“Fit-for-purpose” yeast from the AWRI

The Australian Wine Research Institute (AWRI) has created a quick-read summary page on their ongoing project to develop “fit-for-purpose” yeast: yeast strains designed to facilitate specific flavor profiles for specific applications. They’ve already developed and released (through AB Mauri and Anchor) several strains including two interspecies hybrids — Saccharomyces cerevisiae crossed with S. kudriavzevii or S. cariocanus — and low H2S-producing strains. More are being tested in Shiraz and are likely to emerge over the next 3-5 years. The AWRI is making a point that this research is Aussie-focused — their argument is that similar work being done elsewhere is creating yeasts not necessarily suitable for Australian wine styles — but no doubt their results will end up helping non Australian-industry levy payers, too. It’s worth noting that their development strategies rely on good old traditional genetics strategies and not genetic engineering. They’re not inserting genes from other species into yeast; they’re breeding different yeasts together, encouraging yeast to mutate (that is, spawning lots of random changes in their DNA with chemicals and stress) and looking for useful mutations, and using contemporary genetics to understand which genes do what. For a quick explanation of why I’m glad that they’re sticking with traditional genetics strategies instead of creating GMO yeast, check here.

Whether you’re excited about the prospect of using tailor-made yeast to target particular flavors or whether you’re in the don’t-inoculate-my-wine camp and hold that fermenting with yeast from the environment is the only or best way to terroir-full wines, it’s hard to argue that knowing more about yeast is a bad thing. Developing new commercial products may be an increasingly major research driver as scientists need to look for support from private sources. Furthermore, ending up with a new product you can hand to someone is a tangible way of saying, “Here, look; our research really is applicable and relevant to real-life winemaking!” Regardless, projects like these continue to provide an umbrella for basic research on yeast genetics and wine flavor development. And maybe not tomorrow, and maybe not next year, but in the long run, that’s something that ends up helping everyone.

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.

Pairings with pinots and the futility of looking to science for answers

I have a horrible (given my current location) admission to make: Central Otago pinot noir is, to date and as far as I can tell, not my favorite thing in the world. That said, Otago pinot noir is lovely and fulfills a completely different function at table. One of the best meals I’ve ever had with an Oregon pinot was the whole salmon I roasted with a bunch of herbs and various alliums for my last Thanksgiving in the States. A guest serendipitously brought a Lange pinot, and it was memorable. On the other hand, Grasshopper Rock’s example — grown on the Clutha River in Alexandra, Otago — didn’t really grab me on its own, but was just lovely when I tried it alongside some smoked hoki that I’d brought home from the Auckland fish market (yes, in my backpack, on the airplane). Those rather robust smokey flavors emphasized the wine’s structural and savory notes and took the focus off cherry flavors that were a bit more candied than I prefer.

What I just offered you is a lay theory. To make it more than that, I’d need an empirical study or three to examine the interaction of smoky foods with various potential sensory qualities found in pinot noir. The problem with that idea, apart from it having nothing to do with my current main priority, i.e. the PhD, is that food and wine pairing research is obnoxious. .

Food and wine pairing articles (I’m quoting this one) are full of statements like this: “This research found that eating cheddar cheese before drinking Shiraz reduced some of the negative characteristics of the wine and enhanced the preference for the wine. This indicates that consuming food and wine together can minimize some of the less desirable flavors of both.” And hypotheses like this one: “Certain food and wine combinations will be perceived as significantly better than others.” The latter of which, I suppose, points out that food-wine preferences could be completely personal, like favorite colors (except that favorite color preference isn’t random, either).

Perhaps this sort of research really interests sommeliers who could think about the benefits of a shiraz and cheddar pairing in a tasting menu, though I doubt they need reassurance that their choices will work for someone other than just themselves. The question still arises: is science, in all its reductionist glory, really the best way to attack food and wine pairings?

First, let’s get a methodology point out of the way. Apparently, the best way to evaluate food and wine pairings is to ask people to eat and drink at the same time rather than, say, munching a bit of cheese, swallowing, and waiting thirty seconds before taking a sip of wine or vice-versa. Because that’s the way people usually eat.

Moving on. Research to date says that wine sweetness and astringency, but not its acidity, are significant in determining ideal food pairings. The most recent food-wine pairing article I’ve encountered tried to suss out whether acidity was in fact important, and the role of wine expertise in food-wine preferences, along with moving beyond many previous studies by pairing wine with foods other than cheese. The chosen foods? Chevre, brie, salami, and milk chocolate, paired with an Ontario chardonnay, an Ontario sauvignon blanc, an Argentinian cabernet sauvignon, and an inexpensive LBV Port. Needless to say, this study isn’t going to give me any insight into my pinot noir pairing theories. Or, for that matter, any insight into any real food and wine pairing conundrum anyone ever faces anywhere.

