Nihonshu-do also known as SMV is the way we measure the sweetness to dryness level of a sake. The word nihonshu-do itself can be broken down into three words Nihon Shu Do with the English counter parts being Japan Alcohol Degree (as in position on a scale). So Japanese Alcohol is Sake and Degree or Meter Value taken together represent the main metric used to characterize sake. At first glance this measure is relatively simple and this is as it should be for sake aficionados. A -4 SMV value for a sake implies it is quite sweet while a value of +10 would be very dry. Its use in brewing reflects its more complicated nature.
SMV was originally based on the Heavy Baume scale created by a Frenchman in the late 1700s. However, the heavy baume scale is only valid for liquids that are equal or heavier than water and this is not the case for sake. For this reason nihonshu-do has the same slope as the heavy baume scale but is not the same. When nihonshu-do and heavy baume are used to evaluate the degree of sugar in water they directly represent the amount of sugar by weight in the solution. While the baume scale is pretty much obsolete today, similar scales like the Balling, Brix and Plato scales are all attempts to measure the amount of dissolved solids in solution with more accuracy, i.e. the grams of solids in 100 grams of water. However, where Baume was working with a sodium chloride solution Balling, Brix and Plato specifically worked with sucrose solutions.
Specific Gravity, a more general concept, measures density relative to water. As dissolved solids in water raise the density when added to water it is often useful to use specific gravity. A common way to measure the SMV, degrees Baume, Balling, Brix or Plato as well as specific gravity is to use a hydrometer. A hydrometer is an instrument that floats in a solution and has values listed on its neck that are read off at the fluid level, that is where the neck breaks through the surface of the fluid. There are hydrometers encoded with all of the above systems and in some cases have more than one system covered. The denser the solution is the higher the hydrometer floats while the thinner the solution the lower it floats.
Pure water will, by definition, have a specific gravity of 1.0 and a percent solids dissolved in solution of degrees 0.0 for SMV, Baume, Balling, Brix and Plato. For each of these, as with degrees Baume there is nowhere to go below zero (you can’t have a negative amount of solids dissolved in water). However, while SMV is also 0.0 for pure water it has values on both sides of this mark as does specific gravity. Why is this?
Well there are a few complicating factors. The first and most prominent of these factors is alcohol. Alcohol has a specific gravity of 0.785. So when mixed with water it lowers the specific gravity of the solution. At 20% Alcohol in pure water the specific gravity is 0.957, at 15% it is 0.9678. See the chart below. Between 15% and 20% is the normal range for the percent of alcohol in sake. However, 0.957 corresponds to a SMV of +65 while 0.9678 corresponds to a SMV of +48. These would be outrageously high SMV for sake, very possibly undrinkable. So how is it that sake has an SMV so much lower than these values?
As I mentioned above, dissolved solids push up the specific gravity. During and after fermentation the dissolved solids include various derivatives from starch that have not been consumed by the ferment, proteins and lipids. These push the specific gravity up to the range of roughly 0.990 (14 SMV) to 0.993 (10 SMV) for sake with 18% alcohol. In this case the contribution of the dissolved solids is 0.033 a very significant amount.
In order to get a better feel for how these metrics relate to each other I have plotted each against the SMV (Nihonshu-do value on x-axis). Recall that neither Baume nor Plato is valid below zero though this is not indicated on the chart.
To get a better feeling about how specific gravity changes with SMV, the following chart plots only specific gravity against SMV.
Here are the formulas and table used for graphing these two charts:
Specific Gravity uses the equation:
S.G. = m / (m + SMV) where m is 1443
several m values exist for Baume but I believe this is the correct one taken for SMV.
Heavy Baume uses the equation:
H.B. = -SMV / 10.
Plato uses the equation:
Plato = 260*(1-((m+SMV)/m)).
The sections of the table in yellow represent invalid entries created by extending the slope of the line.
Another important wrinkle to all of this is that temperature matters. What does this mean? Well, because water, alcohol and other liquids vary in their volume with temperature their density also varies. This means that all of these metrics will vary with the temperature of the solution. For the information above, 60F is assumed. However, as the various solutions are mixtures of their constituent components, each expanding at their own rates making all but gross adjustments for temperature impractical.
To adjust degrees Plato, add or subtract 0.0278 for each degree above or below 60F. To adjust the specific gravity, add or subtract 0.00011 for each degree above or below 60F.
We still need to talk about how to get from specific gravity and degrees plato to SMV.
Also, there is another way to measure these values, with a refractometer. I’ll need to cover this at another time though because this post is already too long.
The thing to keep in mind while using these systems is that SMV measures relative dryness, Plato measures the percentage of solids (percent sugar of solution with no alcohol) and specific gravity measures the relative weight compared to water (i.e., times the weight of water in the same volume).