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This is the second part of the story about tannins in hybrid grapes and wine. To read the first installment click here.
Cue Tannin Binding Now
For tannin extractability, the opposite side of the coin is tannin binding. Greater tannin binding means less extractability. So to help keep the two concepts straight, greater tannin extraction is good for our wines as more grape tannin makes it into the wine. While greater tannin binding means more tannin is lost and less grape tannin is extracted into wine.
Researchers methodically ruled out differences in skin, flesh or grape weights were solely responsible for differences in tannin extraction. But they found a significant positive, correlation between skin tannins and final wine tannins. Meaning that in one way or another the grape skins are the driving force behind tannin extraction in wine. Which begs the question, what is the driving force behind tannin binding?
You might agree that the most likely of suspects was pectin, often thought of as the “glue” that holds the cell wall together. And to some extent grape skin cell wall pectin was loosely correlated to tannin binding. Specifically FA hybrid skins harbor the greatest concentration of pectins, leading to stronger tannin binding.
However, pectin didn’t have the strongest correlation with tannin binding. It turns out that flesh (or pulp) cell walls were the best tannin binders! Just to clarify, cell walls are not just found in the skins of grapes. There are cells in every part of the berry: skins, flesh, seeds, etc. and as such there are cell walls in different parts of the berry.1
And as one would expect, FA hybrid flesh cell walls bind grape tannin the best, while V. vinifera bind tannin the least. And consequently this finding explains why tannin extraction rates are greater in V. vinifera than FA hybrids. The Neo hybrids as a class are largely variable because the sample size was only two, with drastic differences in tannin binding. Noiret flesh cell walls had relatively low tannin binding, whereas Corot Noir had the greatest tannin binding across all varieties.1
So what exactly are these almost mythical-like binders of grape tannin?
It was the flesh cell wall protein that gave the grape its strong tannin binding capacity. Meaning, that a certain class of flesh cell wall proteins (yet to be identified) are likely responsible for all of our wine tannin extraction trouble (oops did I say that out loud?). And you would be right to guess that this exact topic is an area of active research.
So despite all the suspicion, when it comes to tannin binding, it appears as though skin cell wall pectin takes a back seat to flesh cell wall protein that is in the driver seat.
Just Fine and Dandy, but what can I do about it?
In a previous MWP tannin article, we covered how the addition of exogenous tannins can help boost the structure and mouthfeel of dry hybrid red wine. New research has further defined the timing of such additions in order to maximize tannin retention in wine.3
The reason for adding tannins at this stage are not only related to mouthfeel characteristics but also to help promote color stability and to help protect endogenous grape tannins by providing a sacrificial tannin3.
The theory behind sacrificial tannin during alcoholic fermentation is that the first tannins extracted from the grape are the “soft,” native skin tannins whereas the less coveted seed tannin occurs later during the alcoholic production phase. The result of this sequential extraction is that the “prized” native skin tannins are subject to “natural” tannin binding mechanisms, by default leaving a larger percentage of seed tannin extracted into the wine. Therefore if you provide exogenous (fermentation) tannin early on during maceration, it will become bound up first (by sacrifice), thereby protecting the fermentations “soft” skin tannins. 3
However, think about this concept from a big picture standpoint. If the hybrid grape solids are rich in tannin binders, from a wine augmentation standpoint, would it make the most sense to add exogenous tannins at crush?
To test this theory, Dr. Mansfield’s lab researched the topic of adding fermentation tannins during crush. The study looked at effects of adding three fermentation tannin additions (400, 800 and 1200 ppm) in Marechal Foch, Corot Noir, and Cabernet Franc wines. Not surprisingly, M. Foch had the least amount of retained tannin followed by Corot noir and then Cabernet Franc.2
What is of great surprise is how little tannin is actually retained in the wine after a tannin addition at crush, blowing apart our sacrificial tannin logic (sad trombone). This was in part due to the finding that commercial tannin products are at best 50% pure tannin! And this isn’t because manufacturers are purposely trying to take advantage of the consumer. This failure in purification is more a function of not having great, food safe methods of extracting tannin products from their original sources. Also to note, significant lot-to-lot variation has been measured within a given lineup of commercial tannins.
