There are many reasons why the rice is polished, but let's start with why rice is polished for brewing Sake.
Polishing rice means to remove the outer parts of the rice, these layers are the husk, bran layer, and germ. If these layers are not removed they will produce some harsh flavors and aromas. Also, by keeping the bran and hull in place, the time it takes to complete the parallel fermentation process increases.
Polished rice can produce a clean and crisp taste when combined with other good ingredients, such as great tasting clean water, a healthy, high yielding yeast and some Koji that has been grown on some good rice. Another reason rice is polished is that it will go bad in a shorter amount of time if it sits unpolished, which in turn reduces the storage life between growing seasons, this could result in a loss of profit and livelihood in some parts of the world.
The tastes of Sake from rice that isn't polished will be rough, wild, and harsh compared to Sake that has been brewed with polished rice. The more polished the rice the cleaner the taste of the Sake. This isn't to say that you have to only use rice that has been highly polished because to do so would be foolish if you were just starting out. The cost of brewing with highly polished rice is high. Online pricing is extreme due to the miller taking the time to polish the rice kernels to the specific ratio, and because the rice kernel is now small, you need more of them to make the same amount.
You can make some really decent Sake with 90% polished Sushi rice. Whenever I get the chance to get some 70% or more polished rice I hope to report on the taste difference using the same brewing process as the recipe on this website.
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