Friday, March 28, 2003

Hansa yellow light:

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Red wine is often assessed by color. The deep red hues of most red wines are modified by apparent color shifts. New wine seems to have blue, purple or violet tinges; aged wine gives an orange, yellow or brown impression. Wine tasters use a variety of techniques to observe the color of a wine, including looking directly through the wine at a light source, and by the color of a shadow of a thin portion of the wine. These techniques are designed to be done in a tasting setting using no more than a tasting glass, a light bulb and a white tablecloth. Given a spectrophotometer however, a more analytical observation can be made of the color of the wine. The chart above is a spectrogram made from a sample of 2002 Black Swan Shiraz, an inexpensive wine from southeastern Australia. A sample was examined at the wavelengths shown in the chart, and the graph drawn from those data. Additionaly, the shorter wavelengths were examined in diluted samples in order to extract any small spectral features. Although a small transmission peak was found between 450 and 500 nm, the main feature is a change from very low to very high transmission that begins at 600 nm and reaches 83 percent at 700 nm. The curve has an inflection at 648 nm. The blue green deviation is too slight to effect the color of the wine, so the color must be determined by the position and shape of the main transmission shift. It will be worth while to examine other wines with different perceptual colors spectrophotometrically. The shapes of the spectra may indicate some aspect of the wine that could be tested by some other means, such as taste. (Continued ...)

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