Calculations

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This page tries to explain how sake is measured and how that might differ from other types of fermentation. You’ll find conversions and equivalents to beer and wine mostly. While the science is fundamentally the same, the literature you might encounter will often use specific metrics that common in Japanese sake production and so this can serve as a primer to explain those and how to interpret graphs and charts you see.

Tools

Online Forms / Spreadsheets

• Sample Ferment examples (acidity, temperatures, etc.)

• Recipe Calculator

• Yeast Calculator

• Unit Conversions

Measuring Equipment

• See Resources/Equipment

Density

S.G. (specific gravity), SMV (sake meter value), and °B (degrees Baumé) are all relative measures of a solution’s Density. This is an extremely important metric to track the fermentation progress of sake because the increase and decrease of density due to sugar and alcohol concentration helps us make corrections to temperature and water ratios to control yeast health and potential alcohol.

• SMV is measured using specialized hydrometers at 15°C (ex: +15 to -40 SMV). This value is also commonly referred to as the Nihonshu-do

• SG is the most commonly used scale to express liquid density and is measured using digital or analog hydrometer. It represents the ratio of the mass (weight) of a given volume of liquid to the identical volume of water.

• °B is a more complicated measurement and has two separate scales for liquids heavier or lighter than water. For sake, we use the heavier scale because it is used early in fermentation when there are lots of dissolved solids (mostly sugars)

Sugar Concentration

Sucrose

One of the most important metrics we can track in any ferment is sugar content, however, due to the nature of a solid mash containing lots of dissolved solids, the results are not always very accurate. The most common equivalent measurements are: °Bx (degrees Brix), °P (degrees plato), and °Balling (degrees balling). A filtered sample is preferred for all of these because the light will refract differently with dissolved solids suspended in the mash, but the results are still assumed to be more of a relative figure than an accurate measurement.

• °Bx is measured with a specific refractometer at 20°C. One degree of brix is equal to 1 gram of sucrose per 100 grams of solution.

• °P is measured with a specific refractometer at 20°C

• °Balling is measured with a refractometer at 17.5°C.

Glucose

While measuring Brix can estimate total sugar content, a more accurate measurement of glucose levels can help us get a better sense of our koji performance (ex: relative enzyme makeup), sense of flavor balance, as potential decisions for when to press. Not every figure needs to be taken. This one provides more clarity that you may or may not feel is relevant to your process.

The following are ways to measure glucose during your fermentation and also after pressing.

• Fehling Method

• Glucose Meter

• Contour Next One, Test Strips

• Phosphate Buffer

Alcohol Content

*Beer brewers and Wine makers should not skip this section.

Alcohol content is typically measured in terms of Alcohol by Volume (ABV) and is represented by a percentage of the overall liquid. The most common forms of measuring Alcohol are Distillation, Ebulliometry, and Estimation. The easiest and least expensive method (~20USD) is Estimation and is recommended for homebrew. Commercial brewers should use Distillation or Ebulliometry methods because they are more precise for regulatory matters and require more expensive equipment. However, it should be noted, that several commercial brewers in the U.S. just use the Estimation method and based on our side-by-side tests, this is accurate within about a 0.6% ABV margin of error, which falls within the 1% range currently allowed as of 2023.

Unlike wine and beer that have an “original gravity” and “current gravity” that tell us how much sugar has been converted to alcohol, the presence of active koji enzymes in the mash mean that sake’s gravity can actually increase as the fermentation progresses. Please refer to the A/B Line article for more information on tracking your fermentation progress. For this reason, we cannot simply apply the common assumption that fermenting 1 degree Brix of sugar will produce approximately 0.55% to 0.6% Alcohol by Volume (ABV).

Brix measurements are further complicated by that fact that once the level of alcohol begins to increase due to fermentation, independent refractometer assessments no longer yield an unambiguous proxy measure of sugar content. This is because alcohol also contributes to the refractive index of the sake, and thus many different combinations of sugar % and alcohol % can lead to the same refractometer °Bx value.

Estimation

Alcohol Content can be estimated by entering the °Bx (brix) and S.G. (specific gravity) into the following formula:

As stated earlier, ABV represents the percentage of alcohol in a given volume of liquid, °Bx is a measure of sugar concentration, and S.G. represents the density of the liquid compared to the density of water. If there was nothing but alcohol and water in the liquid, the S.G. could tell us exactly the % of alcohol because there is a direct correlation of an Ethanol and Water solution to density. However in sake, there are actually dissolved solids (sugars) which will increase the density of the liquid, and result in an incorrect ABV. Let’s break this formula down which is based on several empirically derived constants:

First we multiply the °Bx by 1.646, a constant that represents the conversion factor between °Bx and ABV accounting for the dissolved sugars in solution.

