Erin Norton
A recent publication in the American Journal of Enology and Viticulture from the Mansfield group at Cornell University details some important results dealing with tannin and some of the current issues we see in winemaking with our cold-hardy hybrids.
The overall goal of the study was to investigate the addition of exogenous tannin to must and the subsequent retention of those tannins in the final wines.
Before explaining the experimental details of the winemaking, it’s important to highlight some of the initial experiments they did to determine the actual tannin concentration in commercial grape tannin products available. In the publication none of the products were identified. The researchers measured the amount of tannin in 1g/L of powdered product. What they found is presented in Table 1. These results emphasize that tannin products are not pure, and contain many other types of compounds that were extracted from grape skin or seeds depending on the product. Winemakers need to be aware of this “extra” fraction of compounds that is being incorporated into their wine. It could have sensory effects in aroma/flavor or taste. These effects may not be negative, but winemakers should be aware of the additional sensory changes besides just structure. I did a quick search, on grape tannin products, and no companies are detailing the purity of their product.
That leads to the real experiment that the researchers did. Working with Maréchal Foch (French-American hybrid), Corot Noir (Cornell-Geneva 
hybrid) and Cabernet Franc (Vitis vinifera) tannin product was added to the must at 3 different addition rates: 400mg/L, 800mg/L or 1200mg/L. It should be noted that the normal maximum recommended commercial dose to red must (based on my quick search results) ranges from 100mg/L to 400mg/L depending on the product. Therefore the researchers were adding a significant excess to the must. The retention of the tannin in the final wines is illustrated in Figure 2. Visually it can be seen that Maréchal Foch had much less retention of the added tannins than the Corot Noir or the Cabernet Franc. One conclusion from this is that hybrid cultivars do behave differently and should probably be evaluated individually. Maréchal Foch is also much more cold-hard than Corot Noir, and this result of low tannin retention in a more cold-hardy cultivar is supported through other works from the Sacks group at Cornell, the Pedneault group in Nova Scotia and here at ISU.
Overall conclusions made by the researchers are:
- It is difficult to predict the amount of tannin retention when adding an exogenous tannin product to must.
- It seems like tannin additions to red hybrid must should be made in much higher rates than for V. vinifera cultivars (which is where the recommended doses come from).
- More research is needed to understand tannin-binding activity in red hybrid cultivars and how to enhance retention.