August 2015 Issue of Wines & Vines
 
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When Plant Species Matter

From hybrids to vinifera, all is not equal in the world of grape tannins

 
by Anna Katharine Mansfield
 
 
Lauren Thomas
 
Cornell University graduate student Lauren Thomas separates polyphenolic fractions in preparation for HPLC analysis of tannins and anthrocyanins.

I’ve worked closely with grape breeders for almost 15 years, so I know the thoughtful pause and faraway, dreamy look they get in their eyes when they pronounce their highest praise for a promising grape selection. “This one,” they say with a nod and a faint smile, “this one is really good. It’s so…vinifera-like!”

Of course, they really mean that the potential hybrid has growth habit and fruit chemistry similar to Vitis vinifera, but that all the desired hybrid traits—disease, pest and temperature resistance—are intact. This is the way we’ve thought about grape breeding for years: as a sequential culling designed to combine the positive production and sensory characteristics from V. vinifera ancestors with the hardiness of other, non-wine-worthy species. If the vines grow like vinifera, and the grapes hit the same levels of sugar, acid and phenolic compounds as their illustrious ancestors, all the winemakers would have to do is apply the same production methods to a hardy red hybrid that they would to a Cabernet Sauvignon, and we’d be making world-famous wines in new and previously unknown regions, right? Basically, we think that if it looks like vinifera and smells/tastes like vinifera, it must act like a vinifera in the winery.

The problem is this: It doesn’t. Hybrid grapes usually don’t shine when processed like their European cousins, and issues of tannin and color are a case in point.

    KEY POINTS
     

     
  • Although red hybrid grapes may have the growth habit, fruit chemistry and sensory characteristics of vinifera grapes, traditional vinifera winemaking techniques do not give those wines good tannin or color.
     
  • Research has found no correlation between berry tannin and wine tannin. It is possible that the problem lies not with the method of extracting tannins, but the binding effect of protein compounds naturally occurring in the grapes.
     
  • Early tannin additions can have limited effect on color stability in red hybrid wines. The solution may be to add tannins late in the winemaking process, when pressing and rackings have removed more of the tannin-binding proteins.

The hit-or-miss of hybrid winemaking
Over the decades winemakers have devised a multitude of ways to extract the polyphenolic compounds that give red wines their tannin backbone and distinctive color, from cold-soak to extended maceration to more complicated techniques like délestage. The effects of many of these methods have been studied in a handful of classic winemaking cultivars, but studies on hybrid red grapes are relatively rare. Subsequently, most winemakers have to guess the right way to make Maréchal Foch or Frontenac or Petite Pearl, trying to combine their knowledge of specific fruit chemistry with the traditional understanding of red wine production.

In 2010, the Cornell Enology Extension Lab (CEEL) performed a survey of hybrid red wine producers in the Northeast and Upper Midwest, and (not surprisingly) found no agreement on preferred tannin-extraction methods. In the same survey, winemakers ranked tannin and color as their top two concerns with wines made from red hybrid grapes (Coquard Lenerz, 2012). The results were illuminating: Despite using every extraction method in the book, no one was seeing the tannins and color that they wanted. CEEL followed this up with a study evaluating the tannin extraction and retention in red hybrids with various red wine processing methods and found little to no difference in final wine tannin concentration (Manns et al., 2013). In both the real world and the lab, processing hybrids like vinifera simply wasn’t working well.

The reason? Because hybrids don’t have vinifera-like phenolic chemistry. No matter how well a grape hides its hybrid nature with upright growth habit and balanced harvest chemistry, its tannins and anthocyanins are going to give it away. The problem has been that, like any good agent of disguise, science has just been one step behind the masquerade…until now!

Tannins
All grapes—vinifera and hybrid—produce tannins in their skins and seeds. In absolute numbers, the concentration of tannins in seeds is much higher than in skins, but only a fraction can be extracted without strong chemicals and physical manipulation. Subsequently, most of the tannins found in wine come from grape skins.

