By: Tom Payette, Winemaking Consultant
Testing Volatile Acidity
Volatile acidity, vinegar production, is an important measurement to obtain when making wine. Wine is a perishable product, from a perishable fruit (notably grapes) for our purpose. Getting early measurements of volatile acidity on the fruit is essential to help measure the “chemical condition” of the fruit and how one may care to handle that fruit moving forward in the winemaking process. It is also useful when negotiating with the fruit producer if the grower does not recognize substandard quality. Measuring the volatile acidity regularly as a systematic process during wine aging is important. The test will confirm the wines are aging well and developing properly.
The cash still is a great tool to measure volatile acidity chemically. There are other ways to measure volatile acidity and many are potentially just as accurate; however, this article will focus be on the cash still and how to operate the unit.
Background
Volatile acidity is a chemical data reading from a raw material fruit or juice to measure the degradation of that fruit toward the unwanted production of vinegar. The cash still will drive off volatile acidity from a wine or juice sample using heat and then recondensing the Volatile Acidity (an acid) into a collection flask. This will be titrated with weak solution of Sodium Hydroxide (a base). This is a simple explanation of what is actually happening.
Tools and Chemicals
• Cash still unit or equivalent with stand.
• Distilled water (pre-boiled and cooled for safe use).
• Small mouth 250 milliliter Erlenmeyer flask.
• 0.1 normal sodium hydroxide, or approximate, standardized for accuracy.
• 25 milliliter Class A volumetric burette with definitive sub markings.
• Source of cold water and a sink for the exiting condenser chilling water.
• 110 volt outlet.
• Phenolphthalein and white backdrop to see the color change in the flask.
• 10 milliliter pipette – class A Volumetric.
Mixing and Standardizing Chemicals
Always wear safety equipment when operating this unit. Eye protection is very important.
1. Pre-boiled distilled water – The night before using the cash still one should boil the distilled water to drive off the Carbon Dioxide and allow it to cool.
2. Purchase or mix a 0.10 Normal Sodium Hydroxide solution and standardize the solution each time you use this test.
Procedure
1. Make sure the apparatus is assembled properly, there are no leaks at the joints, and connections are secure when the unit is in operation.
2. Always make certain water is in the heating chamber, to the proper level, before engaging the heating element.
3. Turn on the water source to the condenser.
4. Rinse the complete units’s interior with distilled water and evacuate any residuals from the interior boiling chamber leaving it empty and ready for a wine or juice sample.
5. Make sure the chemicals and reagents are mixed properly, strengths known and ready for use.
6. Collect a representative sample of wine or juice from a vessel in the cellar.
7. Check the sample for exogenous amounts of carbon dioxide. If the wine is not still – pull a slight vacuum on the sample or lightly heat, driving off the carbon dioxide and then cool to laboratory temp (68 degrees F.) Do use caution not to reduce the amount of volatile acidity with these actions as a false reading will occur.
8. Once the sample is ready, make sure the receiving stopcock on the cash still is positioned so the sample will go into the interior-boiling chamber.
9. Pipette with a class A volumetric pipette, or equivalent, 10 milliliters of the wine/juice sample and deliver it into the interior boiling chamber. Rinse any portion of wine/juice into the bowling chamber from the funnel with pre-boiled and cooled distilled water. Do not rinse out the volumetric pipette as they are made “to deliver”.
10. Close the stopcock to trap all inside the unit.
11. Place a collection flask under the condenser where distillate will be discharged from the unit during operation. Use a small-mouthed Erlenmeyer flask and make sure the connection is closed but loose. This is to limit the possibility that some of the collected sample would revolatilize and evaporate out of the collection flask. Example: Make sure the distillate is not falling into the collection flask and rather the distillate spout protrudes into the flask.
12. Turn the power to the unit on and boiling will soon begin.
13. Double check that cold water is flowing through the condenser
14. Watch the unit and the collection process.
15. When approximately 100 mils of distillate has been collected in the receiving flask – turn the power to the unit off.
