pure, unadulterated, select
粹 centers on the idea of purity and refinement: something unadulterated, carefully selected, or of the highest quality.
粹 combines 米 (rice) with 卒, which likely contributes the sound and the idea of finishing or completeness, suggesting rice that has been refined to its purest form.
Rice 米 that has been fully processed 卒 is pure and select: only the best grains remain.
For スイ, imagine pure Swiss water used to wash the rice: Swiss -> スイ, and the rice becomes perfectly clean.