Tannins are found throughout the plant world, and at least one of their properties has been known for some time. The word ‘tannin’ is derived from the process of using plant extracts to cure leather (tanning). This highlights one of the principal chemical aspects of tannins – they are highly reactive with proteins. Tannins play an important role in both grapes and wines. In wine, the perception of astringency on the palate is attributed to tannins. In your mouth they bind with salivary proteins and cause the proteins to precipitate. The end result is that your mouth will lack the lubrication that saliva provides. Thus, astringency caused by tannins are very much a tactile sensation in your mouth. This is why we often will describe the sensation of tannins as silky or rough. The British learned long-ago that a splash of milk in their black tea can make it more palatable. This works because instead of reacting with the salivary proteins in the mouth, the tannin extracted from the tea leaves reacts with milk protein (casein), resulting in a beverage that is less astringent. The same thing occurs when one consumes red wine with cheese. The proteins in the cheese react with the tannins in the wine, making the wine seem less harsh. Tannins are what make drinking red wine with a high-protein food like steak such an enjoyable experience.
In nature, one role they play is a protective role against predation. The puckering sensation one feels from eating unripe fruit is usually the result of under ripe tannins. In grapes, the early formation of tannins coupled with the high acidity in the green grapes causes them to be unpalatable to birds and other animals until the seeds within the grape are ripe. The color change, sugar accumulation, diminishing acidity and astringency all work to make the berries more enticing so that animals will eat the fruit and help distribute the seeds far and wide!
In wine, tannins undergo several different physical and chemical changes and are important factors in the mouth-feel and astringency of a wine, its color, as well as its aging potential. However, the purpose of this post is to not discuss the properties of tannins in detail (which would take an entire post in itself), but to discuss different methods that we trialled this past year to try to enhance tannin concentration in Marquette wine. While Marquette does have some perceptible tannin on the palate, the overall concentration is quite low – much less than even Pinot Noir (which is one of the few red wines that can be enjoyed with fish due to its low tannin concentration). While this isn’t necessarily a bad thing, an increase in tannin can help add structure to an otherwise thin and/or weak wine. It may also help to make the wine more stable (ie: age-able) over the long-term.
Tannin essentially exists in the skin, seeds, and stems of grapes. Fermenting wine with oak chips or aging wine in oak barrels can also be a source for tannins in wine. There are also a number of different commercial tannin additives that winemakers can use that contain tannins derived from grapes, oak, and/or exotic woods.
During the 2011 harvest, I was more interested in determining how different vinification techniques might enhance the tannin concentration in Marquette wines without any addition of commercial tannins or oak. In summary, I used three different techniques: 1) Fermentation with 50% whole clusters, 2) Saignee – I removed 20% of the juice before fermentation started, 3) I froze the grapes prior to fermentation (I would have preferred to try a heat treatment, but it seems like our industry is far from investing in thermovinification or flash-detente systems, and I couldn’t simulate it in our winery anyway. It seemed like freezing was a technique relative to the current state of our industry).
Here is a graph displaying the difference in the tannin concentration of the 3 different fermentations (Quantified using HPLC):
The only technique that really had an impact on the tannin concentration was the 50% whole cluster fermentation. None of the different methods showed a difference in anthocyanin (colored pigments) concentration. We saw similar results with our tannin trials in 2010.
Using whole clusters when fermenting wine is a fairly common practice in certain cultivars, such as Pinot Noir and Grenache, with low quantities of phenolics (tannins and anthocyanins). Besides increasing the tannin content of the finished wines, using whole clusters when fermenting can benefit the wine in other ways as well.
One caveat to using this technique is that the stems must NOT BE GREEN. Green stems will release compounds that will taste green and herbaceous. Ripe clusters with brown, lignified stems are what you’re looking for. The grape clusters should be placed whole into the fermentor without crushing.
Grape stems have the ability adsorb/absorb certain compounds, as well as releasing other components into the wine over the course of fermentation. They will release tannins, but may also bind some of the colored compounds of your wine, which can result in a wine that has a lower color intensity. However, whole cluster fermentations can also help to aerate the cap as well as provide channels for wine to traverse during pumpovers. Oxygen helps bridge tannin-anthocyanin complexes, which can stabilize the color of the wine, while the channels provided the stems helps facilitate extraction of anthocyanins from the skins. Both will result in a wine with an intensified color. Another benefit of this increase in oxygen is that the temperature of the cap is easier to manage, and fermentation time is shorter.
Nonetheless, perhaps the most important thing to consider when deciding whether or not to employ this technique (besides making sure stems are not green) is the effect it can have on the total acidity and pH of your finished wine. Generally, wines fermented with whole clusters will see a decrease in total acidity, and an increase in pH. In years when high pH is already a concern, you may not want to use this technique unless you are prepared to acidify your wine.
All-in-all the tannin concentration in Marquette is very low – so low that we have had difficulties quantifying it using classic methods. So far, it seems as though whole cluster fermentation could be an option for increasing the structure in Marquette wines. This coming vintage, we will continue to trial this method. In the next few years we will learn much more about the the types and location of the tannins in Marquette, as well as how they develop over the course of ripening thanks in part to work in the Northern Grapes Project. This will help guide recommendations for winemaking protocols over the next few years.
I would like to give a special thank you to Dave Manns, a post-doctoral research fellow at Cornell University who assisted in HPLC analysis of this tannin trial.
Hashizume, K. and Samuta, T. 1997. Green Odorants of Grape Cluster Stem and Their Ability To Cause a Wine Stemmy Flavor. J. Agric. Food Chem. 45, 1333-1337