Application Of NMR Techniques In Quality Evaluation Of Instant Rice

Instant rice is a newly emerging convenience food. There were great difference between instant rice and fresh rice. Its rehydration capability and texture are always poor. In the process of producing instant rice, the moisture content change significantly impacted on its quality. This project is the first attempt to use nuclear magnetic resonance (NMR) technology to evaluate the instant rice production and storage processes, this reseach should also enhance the application of the NMR technology in food qulity improvement. The main results of the reseach are showed below:1. The physical and chemical properties of the raw material rice, were analyzed first. And there were clearly differences in the two varieties of rice in terms of the amylase content. From soaking and cooking of different rice with different glutinous rice content, it was found that the water absorption and expansion rate increased with the increase of the glutinous rice content. The rice with5%of glutinous rice content were moderately soft, the palatability was good, and it was not easily retrogradated.Through NMR analysis, after soaking, with the glutinous rice content increasing, the corresponding values of T2and the proton density related to T2were rising also. So the trends of water absorption of rice could be monitored mondestructively by the NMR technique.2. The effects of different factors on the quality of rice were studied through single factor tests, and it was found that the instant rice quality was affected by initial rice/water ratio, soaking time, steaming time, dispersing method, drying time and temperature. The optimum processing parameter was determined to be0.2%salt water soaking for60min at room temperature, water to rice ratio of1.4, steaming of12min, dispersing with40℃water, then drying at100℃for120min.3. Based on the results of single factor tests, the orthogonal experimental design was also used and tested. The results showed that the optimum processing parameter were:soaking time of60min, water to rice ratio of1.4, steaming time of14min, dispersing with40℃water, then drying at100℃for100min.4. Through the analysis of NMR relaxation properties, it was observed that in the process of soaking, rice water content and T2was significantly correlated. When the sample’s T2corresponding proton density was biggest and the T11and corresponding proton densities were maximum, the rehydration rate also was maximum. Therefore we can use NMR techniques to determine the rehydration instant rice’s T2corresponding proton density and T11and T12corresponding proton densities to measure the water content, and rehydration rate of instant rice.5. During the storage period, as the time of rehydration of the instant rice increased, rehydration rate decreased significantly, hardness increased, springiness, chewiness and sensory scores declined. After long time (over90days) of storage, the rice aroma was owered, rice was dry and hard, rice integrity and appearance were poor. During storage, from the changes of T2, T11and T12obtained, we could see the trends of water in the rehydration process in the instant rice. With the extension of storage time, the total water content in the rehydrated instant rice was reduced. Room temperature is appropriate for instant rice storage, the storage time could reach90days.