Spectral evaluation of total phenolic components in Vitis vinifera: Grapes and wines

Whereas high levels of flavonoid extractives are responsible for the characteristic λDmax at about 280 nm in the ultra‐violet spectra of red wines, there is wide variability in the spectral features of natural white wines, in which the phenolic content is often minimal. For the latter, λDmax may occur anywhere within the range 265–285nm, with relatively strong absorbance at 320 nm arising from the presence of hydroxycinnamate esters. Massive fining of many white wines and juices with polyvinylpyrrolidone (10% w/v) has shown residual non‐phenolic u.v. absorbing constituents to be sufficiently uniform to enable direct spectral measurement of total hydroxycinnamates in white juices, white wines and rosés as (E320‐1.4) absorbance units. For white grape juices and white wines, total hydroxycinnamates are directly quantitated as mg litre−1 ‘caffeic acid equivalents’ and total flavonoid extractives may also be estimated. Total phenolic components of red wines are accounted as (E280‐4) absorbance units. The spectral interpretations enable comparative estimation of flavonoid extractives in all natural wine types.