Issue link: http://read.dmtmag.com/i/197178
less labor. Skin tannin extraction can be quick to plateau, whereas seed extraction is slow to plateau. Another issue is that extraction does not equal retention, because tannins will be lost. Using saignee (juice runoff and separation pre-fermentation) can concentrate and enhance tannins and anthocyanins, although its impact may be influenced by berry size. Harbertson said extended maceration extracts more tannin, but primarily smaller-sized polymers, to make wines more astringent and bitter. Extended maceration tends to extract more seed tannins. Research to date indicates that the amount of tannin extracted during the maceration is independent of grape maturity and season. Using saignee in combination with extracted maceration can be more effective for overall tannin extraction. 70 V I N E YA R D & WIN E RY MANAGEM ENT | Ethanol content can affect tannin extraction from seeds, as ethanol degrades the outer seed layer. The amount of ethanol may influence sensory character. Catechins may seem more bitter with higher ethanol content, and higher alcohol can diminish fruit aroma. OAK AND EXOGENOUS TANNINS Recent research to evaluate hydrolysable tannins in oak cooperage and their potential impact on tannin levels in wine was discussed by Dr. Tom Collins, director of research for the UC Davis Food Safety and Measurement Facility. Much of this research was performed by Collins, formerly of Treasury Wine Estates, as a cooperative project between Treasury and UC Davis. Tannins in oak wood are primarily hydrolysable, composed of gallic acid esters with glucose. They are primarily ellagitannins, with the most common being castalagin and vescalagin. Collins said ellagitannin content in oak can vary based on the geographic origin of the tree, its species, individual trees in a forest stand, and the location on the tree from which a barrel stave is milled. Seasoning of oak wood prior to coopering, and toasting of oak during coopering, can impact ellagitannin content. "Air seasoning generally decreases ellagitannin levels," Collins said. Toasting can have significant effects on tannin levels, in part due to variability inherent in the toasting process. Collins' research evaluated variability resulting from temperature differences by toasting using an open flame from a firepot at a commercial cooperage, by embedding wireless temperature sensors in wood staves. Samples of toasted wood near these sensors were then chemically analyzed using gas chromatography. "At the extreme, there can be a 50°C temperature difference between the ends of a stave from top to bottom over the toasting fire," he said. There can be a 72%-99% loss of ellagitannins during toasting, he added, and generally, lighter toasts Nov - Dec 2013 resulted in less ellagitannin loss. Toasting also reduces castalagin levels. "We are seeing losses in toasted wood versus untoasted wood," Collins noted, "and a lot of variability in the toasted wood based on the position on the stave." Exogenous tannins are preparations that can be added to musts or wines during the winemaking process. They include tannins from oak cooperage, commercial tannin products extracted from grape seeds or skins, and tannins extracted from other types of wood such as quebracho, or tree galls. Many exogenous tannins are commercially available, and some preparations include combinations of hydrolysable and condensed tannins. Exogenous tannins can affect astringency and mouthfeel, and they sometimes help clarify wine. Winemakers can add these products pre-fermentation to improve color stability, precipitate grape proteins, act as antioxidants, and to modify "green" character. In a trial at Treasury, several exogenous tannin products were evaluated following their additions early in the pre-fermentation process. The wine was evaluated twice after fermentation using the Harbertson-Adams Assay and through sensory evaluation. "In most cases there was an increase in color and tannin levels in November during the first evaluation," Collins said. "However, there were no significant differences in color and tannin levels when evaluated the second time in May, during cellar aging. There was some increase in astringency." Although future trials must be done to provide more data, Collins suggested that these products may provide better results if they are used at different points in the winemaking process, rather than just during pre-fermentation. Ted Rieger , CSW, is a writer and photographer based in Sacramento, Calif., and has been a contributing editor for V&WM since 1990. Comments? Please e-mail us at feedback@vwmmedia.com. w w w. v wm m e d i a . c o m