One question that comes up over and over in sake brewing is how to hit the SMV value desired. Puzzling over this question I asked one of the brewers from Yoshi-no-gawa when the opportunity presented itself. This was well over a year ago now, but what he told me was that they monitored the moromi (main ferment) and when it reached their desired SMV value, it was time for Shibori or Joso, that is time to press the lees from the sake. Using an assaku-ki machine (an accordion like press), often referred to as a yabuta, they are able to remove virtually all of the lees and even the yeast. Because this leaves only trace amounts of the yeast, fermentation is stopped and the SMV value is stabilized.
Very nice! However, if you have seen my videos of the pressing process you may be wondering if something is amiss. Well, yes, something is amiss. My pressing method leaves a large amount of the lees in my sake so there is no way I am removing the yeast. This is true for most homebrewers. There are filters used for brewing that may be able to do what is needed. I have one but have not used it yet. In any case to use it I will have to first press and let settle or fine before filtering because the amount of lees would hopelessly clog the filter if the sake isn’t pretty close to clear. So, for now, I can’t use their method.
Another method is similar but with a twist. When the moromi gets to the SMV desired, it can be pressed and then stored with its temperature lowered enough to stop the yeast from fermenting; 35F to 40F should do the trick. With this method the entire clearing process should be done at this lower temperature so the fermentation does not resume. After the first pasteurization has been completed the yeast will be dead and the enzymes denatured so the low temperature regime can be halted without fear of the SMV changing.
Another method is to simply let the moromi go until the yeast can ferment no more. In this case, unless your target SMV is very high, the sake will be too dry. So to bring it back to the desired SMV value we can steal a method from wine makers called amelioration. Technically speaking, if you go this route, you will not have a junmai, honjozo or any kind of ginjo sake. This is because, as you may recall, the definition of these special designation sake or Tokutei meishō-shu (特定名称酒) is that they have nothing added but rice, koji, water and yeast for the junmai and only a little pure alcohol added for the rest. Anyway, if you are fine with that then the amelioration process with a little table sugar will do the trick.
We could steel other methods from wine makers like adding Sulfite and Potassium sorbate. This creates sorbic acid which stabilizes wine by making a hostile environment for the yeast and disrupts its reproductive cycle. However, there is a noticeable taste produced and it gets stronger over time. Sodium benzoate can be use rather than potassium sorbate if you like. I looked into these some time back but decided that using the above methods were better in that they do not leave any additional tastes and seem more traditional. However, these methods should work to stabilize the SMV just fine.
OK, so how do we go about amelioration? Well, it is pretty simple really. Basically we want to lower the SMV from some level x to another level y. For each unit of SMV per volume of sake we will add a specific amount of sucrose (table sugar). To lower the SMV by one unit we need to add 6.6grams of sucrose per gallon of sake.
If you let your sake ferment all the way until it stops it is likely to have an SMV value in the range of +13 to +18 or so. This is very dry sake. Let’s say, just for the example, that we have fully fermented sake with an SMV of +15 and want to have sake with an SMV of +4. Given this we need to lower the SMV by 15 – 4 = 11 units. So for each gallon of sake we will need to add 6.6g * 11 = 72.6g of sucrose.
A nice way to add this sugar is to make a 50:50 solution of sake and sugar. To do this take a milliliter of sake for every gram of sugar needed, and warm it to around 140F (pasteurization temperature). Stir in the sucrose until completely dissolved. This will then be added to the sake, mixing well. It is that simple.
Now, I don’t really do this directly with the SMV units because I don’t have an actual SMV labeled hydrometer. I use the specific gravity scale which is more common on hydrometers in the US. For specific gravity, we want to raise the specific gravity (sweetness) rather than lowering the SMV (dryness). The conversion between these two is:
Specific Gravity = m / (m + SMV) where m = 1443
Given this we would have had a specific gravity after fermentation in our example of 0.990 (equivalent to 15 SMV) and want to raise the specific gravity to 0.997 (equivalent to 4 SMV). That is we want to raise specific gravity 0.007, let’s call this 7 points. To raise the specific gravity in one gallon of sake by 1 point requires 9.5g of sucrose. So, to raise the sake in our example by 7 points will take 7 * 9.5g = 66.5g.
Now, you may be wondering why we only used 66.5g of sucrose while using specific gravity but need 72.6g of sucrose while using SMV. Well, you caught me, both methods should use the same amount of sucrose. The rounding error is the cause for the difference.
How do you want to hit your SMV target? The answer may depend on how strongly you want your sake to fulfill the requirements for a special designation sake.