Measuring the Acidity of Sake by Titration

Written By Mike Simons

Why measure the acidity of sake?

Acidity plays a significant role in the taste and texture of sake, even at relatively low levels compared with other beverages. Acid balances sugars and other components via our sense of taste, so evaluating acidity alongside SMV and alcohol content adds context to the overall profile of a sake. The acidity level is often listed on sake labels, so it can be an interesting point of comparison.

Testing acidity by titration is simple and inexpensive to do, resulting in a measure called ‘titratable acidity’, or TA. The measurement requires a few supplies that can be obtained from a homebrew or winemaking supply shop.

List of supplies

  • Clear plastic or glass sample container, 20-50mL

  • 10mL filtered sake sample

  • 1mL syringe

  • 10mL syringe

  • pH indicator with dropper or plastic pipette - Link to a combination indicator

  • 0.1M or 0.2M NaOH solution

The sake sample should be reasonably clear with low cloudiness for best results. It is important to be able to view all the way through the sample to judge the color change of the indicator. If the sample is cloudy, allow it to sit for longer to settle and use the top clear portion for your test sample.

Combination indicators use multiple ingredients to cover a wide pH range. The indicator you choose should come with its own reference swatch, which generally looks like the following:

Steps for titration

  • Choose a well lit work area where spills can be easily cleaned. Place the sample container on a white plate or a white piece of paper to act as a reference background.

  • Draw up 10mL of sake using a 10mL syringe, inverting to getting rid of any bubbles. It is recommended to de-gas the sample to remove any dissolved CO2, especially for unpasteurized sake. Transfer the full 10mL into the clean and dry sample container.

  • Add 3 drops of the combination pH indicator to the sake sample.

  • Draw up 1mL of the NaOH (Sodium Hydroxide) base solution using a 10mL syringe.

  • Begin dropping the base solution into the sample jar. Dark blue spots will immediately form. Swirl the jar and the color will even out and change back to orange or red.

  • As more base solution is added there will be a persistent color change. At this point add drops more slowly, swirling the sample each time to stabilize the color. If the sample hasn’t reached the target color after adding 1mL, draw up another 1mL with the syringe and continue dropping.

  • At the point when the color matches closely to the 7.0 swatch on the indicator reference card the titration is complete. Record the total quantity in mL of standardized base used to perform the titration. For example, if 1.0mL of 0.1M strength solution was used and there was 0.2mL remaining from a second time filling the syringe, a total of 1.0 + (1.0-0.2) = 1.8mL was used. The san-do of your sample is 1.8, the same value as the volume of 0.1M solution used.

Left: Starting indicator color shows the acidic pH of the sake sample
Right: Untreated sake sample
Titration in progress, yellow-green color is close to the target
Titration complete.The sample color closely matches swatch for 7.0 (pH 7.2 true end point will be slightly greener). The titration can also be verified using a pH meter to become familiar with the end point color.

The mL of 0.1M NaOH used to titrate a 10mL sample to pH 7.2 is defined as the san-do (酸度). 

If you instead are using 0.2M NaOH, the san-do is 2x the mL of NaOH solution used. San-do is listed on some sake bottles, and if it does you will always see it shown without units. It is a common way of representing titratable acidity, with a unit called milliequivalents/L (mEq/L). The san-do can be multiplied by a factor of 0.59 to get an equivalent g/L as succinic acid, but note that the TA value expressed as g/L succinic is not normally used in the sake industry.

A frequent question that comes up is, “What is the difference between titratable acidity and pH?”. The best short explanation is that pH measures free H+ hydrogen ions in solution, while TA measures both free H+ and bound H+ hydrogen ions in solution.

Tips and discussion

  • If you have a pH meter try using it as a point of reference to recognize the color change of the pH indicator. Once you’re familiar with the process a pH meter is not required since the indicator serves this purpose.

  • Try a titration with a commercial sake and compare your value against the number printed on the label.

  • Below is a titration curve for pure succinic acid, one of the weak organic acids in sake. The shape of the curve is typical for a weak acid reacting with a strong base. A sharp change occurs between pH 8.0-10.0 (the mid-point of which is called an 'equivalence point'). If succinic acid represented the majority share of the acids present, then phenolphthalein indicator with its significant color change from clear to pink at pH 8.2 could be used.

This diagram also points to a challenge – sake contains many types of acid, not just succinic acid. Malic, lactic, citric, and acetic acid are all found in sake in significant proportions. When titrating sake, many such curves are ‘combined’ in our mixed-acid sample, each with its own equivalence point. So rather than a sharp pH increase at any one point we instead see a slow rise in pH as the titration proceeds. This is reflected by a gradual change of the indicator color when titrating sake, so we must select our indicator type and end point carefully.

  • The chart below was produced using a home brewed sake sample and tracking the titration with a pH meter. While each sake will have a different curve, this sample shows that a large spread in measured acidity is possible depending on the choice of endpoint for the titration.

The endpoint pH has to be standardized because there is no single equivalence point for our mixed-acid sample. Acids get neutralized in sequential order from strong to weak, meaning that the weakest acids are still present past the neutralization point.

  • Sake brewers use a combined indicator that covers a wide pH range and addresses the mixed-acid problem. Bromothymol Blue plus Neutral Red has a working pH range of 6.0-8.0, allowing the standardized neutralization point of pH 7.2 to be identified easily and accurately. The specific indicator used in Japan may be hard to source, however aquarium testing or generic pH indicator drops are readily available that cover the same pH range. Look for products that show the same characteristic color pattern (red to green to purple) shown for the universal pH indicator at the top of this page.

References

Mike Simons
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