Measurement of volatile acidity (VA) in wine

This document gives a brief summary of the procedures and equipment requirements for some commonly used techniques for determination of the volatile acidity in wines. There are several techniques of varying complexity that are used to measure the concentration of volatile acidity in wine.

Steam distillation/titration

Description: This method involves the steam distillation (using a modified-Markham or Cash still) of a wine sample that has first had sulfur dioxide removed by oxidation with hydrogen peroxide. The distillate is titrated with sodium hydroxide to a pink end point indicated by phenolphthalein solution and the acidity calculated and expressed as acetic acid equivalents.

Equipment: Modified-Markham or Cash still, flasks, burettes
Reagents: Standardised sodium hydroxide solution, indicator solution, hydrogen peroxide solution
Services: Electricity, water supply, sink, wash up area, natural gas supply
Space required: Bench space

Enzymatic assay – acetic acid

Description: Since volatile acidity is mainly comprised of acetic acid (Buick and Holdstock 2003), it is sometimes useful to measure the concentration of acetic acid alone, particularly for quality control and monitoring purposes during maturation of wines. The conversion of acetic acid by specific enzymes can be monitored directly by measuring the absorbance (340 nm) resulting from the generation of a by-product of the reaction (NADH). The test is quite straightforward to conduct and requires only sample dilution. Kits for this assay are commercially available.

Equipment: UV spectrophotometer and cuvettes, autopipettes
Calibration: Made-up and kit-supplied standard solutions of glucose in water
Services: Electricity, wash up area
Space required: Bench space depending on spectrophotometer footprint

High performance liquid chromatography (HPLC) – acetic acid

Description: High performance liquid chromatography (HPLC) is a technique that offers several advantages for the analysis of compounds including acetic acid in wine, including potential for automation, high precision and it is relatively fast. However, there is a fairly high capital cost and skilled personnel are required to operate and maintain the instrument. Generally, this technique is only used where there are other applications of HPLC being used by the winery on a routine basis (e.g. organic acids).

Equipment: High performance liquid chromatograph
Calibration: Use made-up standard solutions of acetic acid in water
Reagents: Mobile phase, standard solutions for calibration
Services: Electricity
Space required: Significant bench area

References and further reading