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W ine tannin originates from the tannin present in grapes; however the journey from grape tannin to wine tan- nin is not a straightforward one. Grape tannin needs to be extracted from solid grape material into must during fermen- tation, and then undergo chemical rear- rangements to reach its final wine tannin form. This is why it is not a simple task to predict eventual wine tannin by measur- ing grape tannin — something winemak- ers would like to be able to do. Understanding extractability The extractablity step in the process – getting the tannins out of the grape solid material and into must— has been inves- tigated, with a focus on the role played by grape cell walls. 1,2,3,4 A number of factors were discov- ered that influence how easily tannin is extracted: • Skin and seed tannins were found to interact with grape cell wall material, and this interaction limited the amount of tannin extracted during fermentation. • Cell wall material interacted more strongly with seed tannin than skin tan- nin, most likely due to differences in their structure. • Suspended flesh (pulp) material was shown to bind and remove tannin as lees during settling, after the tannin had been extracted during fermentation. • Grapes with a low overall tannin con- centration and grapes with a higher seed- to-skin tannin ratio were more affected by this removal of tannin by pulp. • Seed tannin was shown to be less extractable than skin tannin during fer- mentation, but this varied significantly between seasons. Influence of grape maturity The effects of grape ripening on factors affecting extractability also were investi- gated. 5 Riper grapes were associated with: • increased total skin tannin concentra- tion, which could lead to higher wine • more cell wall porosity which can result in a greater amount of total skin tannin trapped in the pores; • more anthocyanin, which appears to enable extraction of tannin; • more sugar leading to higher ethanol level, which may increase tannin and color extraction. Results from these studies of extract- ability have highlighted the role of cell wall material in influencing tannin con- centration during fermentation. The new understanding gained could lead to the development of methods to predict the extractability of tannin for a particular batch of grapes. WINEMAKING Influence of oxygen on wine tannin and color As soon as grapes are crushed, chemical reactions start taking place that continue through fermentation and élevage and post-bottling maturation that eventu- ally determine the wine's final color and taste. One key example is the reactions that convert grape tannins into more complex wine tannins. Oxygen has long been known to modify the astringency of red wines, but the chemical basis of this observation was unclear. Winemaking experiments were con- ducted to investigate what happened when air was injected into red wine fermentations. Results showed important impacts of air additions on tannin con- centration and chemical structure, with accompanying sensory effects. In par- ticular, air reduced the concentration and AT A GLANCE Recent tannin research has delivered: • Simple measurement of tannin and color in grapes and wine. • Breakthroughs in understanding the extraction of grape tannin during fermentation. • Knowledge about the influence of grape maturity on eventual wine color and tannin. • Practical winemaking techniques that can be used to manipulate tannin. • New understanding of what hap- pens to tannin as wine ages and the importance of tannin structure for sensory properties. • Insights into factors influencing color stability. Studying tannins is challenging. Their behavior is rarely predictable; results can be contradictory, and experiments often seem to generate just as many questions as they answer. But if you step back and take a broader view of the tan- nin research conducted in the past few years, you can see that real progress has been made in understanding tannin at key points across the grape and wine value chain, with practical outcomes for growers and winemakers. Perceived Tannin Colloidal State Direct Oxida on Antho- cyanin Organic Acid Residual Sugar Aroma Compound Poly- saccharide Ethanol Glycerol Oxida on Products Manno- protein Tannin Concentra on Composi on Our percep on of tannin is influenced by many wine components Figure courtesy oF James a. Kennedy Figure 1 Paul Smith, Keren Bindon, Jacqui McRae, Stella Kassara and Dan Johnson, Australian Wine Research Institute BY Impacts and opportunities along the value chain p r a c t i c a l w i n e r y & v i n e ya r d J a n U a r y 2 0 1 5 17 w i n e M A K i n G