One of the key factors that determine the quality of sake is the rice polishing ratio, also known as semaibuai.

Semaibuai refers to the percentage of the rice grain that remains after the outer layers have been removed during the milling process. The higher the semaibuai, the more of the outer layers have been removed, and the more refined the sake will be. In general, high-quality sake is made from rice that has been polished to a high degree, resulting in low semaibuai.

The rice used in sake production is different from the rice used for eating. Sake rice has a higher starch content, which is essential for producing sake with a rich, complex flavour. There are several different types of sake rice, each with its own unique characteristics. However, regardless of the type of rice used, the polishing process is critical to producing high-quality sake.

The polishing process involves removing the outer layers of the rice grain. These layers contain fats, proteins, and other impurities that can negatively impact the flavour and aroma of the final product. The degree of polishing is carefully controlled to achieve the desired semaibuai for the particular type of sake being produced. Different types of sake require different levels of polishing, and even slight variations in semaibuai can have a significant impact on the final product.

The rice is also polished make the starch at the centre of the rice accessible to the koji mould to create fermentable sugars through saccharification. Unlike grapes, which have naturally occurring sugar, rice doesn't have anything that yeasts can utilise to produce alcohol through fermentation. That's why it's critical to polish the rice before so the koji mould can properly do its magic. This will turn our rice into sugary deliciousness that can be converted into alcohol.

The amount polished is called the polishing ratio, or seimaibuai in Japanese. It is expressed as a percentage and is written on most labels. For example, a polishing ratio of 70% means that 70% of the rice grain is left and 30% has been polished away. The more the rice grain is polished, the lighter and more aromatic the sake will become. 

This number is sometimes the reason why people tend to think that sake is a spirit. Indeed, when reading a label written only in Japanese, the polishing ratio is the only number followed by the "%" sign, and the alcohol percentage is expressed with the character " 度". To the eyes of a novice, that 70% would indicate that this is some seriously strong spirit!

These very specific polishing ratio levels define sake grades. To make it easy for you to visualise them, we've made the simple chart below :)