Issue link: http://read.dmtmag.com/i/197178
epicatechin-gallate and epigallocatechin. Wine phenolic compounds can also originate from microbial and oak sources, and include hydrolysable tannins from wood, such as gallotannins and ellagitannins. Proanthocyanidin molecular structure in grapes can be highly variable, with rings, or subunits, of flavan-3-ol compounds that are quite reactive and variable in their degree of polymerization that can combine in different sequences and proportions. In comparing proanthocyanidin structure with red wine sensory properties, the following relationships are found: proanthocyanidin concentration relates to astringency magnitude, and proanthocyanidin structure relates to color stability and astringency perception. Kennedy listed key areas of interest regarding tannin research and analyses, and recent progress made: + Concentration in Grapes and Wine: This has a direct relationship with astringency magnitude. Advances in analytical methods can rapidly provide quantitative information using ultraviolet-visible spectroscopy (UV-vis) absorbance data. The Harbertson-Adams Assay, which uses protein precipitation, is one analysis available to winery labs. Because tannin also binds with polysaccharides, a methylcellulose precipitation analysis is also commonly used. Kennedy said the most significant recent developments are web-based analytical tools, such as Wine XRay, and the Australian Wine Research Institute's Tannin Portal, which enable comparisons of tannin concentrations in different wines, from different vintages and regions. + Size Distribution: Tannin size distribution also influences astringency perception. Analytical techniques include gel chromatography, and recent advances in high-performance liquid chromatography and mass spectrometry (MS) have improved the ability to provide tannin size distribution information. Jim Kennedy of California State University, Fresno, says the challenge of analyzing tannins is that the process requires several different tests. + Composition: Research on the composition of tannins in wines has made considerable progress AT A GLANCE + Grape tannin content can be influenced in the vineyard by managing vine vigor. + Key areas requiring better knowledge and analyses are tannin concentration, size distribution, composition and interaction. + The amount of tannin in grapes is significantly higher than the resulting amount of tannin extracted into wine. + The amount of hydrolysable tannin available to extract into wine from oak cooperage is influenced by oak toast level, and variability in the toasting process. w w w. v w m m e d i a.com N ov - D ec 2013 | V INE YA RD & W INE RY M A N A G EM EN T 67