The Australian Wine Research Institute’s Analytical Service - Analyses

Problem indicators

Acetaldehyde

Wine yeast usually produce acetaldehyde from pyruvate on the glycolytic pathway to ethanol formation during fermentation. Ethanol can be oxidised to acetaldehyde when wine is stored in the presence of air, and a high concentration can also be an indicator of microbial spoilage.

Ethyl acetate

Ethyl acetate is formed by yeast and bacteria during fermentation, and by esterification of acetic acid and ethanol, prior to, or after, bottling. It is not measured during the volatile acidity distillation, but can be quantified using gas chromatography.

Depending on the species of yeast used, ethyl acetate is typically formed in the range 8 – 40 mg/L. The sensory threshold is approximately 200 mg/L, but that varies with the style of wine.

Methanol, acetaldehyde and ethyl acetate can be measured in the one analysis.

ID of Tartrate by Optical Rotation

The chilling of wine and subsequent seeding with the addition of Cream of Tartar for the cold stabilisation process is commonplace in the wine industry. Unfortunately wines can still become unstable over time and this is usually seen as a tartrate precipitation in the bottle.

The AWRI's Industry services team has observed the common types of crystalline instabilities in industry are potassium bitartrate, calcium L-tartrate and calcium DL-tartrate. These instabilities can be caused by wineries using the racemic (synthetic) forms of tartaric acid and of cream of tartar (potassium hydrogen DL-tartrate) during the cold stabilisation process.

Optical rotation is a method used to determine whether the product used is a natural L- isomer or the synthetic D or DL-racemic mixture. The method involves resuspending the crystalline material in a suitable solvent and then a polarimeter is used to shine polarised light through the solution. The rotation of the light is then measured and expressed as degrees (calculated with reference to the specific concentration of 1g of solute in 1 mL of solution).

This method requires a minimum of 50g of labeled raw material to measure optical rotation. The target response time is 5 days.