micro-oxygenation in contemporary winemaking - Cape Wine ...

Micro-oxygenation in Contemporary WinemakingTable 14: Common Tests for the Analysis of Phenolic Compounds in WineCategory Test Measurement PrinciplesTotal PhenolsAnthocyaninsTanninsWine ColourFolin-CiocalteuTestOD 280[Anthocyanin]PVP IndexIonisation indexHPLC[Tannin]LA MethodHPLCMDPHCl IndexDialysis IndexGelatin IndexColour IntensityColour StabilityHueColourCompositionMeasure total phenols by reaction of the hydroxyl groups with a phosphormolybdatereagent which can be measured spectrophotometrically.The benzene molecule characteristically absorbs at 280nm so the opticaldensity at 280nm is a simple measure of [total phenol]. Various corrections havebeen proposed to provide more accurate estimates.There is no simple test to measure [total anthocyanin]. Changes in OD520 afterHCl addition or SO2 bleaching can be used to estimate [anthocyanin]. Both testsmeasure the sum of [free anthocyanin] + [bound anthocyanin susceptible tobleaching].Measures the proportion of free anthocyanins and anthocyanins bound to tannin(T-A forms). Free anthocyanins bind to a PVP column whereas T-A forms donot.Measures the proportion of coloured and uncoloured anthocyanins. Based onthe difference in effect of SO2 bleaching on wine colour at normal wine pH andat a pH of 1.High performance liquid chromatography can be used to separate and quantifydifferent anthocyanin compounds.Under acid conditions tannins can be hydrolysed to form coloured cyanidins(tannins are also termed procyanidins). The change in colour after addition ofHCl is measured at OD550. Various corrections have been proposed to providemore accurate estimates.High performance liquid chromatography can be used to separate and quantifydifferent tannin compounds.The mean degree of polymerisation (MDP) indicates the average chain length ofthe polymeric tannins. Measured by acid depolymerisation in the presence of anucleophilic agent such as toluene-α-thiol (thiolysis) to identify terminal units.Measures the proportion of large polymeric forms. Based on the hydrolysis oftannins in an acid medium where speed of precipitation depends on degree ofpolymerisation. Values range from 5-40. HCl index > 25 indicates highlypolymerised tannins.Measures the proportion of tannins that do not pass through a dialysismembrane under standard operating conditions. Depends on size and charge oftannin molecules. Values range from 5-30.Measures the reactivity of wine tannins with gelatin. Used as an indication ofastringency. Values range from 25-80.Total amount of colour in wine. Sum of optical densities at 420nm (yellow),520nm (red) and 620nm (blue).CI = OD420 + OD520 + OD620Measures change in colour intensity after SO2 bleaching.Development of colour towards orange.Hue = OD420 / OD520Each of three colour components as a proportion of total colour intensitySource: (Ribéreau-Gayon and Glories, 1987; de Beer et al., 2004; Ribéreau-Gayon et al., 2006c)55