Does adding tannin boost aromatic thiols, too? It just might.

Thiols aren’t quite like bacon, but they’re not too far off trend-wise. These aromatic sulfur-containing molecules are highly appealing in small quantities — even low concentrations lend a wine’s aroma fresh fruity notes (tropical in sauv blanc, black currant or berry in reds). Just about everyone wants them, or wants more of them. They’re at work in the expected places (thiols in sauvignon blanc are like the bacon in your pasta carbonara; bland without, and much better with), but also do a fair bit in the unexpected ones, too (thiols contribute to the aroma of Bordeaux reds and Provençal rosés, for example, and bacon, I’m told, does excellent things to cupcake frosting*).

Unlike bacon, we still don’t have an especially good idea of how thiols are formed (we figured this out for bacon a good long while ago, I believe). The amounts yeast transform from various precursors under realistic wine conditions just don’t add up to the final concentrations we find in wine, and how the rest happen remains an open question. Last year’s news was that tannins contain thiol precursors upon which yeast act during fermentation. Now, those researchers (an Italian group, with the aid of a Sauvignon blanc-oriented researcher from New Zealand) have demonstrated what I’m sure they’d hoped for when they published last year’s paper: adding tannins to wine before fermentation increases a wine’s thiol concentrations, specifically 3-mercaptohexan-1-ol (3MH). (For some context on 3MH and other sulfur compounds, Jamie Goode’s blog article on the topic is a good primer).

This study is very much a first step, and a bit of a disappointing one. Tannin was only added at one concentration: 1.6 g per 2 kg batch, compared with a no tannin-added control. Seeing a dose-dependent response — add more tannin, get more thiols — or showing that the relationship between those two variables isn’t linear, anything other than just two points, would have been much more convincing. As would using larger than 2 kg batches for those experimental wines (2 kg ~ 1 750 mL bottle), since the volumes in which experimental wines change yeast fermentation and oxygen exposure dynamics; the oxygen mightn’t be relevant here, but the fermentation parameters are. AND, each wine was only made in duplicate, not satisfying the usual experimental expectation of performing studies in triplicate. With two samples, if one is off you can’t tell which reflects the trend you’d see if you did the experiment a hundred times (and you certainly shouldn’t just average them together); if three samples all group, you can feel better about life (and your results). AND, with so little wine, the authors couldn’t conduct a proper sensory analysis, not that doing so would have been worthwhile in any case with their mini-make-do winemaking technique. In other words, this study is less than convincing on methodological grounds.

All of that said and duly noted, this study points toward some interesting possibilities. For instance, I’ve recently talked with a few winemakers who have been experimenting with tannin additions to good but confusing effect. (I seemed to come across people talking about tannin additives about as often as I did bacon-laden menu items on my most recent trip through Eastern Washington, which is to say, a lot.) They know good results when they see them, and they like what they taste. But tannin assays sometimes seem to yield results that conflict with experience, with the assay saying that the with-addition and without-addition wines contain the same amount of tannin even though the winemaker can taste a difference. All manner of possible explanations exist for that phenomenon, and I don’t want to suggest that thiols are responsible for those sensory differences. Nevertheless, this study is a good reminder that adding anything to wine is bound to have more than just one obvious, direct effect, and that adding tannins could play with wine aromas in ways we hadn’t expected.

*I’m told, because I’m one of three people on the planet who likes neither bacon nor cupcake frosting.

Advertisements

Measuring not just tannin concentration but tannin behavior: Kennedy’s stickiness assay

Why does it always seem that we know the least about stuff that’s the most important? Tannins garner a lot of wine researcher’s attention, and for good reason. No one needs convincing about how important tannins are to wine quality (especially not the consulting companies who’ve correlated high tannin concentration with high wine magazine ratings). The amount of noise made about tannins, though, could give someone seriously inflated ideas about how well we understand them.

