| Summary: | 博士 === 國立臺灣大學 === 食品科技研究所 === 91 === This dissertation was attempted to investigate the effects of extrusion condition, milling method, and particle size of rice flour on the rheological properties of glutinous rice extrudates. From the results, feasible conditions to prepare mochi by using single screw extrusion cooking (SSEC) have been identified.
First, the influence of screw configuration and die opening area were studied. Among three screw configurations, forward, mixing bar & disc, and mixing disc and three die diameters of 10, 20, and 40 mm, the combination of mixing disc and 10-mm die exhibited longest mean residence time and good mixing effect. The extrudate prepared using these conditions yielded high degree of gelatinization with smooth texture. The flow patterns of material in the single extruder were analyzed using three models, including Wolf-White, Simplified, and Complete model. The Complete model was the best one among the three models.
The barrel temperature and water content were also important factors on affecting the properties of extrudates. The volume percentage of water in the extrudate was highly correlated (r^2=0.92~0.95) with the dynamic mechanical spectrum. Four models were used to analyze the data from creep test. It was found that the modified four-element model fitted well the data. From the correlations between sensory analyses and rheological properties, the preference index was found to be a good index for processing. The extrudate with 55% water content exhibited similar properties to commercial mochi. The results indicated that the SSEC could be used for mochi production.
The milling method affected that the particle size of rice flour as well as the physical properties of extrudates. The wet milling yielded a damaged starch much lower than dry milling. The sorage modulus at 95℃(G''95℃) of rice flour with 45% H2O increased with the mean particle size(r^2=0.96). When the water content was raised to 80%, both the mean particle size and milling method affected G''95℃. This was probably due to the phase change of particles during heating, which was revealed by the microscopic observation. From the rheological properties and SEM, a smooth texture was obtained when the particle was smaller than 74μm. This would be helpful for setting up the specification of rice flour.
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