In part one I talked about how to measure the Nihonshu-do or Sake Meter Value (SMV) or your sake. In part two I covered how to measure the Arukoru bun (アルコール度数) or Alcohol percent by volume (%ABV). In this part, part three I will discuss how to measure the san-do or acidity of sake.
To measure the san-do (acidity) of sake at home, there are two very closely related methods available. The easier and less expensive of the two methods is to use a wine acidity test kit. This kit contains almost every think you need to measure the acidity on sake. However, there is a difference getting from the physical test to the interpreted value, but I am getting a little ahead of myself. The second method differs from the first in that a pH meter is used rather than Phenolphtalein to determine the point of neutrality. Often the second method employs more sophisticated equipment for each of the components but this is not strictly necessary.
So, what do wine acid test kits come with? Well, they come with a small beaker to mix the sample and chemicals in, a syringe to measure with, a solution of Sodium Hydroxide, usually at a concentration of 0.1 Molarity (M) and Phenolphtalein. The idea behind the test is that we have some unknown amount of acid in our sample that we want to measure. To do this we add a known amount of base to neutralize the sample pH. When we have neutralized the sample pH with a known amount of base we can then work out the original amount of acid. Clear as mud? Perhaps an example will help.
In my last batch of sake, using the kit shown below, I collected 10ml of sake using the syringe and place the sample in the plastic beaker. I then dripped a few drops of Phenolphtalein into beaker with my sample. The Phenolphtalein reacts by changing color when the sample becomes neutral. Then after, thoroughly cleaning, rinsing and drying the syringe, I filled it with 10mls of 0.1M Sodium Hydroxide. At this point I slowly dripped the Sodium Hydroxide (NaHO) solution into the sample while swirling the sample and watching for the color to change. After a little bit, the color changes momentarily around the drips and then returns to clear. After a bit more the entire sample changes color and the sample remains the new color. At this point the sample was neutralized.
By knowing I carefully loaded the syringe with 10ml of NaHO solution to start and that after titration I had 8ml left, I see I used 2ml of 0.1M NaHO solution. Given these facts we can then plug these values into the equation for Titratable Acidity (TA) for sake given in g/L of Succinic Acid:
TA(g/L) = MoleRatio* (BaseMolarity * BaseVolume * MoleMass) / SampleVolume
TA(succinic g/L) = ½ * (0.1M*2ml*118) / 10ml or a TA of 1.18 g/L Succinic
Where the MoleRatio and MoleMass will be discussed later, the BaseMolarity is the concentration (moles / liter) of NaHO in distilled water, BaseVolume and SampleVolute are the amounts of NaHO and Sample solutions in ml.
Gauntner1 puts the range for sake acidity to be about 1.0 to 1.8 with the vast majority of sakes in the range of 1.1 to 1.2. Hey, at 1.18 we have a winner; smack in the middle of the pack!
You may notice however, that this equation is not what comes with the kit. Two main differences exist: acid type and final units used. In sake as we have just observed, succinic acid is used. Succinic acid, along with malic and lactic acid are the most abundant acids in sake. For wine TA levels, Tartaric acid is used because tartaric acid is the most abundant acid in wine along with malic and citric acids.
Two factors that matter in this substitution of succinic for tartaric acid. The first factor is the mole ratio (MoleRatio) for the neutralization reaction. You may recall that a base, something with a high pH value, when added to an acid, something with a low pH value will neutralize, bring the pH value back to around 7. If one molecule of base reacts with one molecule of acid the mole ratio is 1. If two base molecules are required the ratio is ½, and so on. Simply put the mole ratio is the number of acid molecules over the number of base molecules needed for the neutralization reaction to occur. Both succinic acid and tartaric acid both have a mole ratio with sodium hydroxide of ½ so this, while important, does not change the equation.
The second factor is the molecular mass (MoleMass). The molecular mass of succinic acid is 118 while the molecular mass of tartaric is 150. This is the only difference in the above equation. So to convert from acid as succinic to acid as tartaric requires a factor of 150/118 = 1.2712. However, going the other way would be more natural if you get a TA level based on a wine kit and want to convert the final value. To do this you need to use 118/150 = .78667 if this where the only difference; however, it is not.
Recall, above that I said the final units used are also different. For sake, grams per liter (g/L) are used while in wine grams per 100ml are used. This approximates a percentage acid to the whole. So, to convert a standard measurement made for wine to one for sake we need to multiply the measurement by 10*118/150 = 7.8667.
When using a pH meter to determine neutrality rather than a color change, the routine remains the same with the exception of swirling the meter in the sample and watching the meter value. When the value reaches 8.2 we have reached neutrality for this test. Generally a pH of 7 is considered neutral but because phenolphtalein changes color at a pH of 8.2 and this is the standard, 8.2 is used.
With that we have cover how to measure the acidity of your sake. Next, I will discuss how to measure the amino san-do or amino acid level of your sake.