In Part One we talked about measuring the SMV (Nihonshu-do) of your sake. In this part we will discuss two ways to measure the percent alcohol by volume for your sake. Because Heikou Fukuhakkou (並行複醗酵) or multiple parallel fermentation is used to make sake, we cannot employ the same simple method for determining the percent alcohol as is used for other fermented beverages. In particular, the simplest methods used for both beer and wine depend on knowing the initial specific gravity prior to fermentation. For sake, there is no point prior to fermentation when all the sugar is available for such a measurement. Rather, koji enzymes work side by side with yeast in the fermenting mash. Enzymes create sugar and yeast creates alcohol using the sugar; this is multiple parallel fermentation.
The first way to measure the alcohol level we will discuss uses what is known as the boiling method. It is a modified version of the distillation method that can be done at home with relative ease. The basic idea behind this method is that the alcohol in sake has a known specific gravity and a known effect on the specific gravities of substances it is in. Given this we can measure the specific gravity of our sake to get an initial state, replace the alcohol with distilled water and then take a new specific gravity reading. The difference between these two specific gravities can then be used to determine the percentage of alcohol by volume.
The difference between these two specific gravities multiplied by 1000 is known as the spirit indication. If the specific gravity taken after replacement is SG2 and the initial specific gravity is SG1, then the equation is:
Spirit indication (SI) = (SG2 – SG1)*1000
The best formula I have found to give the percentage alcohol by volume (%ABV) is:
%ABV = (0.008032927443 * SI^2) + (0.6398537044 * SI) – 0.001184667159
Note: There is an equation that has been picked up on the net but is incorrect. I mention it only so you will not be confused by it. Please do not use %ABV =(SI/1000)/2.11*1000.
Jon Musther’s site at www.musther.net has an online calculator that will handle the actual calculations for you so you don’t really need to create your own if you don’t want to.
OK, so that was all well and good but how do you replace the alcohol with distilled water? The way this is done is to begin with the sample of sake, place it in a beaker, flask or other container that can be used to slowly boil the sake to close to ½ its starting volume. It is important that you can precisely return the level of sake to the pre-boil level by adding distilled water. The pre-boil amount should be close to 250ml in order to be able to properly take the specific gravity with a hydrometer. Recall from part one how we used the hydrometer to measure the specific gravity of sake.
Be careful while boiling. Sake, in a small flask, will very easily boil over. It will look like it is not boiling at all and then a series of bubbles will come up the side of the flask and out the top. It may then, also catch on fire. In addition you may have to start over because some of the non-alcohol materials that affect the specific gravity can be lost.
The reason for using distilled water is to ensure the water being added has a specific gravity of 1.0. If it does not the calculations will be thrown off and you will not get accurate results. Also, to ensure accuracy, the post-boil level needs to be exactly the same level as was there pre-boil. The degree to which you don’t get back to the same level drives a difference in the ratio of material that produces a deflection in specific gravity to water which will throw off the final measurement. If we do bring the level back precisely then we will have replaced the water boiled off with water and the alcohols boiled off with water. Hey, that is it; replacing the alcohol with water.
So, to recap, what we need to do is choose a sample large enough to take a hydrometer reading (SG1), place these sample in a flask and boil off ½ the amount, precisely back up to the pre-boil level with distilled water, take the second hydrometer reading (SG2), plug these values into the equation above and voila out pops %ABV.
While not overly challenging the boiling method takes a considerable amount of time. A much faster method that is still quite accurate is done using a hydrometer in concert with a refractometer. As the hydrometer uses the relative density, refractometers use the relative speed of light as it travels through a medium. The difference in speed between the two medium is the refractive index.
Many inexpensive refractometers are used for determining the amount of sugar in a solution. They can do this because the refractive index changes with the amount of sugar in solution just as the density of the solution changes. These refactometers often use a scale marked in degrees Brix. Degrees Brix is the measure of sugar in a solution. 1 degree Brix means that 1% of the solution is sugar. A refractometer can be scaled in degrees Brix using a one to one mapping between degrees Brix and the refractive index. Solutions used for producing alcoholic beverages have other components that also affect the refractive index reading so in these solutions the Brix reading is not entirely percent sugar but includes other dissolved solids.
However, the refractive index for solutions that have both sugar and alcohol is more complex. For any given percent sugar + other dissolved solids, we get a mapping from the refractive index to percentage of alcohol. So to make sense of the refractometer reading we need to know both the specific gravity and refractive index. We can use a hydrometer to measure the specific gravity and then use the refractometer to estimate the alcohol. The chart below shows how a mapping for the refractive index to %ABV for various specific gravities can be done.
The equation for this is given by:
%ABV = 1.646 * RI – 2.703 * (145 – 145 / SG) – 1.794
Refractometers like the one in the picture above have Automatic Temperature Correction (ATC). They are used by placing a drop of sake on the plate and closing the cover. You then look through the eye piece and read off the value where the shade changes colors.
After measuring both the specific gravity with a hydrometer and the refractive index (given in Brix equivalents) with a refractometer, plug these values into the above equation and again, voila we get the %ABV for our sample.
Both of these methods work well with the former taking more time and the latter requiring additional equipment (the refractometer).
In the next part (Part Three) I will discuss how to measure the Sando ( 酸度 ), that is the acidity of sake.
 I got this equation from Jonathan Musther at http://www.musther.net. I don’t know where he got it or if he created it but it matches the table values from William Honneyman,B.Sc.,Ph.D. discussed in http://valleyvintner.com/Tips&Links/MeasuringAlcohol.pdf.