Picture the toji presiding with a stopwatch over kurabito as they steep rice for a batch of daiginjo; 188.8.131.52 pull it out of the water. A 31 second steeping to reach the desired 29% uptake of water into the rice kernels. The precision required is just amazing, but why, why is this so time critical?
We need to back up a little and consider what we are trying to do. The goal is to have our steamed rice weigh 1.38x to 1.4x the pre-steeped rice.1 That is to have the rice take up slightly less than 40% of its weight in water by the time it has completed steaming. Steaming will add another 10% or so water uptake. Backing off to pre-steam levels gives us 1.28x to 1.3x for steeping. So how long will this take and why is it different for different grades of sake?
While there are differences between rice variety and milling rates, it turns out that one of the main contributors to the rate of water uptake during steeping relates to the moisture content the rice starts with. In fact it is this factor that is the main cause for highly polished rice to absorb water so quickly and hence the need for a stop watch.
As rice is milled it loses moisture. Rice milled to 50% seimai-buai can reach a 10% moisture level. The speed of milling can also affect the amount of moisture loss. The faster the milling is done the more moisture is lost. Anyway, the point here is that it is the moisture level of the grain rather than how much of the surface has been milled away that affects the rate and amount of water uptake. The following chart shows this relation between original moisture content and the amount of water absorption over time. Rice with the least moisture absorbs water at a very rapid rate while very moist rice only slowly absorbs water and does not reach the moisture content levels as does rice starting at lower moisture content.
Interestingly, this effect amounts to an increase of 3% in the final, saturated, moisture content for every 1% decrease in the initial moisture content before steeping. Once this was understood in the 1970s a less delicate approach was possible. By bringing the moisture content to the desired level so that a 30 minute steeping will yield the desired water uptake makes the process less fragile and produces rice with less micro-fractures.
Why is there a difference in micro-fractures and why do we care? Well, when the rice has very little moisture before steeping and absorbs water at such a rapid rate the expansion rate of the rice at the surface is different than the expansion rate just below. This causes stresses on the grain which in turn create the micro-fractures. In fact, this is the reason drier rice turns white or more opaque in the first few minutes of steeping. These micro-fractures let in an excessive amount of water and make the surface overly moist and mushy which is not good for koji growth.
Whether we choose the more traditional method with its more strict timing or the newer method which adds time for slowly raising the moisture content but relaxes the timing margins we can hit our steeping targets for good sake. So the method you choose is up to you.
- These targets may vary a bit from place to place. ↵