With the growing popularity of Ginjo and Daiginjo, the degree to which sake rice is polished has gradually increased over the past few decades.
The Seimaibuai (polishing rate) of the rice is a major influence on the taste of sake. Obviously, the more you polish the rice, the more it will take to respect the quantity required for the recipe; the polishing rate therefore acts directly on the cost of production and on the price of the beverage.
The objective of polishing is to rid the grain of its testa, of its pericarp, and of its embryo and to retain as much as possible of the Shimpaku (albumen).
It is that the bran of which the parts enveloping the albumen are made will stay between the koji and the starch; it will interfere in the solubilization and saccharification processes making them more random.
The other intention is to remove proteins, fats and ferrous compounds from rice, concentrated in the outer parts of the grain, which induce unwanted flavours and aromas.
Although mechanized, polishing remains a process that requires a great deal of attention and time. Some brewers prefer to entrust it to a specialist miller to be sure the job is done well.
Others, for the same reason, choose to take care of it themselves.
Brewers who choose to own a mill often locate it in a building separate from the brewery; polishing produces a volatile, highly nutritious and easily putrescible fine rice dust which can attract and support invading microorganisms.
Before the polishing begins, one needs to check the quality of the rice.
Damaged, diseased, dead, immature or sprouted kernels should be removed.
The grains must not be dry; that would make them brittle. Already, polishing dries them out. The heat generated by friction and abrasion causes the evaporation of its water to lose between one and 3% of the mass of the rice.
The moisture content of rice is about 15% for cultivars of the Okute type 晩稲 which are mainly grown in western Japan. For cultivars of the Wase type 早稲 growing in the north, the humidity should be a little higher.
When rice is dehydrated, minerals (including ferrous compounds) tend to migrate to the centre of the grain, making it more difficult to remove by polishing.
One will want the grains to be identical in size so that all need the same cooking time.
A perfect steaming of the rice is crucial to the vitality of the koji. If the rice is overcooked, the mycelium which develops on its surface, which would come off, because it is too easily soluble, will not be able to penetrate the grain. Likewise, if the rice is not cooked enough, the mycelium which develops on the surface (Hazé-Mawari) holds up, but the mycelium which penetrates (Hazé-Komi) the kernels like a root won’t succeed because the Shimpaku is too hard. For an efficient production of enzymes, it takes above all a good Hazé-Komi.
In Japan, yes, like sake, rice is classified and precisely it is according to the homogeneous quality of the grains. The fewer damaged, diseased, dead, immature, or sprouted kernels in the sample, the higher the rice ranks.
For the premium sakes, TOKUTEI MEISHOSHU (特定 名称 酒), it is necessary to use first quality rice (Ittô 一等), or better, of special quality (Tokutô 特等), or even better, of superior special quality (Tokujo 特上).
From the harvest, one avoids using rice from the previous year (古米 Komai). The lipids in this rice will have had time to oxidize and will impart a rancid taste to the sake. And if the grains have become dehydrated, the minerals (including ferrous compounds) will have migrated to the centre of the grain and therefore be more difficult to remove by polishing.
There are two types of polishing. The first is to press the grains together while spinning them in a silo. This is the method used to husk and blanch table rice.
For Sakamai, we can start with this method, but for a deeper polishing, we will use the abrasion method.
During polishing, the Sakamai is fed into a hopper located above the polishing chamber.
A valve controls the flow of kernels into the polishing chamber where the grains rub against the abrasive surface.
Afterwards, a mesh separates the bran from the grains.
The bran is picked up on one side. The refined grains are collected by a bucket conveyor to be reintroduced into the hopper for another round.
Polishing can take a long time. And the duration of the process grows exponentially. With the same quantity of rice, if it takes 6 hours to achieve a rate of 75%, to attain 70% may need 8 hours. To reach 65%, 12 h, and for 50%, it can last as long as 30 h.
The closer you get to the Shimpaku, the harder and more brittle the rice is. Moreover it has been dried out by repeated abrasions. The process needs to be even more gentle.
A breakage of the Shimpaku causes its pieces to fall with the bran on the wrong side of the sieve. The precious part of the grain that we wish keeping is lost.
Also, the partial losses of the Shimpaku by breaking induce uncertainty in the calculation of the Seimaibuai.
The polishing rate is calculated by weight. We want the polishing rate to apply as accurately as possible to each grain. Lighter grains, because they are broken, pull the average weight down by compensating for that of grains that would be less polished than the desired Seimaibuai.
For example, in a batch that we mean 60% ripened, one wouldn’t want half of the kernels to be 20% polished and the other half to be whole kernels; the measure of rice will have lost 40% of its mass, but not necessarily the one you want to remove.
Once the polishing is completed, the rice is bagged and stored for at least one week; This is the Karachi, a period during which the grain gradually regains the moisture lost by the process.