| Summary: | 碩士 === 國立海洋大學 === 水產食品科學研究所 === 75 === Long Glutinous rice and Penglai rice were extruded with Clextral BC-45 twin-screw extruder at barrel temperature 154℃ and screw speed 120 rpm. Feed moisture was adjusted frm 10.1 to 19.1% (w/w) for Long Glutinous rice and 10.2 to 20.3% (w/w) for Penglai rice.
Expansion ratio of rice extrudates decreased linearly as the feed moisture increased. On the contrary, bulk density increased with feed moisture. Water absorption index of Penglai rice was higher than that of Long Glutinous rice. But the solubility of Long Glutinous rice extrudates was higher than that of Penglai rice, possibly due to more degradation of Long Glutinous rice starch during extrusion. Moisture content of rice extrudates increased linearly as the feed moisture increased. Lightness and whiteness of the product had no significant variation (p<0.05) as the feed moisture increased. Changes in lightness and whiteness of the extrudate paralled to each other.
Effect of processing variables including oil addition, barrel temperature and screw speed at five levels on the physicochemical properties of rice extrudates and the optimal processing conditions were studied using response surface analysis.
Soybeanoil addition at 1 to 5% to Long Glutinous rice, the extrudates showed that water solubility index and color difference were related to the processing variables accounting for more than 80% (R2>0.80, p<0.01) of the total variation. Increased level of oil addition from 6 to 20%, in addition to water solubility index and color difference, bulk density, shear force, lightness and whiteness of the product were also accounted for more that 80% of the total variation.
For Penglai rice, oil addition in the range of 1 to 5%, water solubility index, shear force, color difference were related to the processing variables and accounted for more than 80% of the total variation. Increased oil addition to a range of 6 to 20%, color difference, expansion ratio, whiteness, lightness, water solubility index and moisture content of the product were significantly affected (R2>0.80, p<0.01).
Scanning electron microscopy showed that oil addition had no pronounced effect on porosity of Long Glutinous rice extrudates. However, the porosity of Penglai rice extrudates had the tendency decreased as the oil addition increased from 0 to 13%. The thickness of cell wall in the extrudate seemed to correlate with the expansion ratio.
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