Second, we need to calculate the Apparent Extract with (145 - 145 / SG). This is a measure of the dissolved solids in the liquid. The number 145 represents the maximum amount of dissolved solids (sugar) that can be dissolved in water. S.G. is used to compare the density to pure water, so (145 - 145 / SG) calculates the amount of dissolved solids in excess of what water can hold in units of weight per volume (ex: grams/liter).

Finally, to increase the accuracy of the estimate, we need to apply a couple correction factor: 2.703. The number 2.703 is a constant that accounts for the effect of dissolved solids (other than alcohol) on the ABV. The constant 1.794 further corrects the result and is likely derived from empirical observations or calibration based on the specific properties of the liquid being measured.

Distillation

A sample of sake is heated in a specialized distillation apparatus, typically a small still or distillation column. As the sake is heated, the alcohol vaporizes at a lower temperature than water and other components, allowing it to separate from the rest of the liquid. The alcohol vapor is collected, condensed back into liquid form, and collected in a container. The volume of the collected alcohol is measured and compared to the initial volume of the sake sample. This comparison allows the calculation of the alcohol content as a percentage of the total volume.

ABV (%) = (Volume of Alcohol / Initial Volume of Sake) x 100

For example, if the initial volume of sake is 100 milliliters and the volume of collected alcohol is 20 milliliters, the ABV would be:

ABV (%) = (20 mL / 100 mL) x 100 = 20%

Ebulliometry

A specific type of distillation used to measure the boiling point caused by the presence of alcohol in a liquid. It is based on the principle that the boiling point of a liquid decreases as the concentration of alcohol increases.

First, The boiling point of pure water (without alcohol) is also measured under the same conditions. Then the test is repeated for the sample containing alcohol. Both temperatures are recorded. By comparing the boiling points of the sample and pure water, a scale is used to calculate the ABV.

Acidity

• Measuring with Titration

• Atago Sake Brix and Acidity Meter (follow included instructions)

Yeast

Almost everything you do in the yeast starter and the main mash is entirely based around yeast cells. The moto/shubo is all about trying to curate a batch of yeast that are hardened against the extreme (out of their preferred zones) environment that sake fermentation requires in order to produce aromatic, low acid, balanced sake. Low temperatures, sugar level, acidity, alcohol level, and other measures can be compared to the yeast strain’s optimal ranges to see what kind of challenges they will be up against. These will eventually lead to cell death in most cases, but before that will induce stress responses that can produce extra acids, undesired aromas, malformed cells, and other problems. The yeast starter is the best place to apply these stresses intentionally to help eliminate weaker yeast before the main mash.

Once the stronger yeast have been cultured into a high cell count (10^8 cells / mL), you’ll be ready to build the mash. The following Yeast Viability Count will help you do that. If your yeast starter procedures were harsh enough to force difficult environments, then the san-dan-shikomi will protect the mash from contamination because it will allow the yeast time to increase their concentration each time you double the size. The point of doing the three step additions is literally to protect the fermentation from bacteria.

Yeast Viability Count

This is the most common test for yeast quality due to the ease of the procedure, and because while it doesn’t tell the full story of how well yeast are prepared to handle tough situations (ex: high sugar, high acid, high alkaline, low nutrient, etc), we can infer some of that through the yeast growing process, especially if we are growing them in a proper moto, which will stress the yeast and cultivate only those capable of surviving the adverse conditions we create for them.

For a Viability Count, we are just interested in how many cell are actually alive, as compared to the total count, which doesn’t differentiate. This test is simple, yet can be complicated to do correctly because it involves a lot of steps. Basically, you dilute a yeast sample with water, and then dilute it again with a specific dye which the healthy cells either won’t permeate through their membranes, or can metabolize out. Which ever cells are colored with the dye are dead, the clear ones are alive.

Check out this Tutorial to learn how this procedure works.

• Equipment Needed

• Hemocytometer

• Microscope (see equipment page)

• Erythosin B (red dye #3) or Methylene Blue

• Isotonic Water (9g salt / Liter)

  • You can also use Distilled Water BUT keep in mind that some cells will start to die because it is technically toxic to them. Sake yeast shouldn’t have a problem with this, but it can throw off your counts if any die. So, just keep that in mind, especially if you are doing a Vitality count.

Yeast Vitality Count

This is a much more complicated test (depending on which stresses you want to evaluate). Essentially, it is a count of life cells before and after a series of stresses. If you really want to pursue this aspect further, please read about the tests performed by the ASBC.