy rice starch gels prepared by ultrasonic-microwave synergistic and microwave treatments was lower than that by the conventional heat. During the storage of 21 days, the enthalpy change of the samples treated by three methods were 6.5±0.2 J/g,6.1±0.2 J/g,7.7±0.1 J/g, respectively. The results of the X-ray diffraction pattern showed that the type of starch gel crystal was not affected by different heat treatments. The analysis of the starch gel firmness change and the observation of the microstructure during the retrogradation process also proved that ultrasound-microwave synergistic heating and microwave heating can retard the retrogradation of non-waxy starch gels. Nevertheless, there was no significant difference in the retrogradation rate under different heat treatments for waxy rice starch gels. The study indicates that this is feasible for the application of ultrasound-microwave synergistic heating and microwave heating in the elongation of non-waxy rice starch shelf life.For non-waxy rice, the results showed that the total score of sensory evaluation from the rice prepared by ultrasound-microwave synergistic heating and microwave heating were higher than that by conventional heat. Through the TPA test, the results indicated that the firmness, springiness and chewiness of the microwave-heated and ultrasonic-microwave synergistic-heated rice were significantly lower than that for conventionally heated sample. During in vitro hydrolysis experiment of the fresh rice, the hydrolysis rate of the ultrasonic-microwave synergistic-heated and microwave-heated rice was slower, furthermore, the finish glucose content was lower. These accorded with the requirements of the modern healthy diet. The SEM observation showed that there were significant differences in the microstructure of rice obtained from the three heat treatments. During 4℃storage, the enthalpy change of rice prepared by UMSH and MH treatments was lower than that by the CH, which indicated that the internal starch crystallization degree was smaller. The quality of the waxy rice was less affected by different heat treatments. The study proved that the feasibility of microwave and ultrasonic-microwave synergistic treatments applied to rice products processes.