For example, in Marechal Foch with a 400 ppm addition, at best only 200 ppm of that addition was pure tannin. And wait for it, wait…only a 1/10th of the pure tannin added was retained (~20 ppm) in the wine. Retention in Corot Noir wine was slightly better but nothing to write home about either.2
In which case, for hybrid grapes the sacrificial tannin theory takes on a whole new meaning. Forget the sacrifice, those poor tannin souls don’t stand a chance; they’re headed straight to the slaughterhouse.
Furthermore, in the follow-up sensory work on the aforementioned tannin addition experiment, there was little to no significant differences amongst tannin addition treatments.2 So the take home here – it may take a whole boat-load of tannin to have appreciable amounts retained but of what little tannin is retained, it doesn’t seem to translate into substantial sensory differences.
Dr. Mansfield’s data suggests that you can get a better bang for your buck by postponing your tannin additions, specifically after grape solids have been removed from the winemaking process or post Malolactic fermentation. Meaning the later you add the tannin, the greater the retention. Hello cellar and finishing tannins, here we come.
A Look Towards the Future
The discovery of a tannin binding protein has practical ramifications as to how we process grapes for wine production beyond tannin additions.
I have maintained (ok lamented at times) that current fermentation aides on the market are vinifera-centric. For example if the compositional constituents of a hybrid grape (like the flesh cell walls noted above) are drastically different than that of a vinifera grape, how much sense does it make to use custom tailored vinifera pectinase with little to no protease activity?!
I wonder if after seeing this aforementioned research, that commercial wine suppliers realize the huge untapped potential here in the marketplace…but perhaps I’m overly optimistic.
To put this concept in perspective this isn’t going to be an easy task. If the end goal is to render a specific protein ineffective at binding tannin, well how does one only target that specific protein and not all other proteins? One can’t simply bombard an active fermentation with an onslaught of proteases (enzymes capable of degrading protein) because this would be akin to shooting yourself in the foot. Your yeast is going to shut down because it too would be subject to protease attack (all enzymes are proteins but not all proteins are enzymes).
It’s going to take a very specific protease that can not only survive the harsh juice/wine environment (think pH, temperature swings, alcohol production, etc.), but also carry out a very specific task at hand – degrade tannin binding proteins.
In the interim, for red hybrid wine production, if the end goal is to extract as much grape tannin as possible, look to trial enzymes currently on the market that are rated for reds with side protease activity (often found in conjunction with pectinase enzymes). But don’t hold your breath at this point in time. This may very well be an exercise in futility.
The Final Word
So where does this leave us? Well, with regards to tannin binding, this new research does shed some light as to the identity of the potential culprit in hand. And we do have some better defined cellar strategies on how to best combat low tannin extraction, specifically by delaying the timing of our commercial tannin additions.
If you’re like me, you now have more questions than answers. What nefarious protein are we talking about? What specialized cells harbor these evil, vile creators of mass binding (destruction)? Are these proteins inducible in grapes by environmental stimulus or more constitutive (the light switch is always on regardless of the environment)? What protease can I use to maximize tannin extraction? And the list goes on and on. Trust me when I say this is an area of active research.
I was given permission to share this most recent finding out of Geneva, NY. The culprit at hand happens to be a class of grape proteins called Pathogenesis-Related (PR) Proteins4. PR proteins aren’t novel to grapes and wine, but they are novel in their link as strong tannin binders. If true, this finding could have broader implications to the world of hybrid grape breeding, especially since it’s been a feather in our cap that hybrid grapes have better disease resistance than V. vinifera. Perhaps the price we pay for better disease resistance is lower tannin extraction? Ah how the hybrid tannin saga continues, part III anyone? Stay Tuned.
Piero Spada received his MS from Cornell’s Viticulture and Enology program and was previously a winemaker in the Finger Lakes. Piero is now a Midwest vineyard and winery consultant and can be reached via email at pierospada.com
- LF Springer and GL Sacks, 2014. Protein-Precipitable Tannin in Wines from Vitis vinifera and Interspecific Hybrid Grapes (Vitis): Differences in Concentration, Extractability, and Cell Wall Binding. J. Agric. Food Chem.
- AK Mansfield. 2015. Building the Perfect Body: Tannin Strategies for Red Hybrid Wines. Northern Grapes Project Webinar
- 2014 Scott Laboratories Fermentation Handbook
- LF Springer and GL Sacks. 2015. Of Terroir and Tannins: The Role of Pathogenesis-Related Proteins in Red Wine Astringency. Abstract. ACS: Division of Agricultural and Food Chemistry Graduate Symposium.