Excellent wine chemists are, in fact, still thinking about really good, consistently accurate, and every-day-practical ways of measuring a wine’s tannin concentration. The well-known Harbertson-Adams assay went a long way in that direction, but isn’t the last word on the topic. But just looking how much tannin a wine has doesn’t tell us enough. “Tannin” describes a whole group of molecules, and those molecules behave in different ways.

What we really need is a way of measuring not just how much tannin a wine has, but how astringent it’s likely to feel. That’s a tall order — astringency is a complicated sensation affected by alcohol concentration, sugars, polysaccharides, the person doing the tasting, and undoubtedly other factors. Just tasting the darn thing is, without question, the most elegant and reliable way to measure wine astringency. But it would still be useful to have a way of measuring the relative astringency of different types of tannins to correlate with how different production techniques affect those tannins and make some predictions. And, just as importantly, if we’re ever going to figure out what tannins do, how they behave, and how astringency works, having more tools to look at them is important.

James Kennedy’s group at Fresno State is working on a way to go beyond traditional tannin measurements, which just tell you how much tannin you have, to develop analyses to tell you what the tannin you have does and how it’s likely to produce astringency. More particularly, they’ve developed a way to measure the stickiness of any particular type of tannin molecule. Stickiness, as defined in the article, is “the observed variation in the enthalpy of interaction between tannin and a hydrophobic surface.” Or, to put it a lot more simply, stickiness describes how strongly a tannin is inclined to attach itself to something else (without actually reacting with it). This seems pretty commonsensical — if we sense astringency when tannins glom together with our salivary proteins, then we’d like to know how glom-inclined those tannins are. They’ve shown that their stickiness measurement for a particular set of wine tannins remains constant no matter how much of the tannin you test — in other words, they can measure stickiness as a tannin quality, not tannin quantity.

It’s a trickier puzzle than it might seem. How do you measure how tightly two molecules are holding on to each other? And when you’re interested in how different tannins interact with proteins, which are themselves a very diverse group of molecules, how do you choose which protein is going to be the protein that represents all other proteins?

For Kennedy and company, the solution involved choosing something that isn’t a protein at all but polystyrene divinylbenzene, a polymeric resin that holds on to tannin in remarkably the same way as the specific amino acid (proline) that acts as the tannin-attractant in salivary proteins. The resin allows for a standardized stickiness measurement and no doubt has all sorts of advantages in terms of working with it in the lab. It won’t actually behave like real salivary proteins which, being folded up into various shapes with proline more or less accessible along their various crannies, don’t bind tannins in ways so predictable. The upshot is that this is a standardized measure of stickiness (a defined scientific parameter), not an actual measure of astringency (a subjective sensation). Nevertheless, stickiness values and astringency should be related in predictable ways. We’ll very likely see a publication verifying that relationship with human tasters before too long.

Stickiness assessment involve some fairly complex chromatography, improving on a method the lab published last year. The methodological details are less important than realizing that this isn’t something that even a well-equipped winery lab is going to be able to do on their own (unlike that Harbertson-Adams assay, which is pretty accessible for a lot of winemakers). Though some wineries may measure tannin concentrations with that Harbertson-Adams assay, which is pretty accessible for a lot of winemakers, stickiness measurements aren’t going to become the new best thing in figuring out how long your syrah needs to spend on its skins before being pressed off. Too expensive (the chromatography columns needed for this kind of work run hundreds of dollars each), too training-intensive (unless you have a chemistry grad student hanging out in your winery), and too little of an improvement over just tasting the darn thing. This research isn’t likely to change the way anyone makes wine tomorrow or even for the next year or two. But it very well may change the way scientists study and think about tannins, the kinds of questions they can answer — those tricky issues around the relative astringency of various seed and skin tannins, for example — and what they can tell winemakers about targeting specific wine styles a few years down the road. And that’s worth making some noise over.

