I decided to do a quick experiment to determine the amount of moisture in my soaked rice. It had been a while since I looked up how to do this and I miss remembered how much time was required. I was thinking that it required 6 hours but in fact the procedure takes 16 hours for the drying phase. This is more than twice the time I was planning. Well, I got started and weighed out two 30g samples each of Hitomebore 90% seimaibuai, Calrose 90% seimaibuai and Calrose 60% seimaibuai. Recall that the seimaibuai is percentage of rice remaining after milling the outer portion away. Having weighed the samples, I added water to one of each of the types of rice to soak for an hour. At the end of the hour, I drained and weighed the rice again and prepared it all to go into the oven at 265°F.
With 10 hours in the oven, I removed the rice and weighed each sample. While 10 hours is a little short of 16 hours I was not willing to stay up till 3AM to complete the experiment. If 10 hours is close enough I’d expect that the dry weight of the wet and dry samples of each type should be the same. This is close to what I found but not quite. The final weight of the wet sample for the 90% seimaibuai was a little lower than the dry samples. Well, at least I know that there was enough time to remove the water weight from the wet samples.

Init Dry Weight (g)  Wet Weight (1 hr. soak)  Final Dry Weight (10 hr. @ 256F) 
Hitomebore 90% Seimaibuai 
30 

27 

30 
39 
26 
Calrose 90% Seimaibuai 
30 

27 

30 
39 
26 
Calrose 60% Seimaibuai 
30 

29 

30 
48 
29 
Based on this data we can see that both the 90% seimaibuai types had 34 grams of water weight. This works out to be about 13% of the wet basis (Init Dry Weight). This is well within the range of expectation for table rice. Rice milled to 60% seimaibuai would be expected to have a much lower moisture level after milling and until it has had time to absorb enough moisture to come back to the 10% expected level. The 60% rice used here was milled by SakeOne and shortly after bagged bye Steinbart’s in a sealed plastic bag. So, it is not given the chance to absorb much moisture after milling. This is just what we see in the data as well; only 1 gram of water weight. One gram works out to be only 3% of the wet basis.

Init Dry Weight (g)  Wet Weight (1 hr. soak)  Soak % increase  Soak % Water 
Hitomebore 90% Seimaibuai 
30 
39 
30% 
33% 
Calrose 90% Seimaibuai 
30 
39 
30% 
33% 
Calrose 60% Seimaibuai 
30 
48 
60% 
40% 
As discussed in “Steeping to hit the numbers,” this data shows that having a lower initial water weight results in a higher rate and amount of moisture uptake. Both samples with 13% initial water weight gain only 30% while the 3% initial water weight sample gains 60%.
A moisture content of about 38% after steaming is considered optimal for ginjoshu koji. Assuming steaming adds 10% moisture content (this is another experiment), we would like to high 28% water weight after soaking on a wet basis. Notice that, the 90% seimaibuai rice is closer to this ideal than the 60% seimaibuai rice.
Based on this it would be a good idea to lower the steeping time some or raise the initial moisture content of the 60% rice to be more on the order of 15% before steeping. Tweaking this a little could improve your sake.
Did you find out more about how much moisture content is added by steaming? Is it always close to 10% as you assumed? Is the amount of moisture content added by steaming also influenced by the amount of milling and the preexisting moisture content of the rice? Just curious. Thank you for the very informative site.
Catherine,
I did not do this experiment. One way to do it would be to weigh the rice after soaking but before steaming and then just after steaming. If done with enough rice the difference in weight should be pretty accurate.
Thanks,
Will