Spectral evaluation of young red wines: Anthocyanin equilibria, total phenolics, free and molecular SO2, “chemical age”



Red wine colour is an integration of contributions from monomeric anthocyanins and polymeric pigment forms. Because of gross differences between the two fractions in their responses to pH change and to SO2 addition, approximate measures of the states of anthocyanin equilibria in young red wines can be routinely made. Recent correlations of such measures with independent assessments of relative wine quality have prompted further examination and development of the original analytical concepts. The equilibria may be described in terms of two percentage parameters α and α ', the values of which are related to wine pH and to SO2 added after fermentation. Measures of free SO2 and also molecular SO2, anthocyanin content and total phenolics are obtained from the same set of spectral data. The interpretations are based upon a number of analytical premises for which certain approximations have been necessary. These appear to be well justified in comparative evaluations because of the wide range of parameter values observed in surveys of current vintage wines. Observations of ageing changes in the composition of wine colour, with proposed spectral indices of “chemical age”, are also presented.