16. Remove the collection flask with the distillate collected as soon as possible.
17. Add three drops of phenolphthalein to the distillate and swirl.
18. Record the starting volume of sodium hydroxide in the burette
19. Immediately start titrating the sample with the 0.1 normal sodium hydroxide. Titrate until a very light pink is achieved that will last for 45 seconds or more. This part takes practice and lab experience.
20. Record the ending volume of sodium hydroxide in the burette to achieve the total amount used for the titration. This will be used later in the calculation.
21. Open the stopcock on the Cash Still to evacuate the remains of the sample tested from the interior boiling chamber. (Some units do not have this capacity – please disregard this step and perform the same function in another fashion if the unit in your lab is not equipped with this function.)
22. Rinse the inner chamber twice with copious amounts of distilled water (two twenty milliliter rinses) and evacuate both rinses residues.
23. Turn the upper stopcock to readjust the distilled water in the exterior bowl as much water will have been lost during the last test. [Keep in mind we tested a 10 milliliter sample and collected 100 milliliters]
24. Make sure to close the stopcock to stop the evacuation of the inner bowl and start the process for another test. Repeat starting with step 8 above.
25. Turning our attention back to the previous test results and data gathered above.
Calculations
The formula used to calculate the results from the process is as follows:
Volatile Acidity:
(VA g/l) = (Mils of NaOH) * (Normality of NaOH) * (0.06) (1000
10 milliliters of wine / juice
The results are expressed in grams per liter.
Below are potential sources of error not stated above:
Be sure to drive off any carbon dioxide in the wine sample. This may actually change the volume of your sample as well and add condensed carbonic acid to your collection flask giving false readings on the high side.
Use boiled distilled water in the outer boiling chamber to avoid dissolved carbon dioxide in the water giving false results to the test. (carbonic acid would take more sodium hydroxide to negate the carbonic acid therefore giving a potentially false high to the volatile acidity measurement.)
Sorbic acid (potassium sorbate) in a wine may give erroneous measurements of the volatile acidity and may need a correction. [1 gram of sorbic acid is equal to 0.54 grams of acidic acid.]
Run a blank on some boiled distilled water and subtract that reading from your sample. Or run a blank on a 12.5 percent alcohol / boiled distilled water mix. Usually this blank will take 0.2 mils of 0.1 normal NaOH and this number can be subtracted from all future burette readings.
Calibrate the strength of your sodium hydroxide. This is the most important chemical known in this equation. For further accuracy use a 10 milliliter burette in place of the 25 milliliter burette recommended above.
This test is not correcting for sulfur dioxide in the wine. In most cases, with today’s lower sulfur dioxide winemaking, this is not necessary to correct.
To make the operation of the unit easier – one may adapt a way to fill the exterior bowling chamber, with pre-boiled distilled water, by having a source above the unit and a pinch clamp to fill the bowl when needed.
Cleaning the Unit
Over time, one will notice a brown deposit dirt developing on the inner chamber of the unit. This is unsightly and may cause inefficiency to the unit. These steps below can help remove these deposits and keeps the unit sparkling clean for better use and for tourist viewing into the laboratory.
Please wear proper safety goggles and equipment while performing this operation, too!
1. Place 20 milliliters of 2.0 normal NaOH into the interior boiling flask and add 2 drops of dish detergent. Rinse residues into inner chamber.
2. Plug in the unit to boil and allow to boil.
3. Place a collection flask at the outlet to collect the cleaning distillate. Do not breathe the gas and use in a well-ventilated lab.
4. One should notice a sloughing/bubbling of the dirt off the inner chamber.
5. After the dirt is removed, open the stopcock to evacuate the internal boiling chamber.
6. Rinse with copious amounts of distilled water to remove all soap and sodium hydroxide by repeated rinsing.
7. When running the first VA after cleaning note that the results may be “off” and be ready to run the sample a second time if the data seems to be in error from previous lab results.
One may also notice a mineral build up on the exterior of the condensing coils from the water used for cooling. These are not cleaned by the action above. One may clean these by removing that section (the condenser) of the apparatus and soaking in a strong base over a weekend or several days. Inspect the unit after soaking and rinse both the inner portion and exterior portions with copious amounts of water or potentially a very light citric acid and water mix. Once again be prepared to disregard any data from the next analysis run since it may be skewed from cleaning chemical residuals.