I’m poking fun, but I’m not being wholly fair. The authors of this article have more expertise in what they’re doing than I do. It’s obvious to any wine or food nerd which of the above pairings will and won’t work, but that evidence is anecdotal, not scientific, and maybe those assumptions are worth testing. But when the authors begin asserting that this study provides evidence that acidity, sweetness, and tannins are all important in pairings, just from showing that milk chocolate works better with port than with chardonnay? No. Four examples aren’t enough to allow for that conclusion, not near enough to weed through and rule out all of the other things (confounding factors) going on in both the wines and the food.

So we’re back to where we started with pairing food and wine. What says our weight of accumulated, non-scientific wisdom? And does it taste good? The reductionism of sensory science may have useful ways to tackle the hyper-complexity of food + wine (don’t ask me whether that’s more or less complex than, say, the human immune system, which science seems to tackle with at least some success), but I’m not sure they’ve figured them out yet. And when I’m trying to decide what to serve with my next glass of pinot noir — Oregon, Otago, or otherwise — the only research I expect I’ll do will be on my favorite cooking blogs.

**All sorts of other fascinating alternate-scientific approaches have been taken to food and wine pairing, Chartier’s fascinating Taste Buds and Molecules: The Art and Science of Food, Wine, and Flavor being perhaps the most interesting example. What I’m talking about here is the mainstream pairing science found in peer-reviewed journals.

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.

 

Why do yeast make alcohol?

Ever wonder why yeast make alcohol? Probably not, I realize, but you should. Yeast throw off ethanol in the process of metabolizing sugar, so alcohol is a byproduct of survival; fair enough. But alcoholic fermentation is, in fact, a surprisingly inefficient way to get energy. The standard oxygen-requiring way of breaking down sugar used by most cells, our own included, wrings somewhere between 30 and 38 ATP (38 is the ideal number; it’s probably never quite that high in practice) out of a single glucose molecule. (ATP is the cellular currency in which energy is transferred and spent.) Nevertheless, alcoholic fermentation has the distinct advantage of not needing oxygen and so it makes perfectly good, intuitive sense for Saccharomyces cerevisiae to use it when oxygen isn’t available.

Here’s the quirk: S. cerevisiae uses inefficient alcoholic fermentation even when it does have access to oxygen, even though it has the machinery for the much, much more energetically worthwhile aerobic metabolic process. Yeast will only switch to aerobic metabolism when the amount of sugar available for them to eat is very low. Why? A good question, and one microbiologists haven’t had much success answering.

Our best hypothesis according to a brand-new review on the subject comes in two parts:

  1. Alcoholic fermentation lets yeast act fast to use up the “public goods” while squirreling away private resources for later. Every microorganism you’ll encounter in grape juice can consume sugar. Very few can also consume (and get energy out of) ethanol, but yeast can. So, by converting sugar to ethanol, S. cerevisiae can starve out other microbes and leave itself with a food source for later.
  2. As an additional and maybe even bigger benefit, ethanol is toxic to most yeast and bacteria at concentrations that Saccharomyces can tolerate with relative ease

Possibly the most bizarre thing? We don’t know much about what determines the circumstances under which S. cerevisiae, our long-time compatriot and coworker, produces alcohol versus making energy in some other way. We’ve looked at when and where different yeast genes are expressed and when and where it makes different byproducts but, like so much else in the wonderful and frustrating world of modern-day genetics, putting together the whole story is still a work-in-progress.

White grapes: more colorful than you think

Our society has a bizarre habit of mislabeling things by color. The familiar case in point: white people are never white but always various tints of pink, peach, and yellow; black people are invariably not actually black but some shade of brown or tan. Less familiar case in point: white wines are really always somewhere in the yellows, and the grapes themselves range from green through yellow to pink. (Red wines are, at least, red, even if the grapes which give them birth are more aptly blue, black, and purple).

We do talk about the color of white wines, from the pale straw of a light-bodied sauvignon blanc to the amber of an elderly riesling. Anything not firmly lodged on the green-yellow to brown-yellow spectrum, though — including forays into pink — is considered a fault. Now, that judgment (like some other “wine fault” decrees) seems a bit arbitrary to me: would I really mind sipping a pinkish chenin blanc? (No, I would not). But “pinking,” as it’s called, is a problem, if for no other reason than consumers might have a hard time coming to grips with it. Gewürztraminer grapes are unquestionably (and beautifully) pink, but gewurztraminer isn’t one of the grapes prone to pinking and, in any case, we’re not talking about color derived from skin contact — those wines are “orange,” not pink. Real pinking qua pinking can show up before bottling or suddenly after pouring and seems to be the result of exposing a reductively-made wine to oxygen.

A group of chemists from Portugal have done a convincing job of demonstrating that — at least in the Siria grapes they tested — pinking is caused by…anthocyanins. Yes, the very same pigmented molecules that make red wines red. But, what are anthocyanins doing in white wine?

In some sense, they were there all along. All grapes have the genetic machinery to be red. White varieties are mutations, the result of genetic changes in the genes responsible for the production of red anthocyanin pigments in grape skins. The same mutations seem to exist in all white grape varieties, which suggests that they probably all descended from a common ancestor.

