The Australian Wine Research Institute’s Analytical Service - Analyses

Spectral analysis of red and white wines

Red wines

Red wine colour profile

Red wine colour measurements are based on the absorbance of monomeric anthocyanin pigments and polymeric pigment forms in the visible and ultra-violet regions.

Measures of wine colour density, wine colour hue, total phenolics, total anthocyanins, degree of ionisation of anthocyanins and free and molecular sulfur dioxide are included in this profile:

• Wine colour density (a.u.) = A₄₂₀ + A₅₂₀
• Wine colour hue = A₄₂₀ / A₅₂₀
• Total phenolics (a.u.) = ^HClA₂₈₀ - 4
• Total anthocyanins (mg/L)
• Degree of ionisation of anthocyanins
• Degree of ionisation of anthocyanins after abolishing SO₂ effect on wine colour
• Free SO₂ (mg/L)
• Indices of chemical age

Refer to Somers and Evans (1977) and Somers and Vérette (1988) for a discussion of typical values of each of the parameters introduced above.

MCP tannin assay

The AWRI has developed a new method allowing tannin quantification in dry wine samples. The method, called the MCP (Methyl Cellulose Precipitable) tannin assay, is based on precipitation of tannin with methyl cellulose and a UV–Vis measurement at Abs 280 nm (for total phenolics). The results are expressed as g/L epicatechin equivalents.
Note: Tannins are polymers composed of monomer subunits such as catechin and epicathechin.

Which tannins are measured?

In general terms, tannins refer to polyphenolic compounds which can be divided into two classes; hydrolysable tannins (oak derived) and condensed tannins (primarily grape derived). In most wines, the grape derived tannins originating from skin and seeds will contribute the majority (> 90%) of the total pool of tannins. The level of tannins is very important to red wine development and has a major impact on sensory properties. In a recent study carried out at the AWRI investigating a set of commercial young red wines, the drying astringent sensation rated by a trained sensory panel was closely correlated with the tannin concentration as quantified by the MCP tannin assay. Previous studies have also shown a correlation between wine colour density and total wine tannin, linked to the ability of tannin to stabilise wine colour by interacting with the anthocyanins.

The MCP tannin assay is a robust selective method to quantify the total pool of tannins in young dry red wines.

White wines – phenolic composition

This method quantifies the phenolic composition of white wines, and could also be used for juices and light rosé wines. The method provides measurements for total phenolics, total hydroxycinnamates, flavonoids and relative brown colour.

Optical density of wine at 420 nm

The optical density of wine at 420 nm is sometimes a useful indicator of browning and, therefore, related to oxidation of the wine.

Grapes

Red grape berry colour measurement is performed after extraction of the berries and involves measurement of the total extractable anthocyanins or ‘colour’. The total phenolics is also measured in this assay. Red grape tannin level can be measured using the MCP tannin assay (For more information, see section on Grape analysis).