All grape-derived or “condensed” tannins are long chains of smaller phenolic subunits, or monomers, called flavan-3-ols. There are specific types, combinations and ratios of monomers found in wine, and individual tannin polymers vary by types of initiator units (the flavan-3-ol subunit that starts the polymer chain), elongator units (the subunits that build on to the initiators) and the ways that the polymers branch (or don’t). One frustrating thing about these condensed tannins is that there’s no good way to identify and quantify them. At present, all we can do is run some chemical reactions that cut the chains into little pieces, tag the ends of the pieces, and then measure their average size. This analysis gives us an idea of the mean degree of polymerization (or mDP) of all the tannins in a wine, but it doesn’t tell us the concentration of any given size of tannin.

Why hybrids are different
Hybrid red wines tend to have low tannin, and the tannins that they do have are generally smaller (less than 4 mDP) than those found in red V. vinifera wines (7-13 mDP). Since polymers with an mDP less than 5 are usually more bitter than astringent, the sensory impact of these small-chain tannins is likely to add unpleasant sensory characteristics to the wine.

Wine tannin ≠ grape tannin
The lower tannin concentration in red wines led to a long-held belief that hybrid grapes also had low tannin concentration, because if the tannins were there, they would eventually be extracted with extended maceration, right? In fact, analysis shows that while many hybrid grapes may have lower tannin concentration than V. vinifera cultivars traditionally responsible for “big” red wines (e.g., Cabernet Sauvignon), there’s no correlation between berry tannin and wine tannin (Springer & Sacks, 2014). To work around this difficulty, Springer and Sacks propose that cultivars be evaluated in terms of tannin extractability using the following equation: 

(wine tannin/grape tannin) X 100

When tannin concentrations are plugged into this equation, the data makes a bit more sense. Cabernet Sauvignon, for instance, has very high tannin extractability, and almost all V. vinifera have higher tannin extractability than hybrid cultivars.

All bound up; no place to go
The question is: What dictates tannin extractability? If there’s not a direct path from grape tannin and wine tannin, understanding the factors that govern the movement of tannin from grape to wine can make a big difference in winemaking choices. Recent research suggests that the biggest factor isn’t extraction method but rather the re-uptake of tannin by compounds that occur naturally in the grape. Pectin plays a small part, but proteins—and likely the same ones that provide disease resistance—seem to have the largest binding effect. If this is found to be true, it suggests a big problem: that the same attribute that allows hybrids to thrive in challenging environments inherently handicaps the winemaker.

The solution seems easy: just add tannin! As it turns out, just adding more tannin isn’t enough. If the binding compounds are still in the wine, any tannin that’s added will just be sucked right back out. In order to make good red hybrid wine, then, there are two challenges: First: Tannin has to be added to the wine, and second: Sorption activity has to be avoided or defeated so that the tannin stays in the wine.

How much tannin to add?
Tannin vendors have always talked about “sacrificial tannins,” or the percentage of any tannin addition that will be lost during wine processing. Empirical data from winemakers suggests that additions to hybrid wine fermentations should be anywhere from two to four times higher than the manufacturer’s recommended addition, which is geared for V. vinifera wine production.

In a study with Maréchal Foch, Corot Noir and Cabernet Franc (used as a representative V. vinifera control), a condensed tannin product was added at crush at the recommended rate and at two and three times the recommended rate. Results showed that Cabernet Franc retained more tannin at all levels than did Corot Noir or Maréchal Foch, with the latter only retaining half as much as Cabernet Franc for the same addition concentration.

Further, a sensory panel showed no sensory difference in any of the wines, despite the vast range of addition, which suggests that higher additions can be made without contributing unpleasant flavor compounds. Appropriate addition rates and sensory impact will vary by grape cultivar and tannin product, however, so bench trials are always a good idea.

