Measuring your Sake – Part One: the Hydrometer and SMV

In this series on Measuring your Sake I will cover how to measure all the key characteristics of sake. These include:

–          Nihonshudo a.k.a. Sake Meter Value (SMV) and specific gravity
–          Alcohol percentage by volume
–          Sando or Acidity
–          Amino Sando or Amino acid levels

For Nihonshudo or SMV all that is needed is a hydrometer. The most commonly available hydrometers are for specific gravity though you can find other metrics. To simplify this discussion I will stick to specific gravity and SMV. A discussion of these other metrics is in the article: Nihonshu-do (日本酒度) or Sake Meter Value (SMV).

So, what are we talking about when we talk about specific gravity? Well, specific gravity is a measure of the density of a liquid compared to the density of distilled water at 60F. But what does that mean?

Let’s back up just a bit here. Distilled water is pure water. The distilling process is a means of removing a liquid from a substance by converting it into a gas and then condensing it. When the liquid involved has a boiling point that is distinct enough from that of the other substances involved the distilling process will separate the substances based on their boiling points. So distilled water will have none of the other minerals, salts or other substances that may be found in water. These, or any other substances in water, will change its weight per unit volume. Also, because water expands and contracts with temperature its weight per unit volume will change with temperature. So to measure a substance’s density or weigh¹ per unit volume relative to water we need to specify the water temperature and that it contains no added substances.  So, we define distilled water at 60F as our standard and that specific gravity of a substance to be the density of the substance divided by the density of distilled water at 60F.

S.G. = (measured substance mass/volume)/(distilled water mass/volume) both at 60F

Because the units in this formula cancel out, specific gravity is a unitless metric.

An easy way to measure the specific gravity of a substance is by using a hydrometer. A hydrometer is a glass float with its units marked on its stem. When floated in distilled 60F water the value at the point that the hydrometer pokes through the surface of the water is 1.000. SMV can be calculated from the specific gravity by:

SMV = m/S.G. – m where m is 1443.

Notice that if the specific gravity is 1.0 then the SMV is zero.

Below is a hydrometer and its jar. The hydrometer is place in the jar filled with a sample of the substance to be measured. The sample size needed is roughly 250ml.

To measure the substance it is important to take the reading across the surface of the liquid to the markings on the hydrometer rather than the top of the lip where the liquid rises around the stem of the hydrometer. It can be tricky until you get the hang of it.

Once you have read the specific gravity you can convert it to SMV using the formula above. Values of SMV above 1.0 represent specific gravities below 1.000 and visa versa. As such there must be something in sake samples that is lighter than water and this is alcohol. Alcohol has a specific gravity of 0.785. So, despite the heavier substances, like sugars, in a sake sample the levels of alcohol can compensate. Negative values of SMV represent sweeter sake while positive values move towards drier sake.

Well, that is about all there is to measuring the SMV of a sample of sake with a hydrometer. In part two I will describe how to measure the percentage of alcohol by volume.