 

Astringency is not a flavor (thank you, wine physiology)

It’s generally agreed that we have five basic tastes — sweet, salty, bitter, acidic, and umami — all of which make appearances in wine.** The nuances described in baroque tasting notes — fruits and flowers and tar and tobacco and the rest — are, of course, smells. But where does that leave astringency? In the hands of physiology researchers, evidently. Anatomy is the science of labelling the parts of the body and where all the bits are. Physiology is the science of understanding how those parts work. So when we ask questions about how wine triggers responses in the mouth, we ask physiology.

Astringency is the dry, rough, puckery feeling left in your mouth by a sturdy red wine, strong black tea, dark chocolate, or (best example ever) an underripe persimmon. Some astringent molecules also taste bitter, but that’s not what we’re talking about. Astringency doesn’t seem to be a taste. It’s definitely not a smell. It’s…something else. But since descriptions like “something else” leave scientists (and wine drinkers, maybe) feeling unsatisfied, physiology researchers at Ruhr University in Germany have been trying to pin down astringency more precisely.

Research on astringency isn’t new, but it’s been confusing. Astringency triggers the same nerve that’s used to carry flavor sensations in mice; since flavors and feelings (like touch and temperature) are carried by different nerves in the mouth, that’s a useful observation. But astringency can be sensed by parts of the mouth that don’t have taste receptors. We know (or we think we know) that tannins are responsible for red wine astringency, and what we’re taught in food science classes is that tannins bind to the proteins in your saliva and cause them to glom together, which simultaneously decreases the slipperiness of your saliva — making your mouth feel dry — and creates a bunch of big rough tannin-protein blobs that themselves feel rough. The problem with that explanation is that the intensity with which we sense astringency doesn’t seem to be related to how much protein gets bound up, and not all molecules that seem to cause astringency bind up proteins at all.

This new German study, unfortunately, doesn’t help resolve most of those conundrums. But it did use a simple, elegant little trick to pretty firmly say that astringency isn’t a taste, and that it is a feeling.

Since completely different nerves carry taste sensations and mechanical feelings like pressure or roughness back to the brain, these researchers used anaesthetic — the same injectable kind you’d get at the dentist — to numb up either the taste nerve for the front of the tongue alone or both the taste and the feeling nerve of some real live humans, then subjected them to astringent things like quinine (the stuff in tonic water) or powdered chestnut. They also found some folks whose mechanical feeling nerve had been cut in the course of middle ear surgery, which means that they couldn’t taste on half of the front of their tongues (these and most nerves are paired with one for the right and one for the left side of the body). The folks who couldn’t taste — either because of surgery or because of anaesthetic — had no trouble detecting astringency. But the folks who couldn’t feel were numb to the astringent sensations. That suggests that we don’t sense astringency in the same way as mice, but that’s not outside the realm of possibility.

The study also included tests on isolated nerve cells (from mice, not those human subjects; don’t worry) to look for exactly what molecules were triggering the mechanical nerves and what kind of triggering was going on. Those experiments showed that astringency isn’t just the roughness you feel when you move your tongue around, but that nerve cells are being activated directly. In other words, you could still feel astringency if you couldn’t tell whether the inside of your mouth was rough by moving your tongue and cheeks around.

But, still, that’s an open question: is the sensation of astringency caused just by chemicals triggering nerves, or is it also the product of rough feelings when we move our mouths around? Since knocking out the mechanical sensation nerves knocks out both of those feelings, these experiments couldn’t say.

So when astringency comes up over a glass of tannic red, you can continue to confidently say that the wine feels astringent rather than tasting astringent. Cocktail-party trivia, sure. And maybe this research has other functions, in understanding and knowing how to fix peculiar diseases of messed-up mouth nerves. But in some ways, it’s about what science has always been about: looking at something — in this case, our own bodies — and asking, “Well gosh, how does that work?” The earth is a giant puzzle book, and we’re certainly in no danger of reaching the last page any time soon.

 

**Yes, yes: what constitutes a basic taste is a matter of debate, but that’s an interesting topic for another day.