And when to add it?
Traditional wisdom dictates that winemakers apply all additions as early in the process as possible to allow wine components to “integrate.” While this seems to make sense, in the case of tannin sorption, it may run contrary to best practices. If the proteins that bind and remove tannin come from the grape, tannin sorption should decrease as more grape material is removed from the wine. In other words, sorption activity is highest at crush and will decrease as pressing and subsequent rackings remove grape material. A series of studies at CEEL has shown this to be true, and that later tannin additions will result in a greater percentage of tannin retained. Sensory studies are planned to see how addition time impacts wine aroma.

But what about color?
In addition to integration, winemakers often voice concern about the formation of polymeric pigments, the anthocyanin-tannin complexes that are thought to provide stable color during wine aging. Since anthocyanins are at their highest concentration during the first few days of on-skin fermentation, one would deduce that tannin additions made early in the process would interact with more anthocyanins, leading to the most stable color.
This may be true in V. vinifera, but we’re unsure how stable color formation occurs in red hybrids. In addition to having different tannin compounds, most hybrid grapes have different anthocyanin compounds, and their binding affinities and mechanisms are unknown. Until more research has been done, it’s impossible to know whether early tannin additions will have any effect on stable color formation in red hybrid wines.

Know your tannin additive
Exogenous tannin additives are not created equal but fall into roughly three categories: grape-derived condensed tannins, plant (but non-grape)-derived condensed or hydrolysable tannins, and any mixture of the two. Hydrolysable tannins are commonly extracted from oak, acacia or the South American Quebracho tree.

Producing high-purity tannin extracts is very difficult, so exogenous tannin products range from 10% to 45% tannin, with the remainder consisting of inseparable non-tannin phenols, drying and solubility agents, and non-phenolic plant material. This residual plant material may have unknown sensory characteristics, and the tannin:non-tannin ratio will vary from batch to batch. Consequently, it’s important to run bench trials with any tannin product.

Treat your hybrids like hybrids
While the complexity of polyphenolic chemistry means that we have a long way to go before we find a foolproof method of boosting tannin in red hybrid wines, there are some general rules taking shape:

 

  1. Red hybrid grapes have fewer and smaller tannins that are harder to extract and retain than do red V. vinifera grapes.
  2. Add tannins as late as possible in the winemaking process to boost retention rate.
  3. Double or triple the rate of addition, but with caution.
  4. Understand the contents of the tannin you’re using, and always perform a bench trial.
  5. Don’t bank on early tannin additions stabilizing hybrid wine color.

The larger lesson, though, may be this: No matter how vinifera-like a hybrid cultivar may seem, there are a multitude of chemical differences that we can’t easily measure, but that still have to be accommodated during winemaking. Instead of looking to traditional red winemaking practices for the answer, it’s time to consider hybrid red production as a different ball game, and play with different rules. 


Anna Katharine Mansfield is an associate professor of enology at Cornell’s NYSAES in Geneva, N.Y. Her research focuses on hybrid wine phenolics and fermentation nutrition. This article is drawn from a webinar entitled “Building the Perfect Body: Tannin Strategies for Red Hybrid Wines,” which was presented March 10, 2015, by the Northern Grapes Project.

For more information


Coquard Lenerz, C.T. 2012. Phenolic Extraction from Red Hybrid Wine Grapes. Cornell University MS Thesis.

Manns, D.C.; C.T. Coquard Lenerz; A.K. Mansfield. 2013. “Impact of processing parameters on the phenolic profile of wines produced from hybrid red grapes Maréchal Foch, Corot Noir, and Marquette.” Journal of Food Science, 78:C696−C702.

Mansfield, A. K. 2013. “When Species Matters: All Is Not Equal in the World of Wine Tannins.” The Northern Grapes Project. http://northerngrapesproject.org/wp-content/uploads/2015/05/NG-News-Vol4-I2-May-2015.pdf

Springer, L.F., and G.L. Sacks. 2014. “Protein-Precipitable Tannin in Wines from Vitis vinifera and Interspecific Hybrid Grapes (Vitis ssp.): Differences in Concentration, Extractability, and Cell Wall Binding.” Journal of Agricultural and Food Chemistry, 62(30):7515-7523.

 

 
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