Tannin Structure-Activity Relation to Red Wine Astringency

Optimizing wine quality with regard to mouth feel is a quest for wineries that are trying to fine tune the astringency of their wines to a desirable level. Consistent with that, various wineries had agreed and participated in the extended maceration project in order to understand the driving force behind astringency. With that in mind, this project was born to comprise different analytical techniques, which are necessary to construct a conclusive understanding of tannin’s activity. During the 2014 vintage, maceration trials were conducted in five Napa Valley wineries. Winery selection was based upon winemaker interest. For each winery, fermentations were set up according the the specific cooperators protocol. At various times during the course of fermentation/maceration, samples were collected following pumpover operations. Following collection, samples were transported and stored at 7 degrees Centigrade until processed. For tannin isolation, each sample was filtered using a 9.0cm Whatman filter paper (20-25 μm pore size), diluted with milli-Q water (50:50 v/v) and then loaded onto a preconditioned column containing Toyopearl chromatography resin (HW 40C). Purification of tannins was conducted according to Aron et al.1 Briefly, and following application to the column, the column was washed with water followed by 50{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} v/v aqueous methanol and then tannins were eluted with 66{aed9a53339cdfc54d53cc0c4af03c96668ab007d9c364a7466e3349a91bf0a23} v/v acetone in water. The acetone was removed by rotary evaporation and the aqueous portion containing the tannin was lyophilized to a powder. Gravimetric yields were determined and the tannin powders were stored in glass, air tight, vials after being sparged with N2 and stored under -20°C. Tannin isolates underwent analysis using a variety of analytical techniques including gel permeation chromatography, phloroglucinolysis and stickiness. The analysis of tannins’ stickiness has recently been introduced by Barak et al.2 and further developed by Revelette et al.3. Samples were prepared according to the above methods and the enthalpy of interaction was determined for tannin polymers absorbing at 280 and 520 nm. Secondly, tannin powders underwent acid catalyzed degradation in the presence of excess phloroglucinol (phloroglucinolysis) to determine the subunit composition, average degree of polymerization and conversion yield following the method of Kennedy and Jones.

Following phloroglucinolysis, tannins were analyzed by gel permeation chromatography which provided information on size distribution5. Across all experiments, a total of 104 tannin isolates were prepared. Given that these isolates represent tannins collected at various points during fermentation and maceration and across different blocks and varieties, it is believed that a realistic picture of commercial structure variation for Napa Valley has been achieved. Results to Date are as follows: Winery 1 investigated the effect of extended maceration on tannin composition and activity. One Cabernet sauvignon block was harvested and equal portions of fruit were contained in two identical fermentation vessels. The must underwent a prefermentation soak for four days before inoculation. Samples for the control tank were collected on a semi-daily basis until press day (total soak time was 15 days). On the last day (day 15), the first extended maceration sample was collected. Similarly, extended maceration samples were taken on a semi-daily basis until press day (total soak time was 26 days).