Abstract
Polished rice (polishing rate 70%) was cooked by a twin screw extruder under several conditions (material temperature 100-140°C, and moisture content 20-40%) to compare with ordinarily steamed rice and to find suitable cooking conditions. A pilot scale brewing (total amount of rice 100 kg) was also carried out with extruder-cooked rice.
The degree of gelatinization and digestible direct reducing sugar (DRS) increased and digestible formol-nitrogen (F-N value) decreased in proportion to the increase in material temperature and water content. The most appropriate cooking condition was determined as follows: material temperature 110°C, and water content 25%. The pre-cooked rice obtained under this condition indicated the same analytical values as those of the ordinarily steamed rice.
In the moromi mash prepared with pre-cooked rice, the acidity, amino acid value and alcohol concentration showed almost the same progress as those of the control using ordinarily steamed rice, but specific gravity (Bé) and direct reducing sugar content proceeded a little higher. Sake obtained from pre-cooked rice contained higher content of aroma components than the control, and sensory evaluation by 12 panels showed no difference in quality between the two sake.
The degree of gelatinization and DRS value of pre-cooked rice was decreased little during storage, and the molecular size distribution of rice was shifted to lower molecular side as compared with that of ordinarily steamed rice. This suggested that dextrinization (degradation of starch) occured during extruder cooking.
