Investigations of wine and grape skin tannins from the Okanagan valley

• Main Author: Visintainer, Dawn Michaela
• Language: English
• Published: University of British Columbia 2013
• Online Access: http://hdl.handle.net/2429/45652

A grape skin tannin profile was established throughout berry development in Oliver, Naramata, and Osoyoos, three sites in the Okanagan area of British Columbia, using both 70% acetone and 12% ethanol as extraction solvents. Tannins were analyzed by ultra-high pressure liquid chromatography (UHPLC) to determine how total tannins, mean degree of polymerization (mDP), and percent subunit composition varied. Tannins for each sample did vary by more than 20% with the exception of one sample set. All of the samples’ mDP did not vary by more than 20%, with the exception of two sample sets; most of the percent subunit composition also did not vary by more than 20% with the exception of one sample set. These same samples were then analyzed by methyl cellulose precipitation assay (MCP) to determine tannin content in relation to astringency. It was predicted that as more tannins bonded with polysaccharide over the course of berry development, the astringency would decrease. We found a low correlation between total tannins quantified by the MCP assay and by UHPLC (r²< 0.52), and between total tannins quantified by the MCP assay and mDP calculated via analysis by UHPLC (r²> 0.37). A new gelatin adsorption assay to determine tannin quantity in wines was also developed. This method allows tannins in wine to bind with gelatin. Then, by recording the absorption of phenolic compounds at 280 nm before and after binding, we ascertained a value related to astringency. There was a good correlation between the Gelatin Adsorption Assay and the established MCP assay: r²= 0.99 when using model wine with the addition of grape seed extract, and r²= 0.98 when using real red wine samples. This method will allow for a cheaper way to evaluate tannins in red wines.