Definition & Relevance
Volatile Acidity refers to the steam-distillable acids present in wine (mostly acetic acids). In small, barely detectable, amounts below 600 mg/L, Volatile acids are believed to add complexity, but above 1g/L they are considered spoiling the wine (vinegar).VA is a natural byproduct of fermentation and tends to go up over time as a consequence of microbal activity. In concentrations above 900mg/L it affects the nose negatively: either ethyl acetate esters (acetone) which smell like nail polish or remover or acetic acids which smell like vinegar.
Early detection of rising VA is important as it indicates excessive exposure to oxygen and unhealthy microbal activity. While excessive VA can be removed it is important to treat its root causes – the earlier the better.
To measure VA we use a Cash Still manufactured by Research & Development Glass Products & Equipment, Berkeley, CA. The procedure is as follows:
- Step 1: Fill the boiler. Fill the boiler reservoir to the middle of the round flask with distilled water through the funnel; for this the green handle on the funnel-stopcock must point down. This takes about 500 mL.
- Step 2: Start the Condenser. Connect the cooling water coming in to the lower end (inlet) of the condenser. Connect the upper end (outlet) of the condenser to the upper end (inlet) of the aspirator and connect the cooling water discharge hose to the lower end (outlet) of the aspirator. Turn on the cooling / tap water and adjust the water-flow so it creates suction in the aspirator. (To check the suction, add a little distilled water to the sample chamber [through the funnel, with the red handle on the funnel-stopcock down]. Then open the aspirator-stopcock. If the water in the sample chamber is sucked out, the water-flow is sufficient).
- Step 3: Add the sample. Turn the red handle on the funnel-stopcock down. Close the aspirator stopcock and add exactly 10 mL of the wine sample through the funnel to the sample chamber.. Add 2 drops of Antifoam B and rinse the funnel into the sample chamber with a few mL of distilled water.
- Step 4: Degass the sample. For this the red handle of the funnel-stopcock needs to stay down. Turn the electric power to the heating coil on. Place a 300 mL Erlenmeyer flask under the delivery end of the condenser. Wait 20 seconds after the water starts boiling, and then close the funnel-stopcock (these 20 seconds allow the sample to be degassed from CO2)
- Step 5: Distill the sample Collect approx. 125mL of distillate in the Erlenmeyer flask for the subsequent titration.
- Step 6: Clean and ready for next sample. Open the aspirator-stopcock to suck out the remains of the wine sample. Close the aspirator-stopcock. Turn the red handle on the funnel-stopcock down and add distilled water to cleanse the sample chamber. Open the aspirator-stopcock to flush out. Repeat until the sample chamber is clean. Before going back to Step 3 for the next sample, check the water level in the boiler reservoir. It must be well above the heating coil. If the reservoir overheats due to lack of water it may break. Never add water to a dry and hot reservoir – instead turn the power off and wait until the glass has cooled down.
- Step 7: Titration. Take the 125 mL of distilled sample (S) and add 3 drops of 1% Phenolphthalein and titrate in Titrator #1 with 0.1N NaOH until the distillate turns pale pink and stays pink for 15 seconds. Calculate amount of NaOH used (AN = Startpoint less Endpoint). Then add 1 mL of Starch Indicator and 5 mL of 25% Sulfuric Acid and titrate in Titrator #2 with 0.02N Iodine to a faint blue color. Calculate amount of 0.02N Iodine used (AI = Startpoint less Endpoint)
Result: VA [mg/L] = 600 * AN[mL] - 12 * AI [mL]
For a video on the process go to this page