The Calculation of the Seimaibuai
Updated: Mar 12, 2022
How is the Seimaibuai, the rice polishing rate, calculated and guaranteed?
We often hear that Seimaibuai is the percentage of the mass of brown rice that remains after polishing. For example, if we start with a ton of whole rice and there is 600 kg of rice left after polishing, we could claim that the Seimaibuai is 60%.
It would be too simple.
For an estimate, that's fine. But the result could even be wrong.
First, if after polishing you add together the weight of the polished rice and the weight of the bran, you don't get 100% of the starting mass. The heat generated by friction dehydrates the grain and causes it to lose up to 3% of its mass.
Also, a grain-to-grain consistency of the polishing rate is sought. For a Seimaibuai of 60%, each grain should have a rate as close as possible to 60%.
With the calculation method which consists in measuring the percentage of the mass of brown rice that remains after polishing, we would have a Seimaibuai of 60% even if we reduce 40% of the kernels to dust while keeping the remaining 60% as whole grains. This calculation method does not differentiate between having 60% of the whole grain and having 100% of the grains polished at 60%.
For a reliable measurement, one proceeds differently.
Before starting, a sample of 1000 whole rice grains is weighed.
Depending on the cultivar, a thousand kernels of brown Sakamai weigh between 25 and 30 grams.
In principle, because the rice is minimally premium (Ittô 一等), the grains are similar in size. We can then say that each grain weighs roughly one thousandth of the weight of the sample.
After polishing, one or more samples of 1000 polished rice grains are weighed. Then we calculate the weight of the sample of 1000 grains of polished rice divided by that of the 1000 whole grains and we multiply by 100, we get the polishing rate (weight of 1000 polished grains / weight of 1000 whole grains x 100 = polishing rate).
But still, grains could have been broken and lost their fragments. Their lesser weight would decrease that of the sample; thus other grains would not be as polished as the weight of the sample would falsely suggest.
So the third step is to measure the variations that there may be in the size of the polished grains. For this, 300 grains are weighed individually and the values are distributed on a board.
If the weight of each kernel approaches the target, a threadlike bell-shaped graph is obtained.
On the other hand, if the values are scattered, if there are two or more peaks and they lay on either side of the target value, one cannot claim to have the Seimaibuai suggested by the weight of the sample.
Polishing to a satisfactory result and calculation of the rate afterwards.
Rather than polishing the kernels to achieve a target rate, some millers grind them to a desired state.
There is no point in polishing up to 40%, if already at 50%, only the starch core remains.
The miller will polish and repolish, but after each round, he will take a few grains, wet them with a reactive dye that colours the Shimpaku pink and the ball, the embryo and even the base of the embryo, blue.
It is after the miller has judged the result satisfactory with regard to the remaining bluish parts that he calculates the Seimaibuai.