Color Measurement of Wine

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Wine is a traditional, natural product where color variation is expected and accepted. High color comes from high anthocyanin content and high tannins associated with red wines. Color varies with wine processing practices, particularly fermentation temperature. Co-pigmentation in wine and berry colors (related to presence of anthocyanins) enhances the wine color.

 

White wine can also be measured for lot-to-lot consistency in color. Of concern in white wines is “pinking”, a slight reddish tint to what is typically a yellow/green product.

Primary Color Quality Concerns of Wine Color

  • Verification of lot-to-lot consistency in wine color, particularly with varietal blends, over a season and from season to season.
  • Monitoring the change in wine color over time.

Other Related Applications

Verification of lot-to-lot consistency in bottle color, natural and technical cork, caps, and metal foil covers.

“Anthocyanin pigments are responsible for the red, purple, and blue colors of many fruits, vegetables, cereal grains, and flowers…. There is considerable anecdotal and epidemiological evidence that dietary anthocyanin pigments and polyphenolics may have preventive and therapeutic roles in a number of human diseases. Through the much publicized “French paradox”, the public has become aware that certain populations of red-wine drinkers in France and Italy have much lower rates of coronary heart disease (CHD) than their North American and Northern European counterparts. It is widely accepted that red wine phenolics contribute at least partly to this beneficial effect…” [1]

Change in wine color related to how red wine changes over time. Quality concern is change in color over years relative to original color. The sweet time spot is 20 years and consistency in color at that point is important. Over than time anthocyanins (mostly found in skin of red grapes) change in color over time, starting out as a deep red and gradually reading a fine ruby red at 20 years and will be yellow (very red desaturated) after 100 years as anthocyanin polymerizes over time. [2] [3]

As anthocyanins become increasingly polymeric with time, they break down to phenolics, which are also red pigmented. So the red wine starts out red and gradually changes to phenolics that are also red, but not the same red. What also happens is the breakdown of anthocyanins by oxidation, which creates a yellow color, not red. [4]

Traditional Metrics for Color Analysis of Wine [5]

There are a couple of traditional metrics based on spectral data that are used to quantify color in red wine over time. The quality concern is change in color over years relative to original color. The sweet time spot is 20 years and consistency in color at that point is important. Anthocyanins (mostly found in skin of red grapes) start out as a deep red and gradually reading a fine ruby red at 20 years and will be yellow (still red but very desaturated) after 100 years as anthocyanin become increasingly polymeric with time.

  • Undiluted wine but filtered (2 micron filter) to remove any residual solids
  • Sample at natural pH
  • 10 mm path length transmission cell is typical for light reds, rosés and most whites. Extremely

Standardize instrument using a transmission cell + DI water such that all spectral values across the visible spectrum are set to 100% transmission.

As a PQ Performance Qualification step, read back the cell + DI water and verify transmission at 420 and 520 and 620 nm wavelengths = 100%.

Fill cell with wine and measure spectral transmission [%], and convert to absorbance values, sums and ratios as follows:

  • A420-nm and A520-nm and A620-nm
  • Absorbance at A420-nm + A520-nm
  • A420-nm/520-nm

These spectral points ar used in the calculation of tradition metrics based on the Sudraud Method.

Red Wine Colour Intensity (also called Red Wine Density) = A420-nm + A520-nm

Red Wine Colour Tone (also called Red Wine Hue and Red Wine Tint) = (A420-nm / A520-nm)

where: Absorbance at 520 nm will be smaller as the red wine ages and oxidizes.

Typical transmission cell path length for these metrics is 10 mm.

Some academic food scientists use L*, a*, b*, C*, h D65/10 to measure wine color when exposed to different processes.

Large companies can measure lot-to-lot production differences in L*, a*, b*, dL*, da*, db*, dE*, dEcmc D65/10 as well as Haze% to measure scattering. [1]

  • CIE L*, a*, b* D65/10 (Of all illuminants, D65 shows purple values in wine the best)
  • C* chroma indicative of the amount of color
  • h hue angle indicative of a hue shift
  • dE* total color difference relative to seasonal average or fixed standard color

FAQ: “Can you analyze the color of turbid or cloudy wines?”

The scattering of light by internal scattering centers can interfere with accurate color measurement. Sphere-based colorimetric instruments are robust in measuring the color of wines that are transparent in color but may exhibit some slight scattering.

Particulates in turbid wine samples- such as fermenting wines can be centrifuged or if the sample (some juices) are very turbid, they may require filtration after centrifuging. [2]

If quantifying slight scattering in a nominally transparent wine is a quality concern, this can be measured as %Haze or correlated NTUs on a sphere colorimetric instrument.

FAQ: “There has been reference to using an NIR instrument to measure the concentration of anthocyanin which apparently can be measured in the NIR but visually shows up as the degree of redness or “color” in wine and grape juice[3]

See Red Wine Colour Intensity and Tone.

FAQ – “Are there any visual standards for wine that can be used to check a colorimetric spectrophotometer?”

There are no industry standards for wine. The only suggestion I can make is to purchase glass filters from a glass supplier with similar L, a, b values to wine. Verifying how stable these filters would be over time would be a question. [4]

FAQ: “What is the difference between the 420:520 ratio measurement of hue and CIE hue angle?”

The 420:520 hue ratio is based on two wavelengths: the 420 nm for indicating yellow and the 520 nm for red. In contrast, the CIE hue angle is based on the entire wine spectra. Hue angle is more descriptive of color nuance and, as a result, is more sensitive to subtle changes in wine chemistry and a more accurate representation of color present. What’s more, the CIE hue angle can describe any color. The 420:520 ratio is only appropriate for red wines. [5]

FAQ: ”Can you compare the same wine at two different dates?”

Yes, wine color information can be stored in a database, then retrieved for comparison against itself at different points in its life cycle. The key lies in the goal and the timing of the initial sample. For color development at the winery, a good time to sample is when the wines are going to barrel. For tracking changes in color after shipment, samples should be taken at bottling. [6]

Commercial Testing Lab for Wine Color

ETS Laboratories,

St. Helena, CA USA

 www.etslabs.com

References that cite Wine Color

Pérez-Caballero, V., F. Ayala, J.R. Echávarri, and A.I. Negueruela. 2003. Proposal for a new standard OIV method for determination of chromatic characteristics of wine. Am. J. Enol. Vitic. 54:59-62.

There are some OIV methods in the Compendium of International Methods of Analysis of Wine and Musts – search on “wine color” at www.oiv.int .

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