Abstract
To intensively analyze and examine the changes in palatability of cooked rice and structural properties of rice grains influenced by increased nitrogen topdressing during the panicle formation and ripening stage, a series of experiments were carried out using three japonica rice cultivars with contrasting rice quality characteristics. The protein content of brown rice increased maximally up to $35{\sim}47%$ of that in standard N6 plot with the increase in nitrogen topdressing fertilizer during 20 days before heading to 10 days after flowering. The high-yielding rice cultivars showing poor palatability of cooked rice revealed larger increase in protein content of rice grains by increased nitrogen topdressing. Under the same nitrogen level of 15 kg per 10a with nitrogen topdressing at 3 kg/10a, high-yielding rice cultivars, Yumehikari and Reihou showed the significant increase in protein content of brown rice when topdressing was applied at 10 days after flowering as compared with when it was applied at 30 days after transplanting. Although the variation in amylose content of milled rice as affected by nitrogen topdressing level was relatively small, it decreased within 1% with the opposite tendency against increased protein content of brown rice by increased nitrogen topdressing. The total score of sensory evaluation was higher in the order of Hinohikari < Yumehikari < Reihou in panel test. It decreased significantly by increased amounts of nitrogen topdressing during 20 days before heading to 10 days after flowering when nitrogen level was higher than 12 kg/10a. The more poor palatable rice cultivar in panel test revealed the larger decreasing in total score of sensory evaluation by higher nitrogen topdressing rates. All sensory evaluation components were largely affected by the change in protein content of brown rice rather than amylose content of milled rice. The influence of protein content to palatability of cooked rice was larger in poor-palatable rice than in high-palatable rice. The protein content decreased drastically from outer layer to inner layer of rice grains, while the amylose content increased on the contrary. The high-palatable rice exhibited higher distribution of protein content on bran layer but lower distribution of protein content on the layer of polished rice as compared with the poor-palatable rice. Especially, the high-palatable rice showed also significantly lower distribution of amylose content on the outer layer of polished rice as compared with the poor-palatable rice.
질소수비 및 실비 시용량을 증시하여 미립내 단백질함량을 증가시킴으로써 이에 따른 식미저하와 쌀의 구조적 특성의 품종별 반응을 정밀하게 분석 검토하였으며, 그 결과를 요약하면 다음과 같다. 1. 현미 단백질함량은 질소 수비 실비의 증량에 따라 표준구(N6) 대비 $35{\sim}47%$까지 증가하였으며 식미가 낮은 품종일수록 증가 정도가 컸고, 동일한 질소 15kg/10a 수준으로 1,2차 수비가 충분히 준 조건에서도 1차 추비에 비해 실비를 준 경우 유메히까리와 레이호우는 현미 단백질함량이 유의하게 증가하였다. 2. 아밀로스함량은 질소 수비 실비의 증시에 따라 대체로 단백질함량과 반대로 변화정도는 품종간 차이보다 미미하지만 감소하는 경향을 나타내었다. 3. 식미 관능총평은 히노히까리 > 유메히까리 > 레이호우 순으로 높았는데 N 12 kg/10a 이상에서 질소 수비 실비 증시에 따라 유의하게 감소하는 경향을 보였으며 식미가 낮은 품종일수록 저하 정도가 컸고, 모든 식미관능 특성이 아밀로스함량보다 단백질함량 변화에 따라 크게 좌우되었으며 식미가 양호한 품종보다 식미가 낮은 품종에서 단백질함량 변화의 영향이 컸다. 4. 미립의 층위별 분포를 보아 외층에서 내층으로 갈수록 단백질함량은 현저히 감소하였고 아밀로스함량은 증가하였다. 식미가 양호한 품종이 낮은 품종에 비해 단백질함량의 분포가 겨층에서 높은 반면 백미층에서 크게 낮았으며 특히 아밀로스함량의 백미 외층 분포비율이 낮았다.
