Wine colour and tannin

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:

Modified Somers reports as below

  • 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.

Modified Somers

The principle of this spectroscopic analysis is based on the methods of Somers and Evans1. The Somers color assay was modified to allow the standardisation of pH and ethanol concentrations of wine samples in a simple one-step dilution with a buffer solution, thus removing inconsistencies between wine matrices2. Tannin measurements are derived from a calibration developed from the methyl cellulose precipitable (MCP)3 ,4 tannin assay.

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. 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.

White wines phenolic composition

This method quantifies the phenolic composition of white 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.

Price per analysis
Analysis Per Sample (excluding GST) Volume required Target response time
Modified Somers $20 20mL 48 hrs*
Modified Tannin $10
Modified Somers and Modified Tannin $25
White wine spectral (OD280/320/420nm) $20 24 hrs
Optical density at 420 nm $6

* Response time may vary depending on sample numbers submitted and the current workload in the laboratory, and refers to days after receipt of samples.

For more information, please contact AWRI Commercial Services on (08) 8313 6600 or e-mail commercialservices@awri.com.au

References

  1. Somers, T.C. Evans, M.E. (1974) Wine quality: correlations with colour density and anthocyanin equilibria in a group of young red wines. J. Sci. Food Agric. 25 : 1369-1379.
  2. Somers, T.C. Evans, M.E. (1977) Spectral evaluation of young red wines: anthocyanin equilibria, total phenolics, free and molecular SO2, “chemical age”. J. Sci. Food Agric. 28 : 279-287.
  3. Sarneckis, C.J. Dambergs, R.G. Jones, P. (2006) Mercurio, M. Herderich, M.J. Smith, P.A. Quantification of condensed tannins by precipitation with methyl cellulose: development and validation of an optimised tool for grape and wine analysis. Aust. J. Grape Wine Res. 12(1) : 39-49.
  4. Smith, P.A. (2005) Precipitation of tannin with methyl cellulose allows tannin quantification in grape and wine samples. Tech. Review 158 : 3-7.