Anthocyanins would seem the obvious culprit for pinking, even if it does seem odd to think of them being found in whites — that genetics of grape color research isn’t especially new. One of the classic wine science textbooks, Ribéreau-Gayon and company’s Handbook of Enology, says that pinking is caused by unknown compounds that can’t be anthocyanins because they don’t respond to sulfur dioxide and pH in the expected ways. That book was published in 2006, though, and folks like James Kennedy at Fresno State University and Jim Harbertson at Washington State University (along with a good many other researchers) have, since then, made a fair bit of headway into figuring out how anthocyanins react with each other and other wine components. (It remains a terribly complex and incompletely understood topic.) This team of Portuguese researchers could still observe that the pinking-related anthocyanins they observed didn’t act exactly like “normal” anthocyanins because they polymerize over time in the wine, which makes them more resistant to the color-bleaching effect of sulfur dioxide. Suffice it to say that they go through some complicated chemical acrobatics to show that the molecules they isolate from their pink Sirias are indeed anthocyanins.

The researchers responsible for this study speculate that Siria, the rather obscure Portuguese white grape variety with a persistent pinking problem that they chose to examine, may have regained the ability to manufacture some anthocyanins. Not enough to make the grapes overtly pink in the vineyard, but enough to belie their presence after at least some kinds of winemaking operations. (Anthocyanins are unstable molecules susceptible to changing in the presence of oxygen and other molecules.) Though they haven’t substantiated that speculation with molecular analyses, it’s not out of the question that additional mutations in those anthocyanin-producing genes might restore some of their functionality or cause them to be transcribed under specific circumstances.

If you’re not a winemaker with pink problems, why is this research interesting? It’s a good reminder that white grapes aren’t necessarily simpler than red ones, as it’s so easy to imagine, and that we’re still learning a lot about the very complicated pigments that make wine color happen. But it also makes me stop and think about how flexible plants really are. We can select for and preserve features we want through careful clonal selection of the most highly desirable plants, but vines are still going to change and mutate and do new (or redo old) things on the sidelines.

Will magnetic yeast make better Champagne?

Wine Searcher ran a story this past week about new technology from the University of Ljubljana that speeds traditional sparkling wine processing times by magnetizing yeast cells. Magnetic nanoparticles affixed to the cells’ surface don’t interfere with fermentation and let winemakers literally and near-instantaeously pull the yeast into the neck of the bottle by applying a magnetic current. Since riddling — slowly inverting and rotating bottles to remove (unattractively cloudy) dead yeast after the secondary in-bottle fermentation responsible for effervescence-generation — traditionally takes a few months and a LOT of hands-on work, a 15-minute flip-a-switch solution looks pretty attractive. BUT:

Interesting fact #1 – This technology isn’t new, though applying it to the sparkling wine industry is. Bioengineers came up with magnetic yeast in 2009.

Interesting fact #2 – If actually adopted by the industry, magnetic yeast will be far from the only use of nanoparticles in food. Quite the contrary, which you know if you follow the American health and science news. Titanium dioxide nanoparticles are common additives to everything from chewing gum and toothpaste to yogurt and soy milk, generally to the effect of making whateveritis whiter. Nanosilver particles are common both as agricultural pesticides and in antimicrobial coatings for household goods, and nanolipids and nanoproteins and assorted other nanostuff finds its way into all manner of food-related items. The consensus is that we don’t yet have a consensus on whether and to what degree ingesting nanoengineering is safe (a peer-reviewed take on that question here; a more accessible and more inflammatory story from Mother Earth News here). Logically, magnetic force should effectively pull all of the magnetic particles (made from magnetite, if the Ljubljana authors are using the same general strategy published in the 2009 paper) out of the wine, but nothing is perfect. If residual particles remain, drinking them might be a health risk, but it won’t be a unique one.

Interesting fact #3 – Alright; this one isn’t a fact. It’s a speculation based on fact. I speculate that we needn’t worry too much about magnetite in our celebratory libations. Champagne in particular and high-quality, methode champenoise sparkling wine in general, is not about fast. Exactly the contrary. Champagne legally has to spend at least 15 months in bottle and at least 12 months on the lees, and usually exceeds that by a year or two because age on the lees is vital to the flavor profile of high-quality sparkling. I reviewed some of those considerations in this article for Palate Press.

The problem with riddling isn’t the time per se so much as the labor: some poor guy has to spend his days jiggling bottles (and if champagne riddlers don’t have a high incidence of occupation-induced carpal tunnel syndrome, I suspect that it’s just going undiagnosed). The gyropalette solves that problem by loading a box full of bottles onto a modified forklift and letting the machine jiggle them for you. That bit of technology has been popular and successful, but it seems to me that it’s also a lot less expensive than magnetic yeast.

Think about it. Yeast reproduce in the bottle, a lot. So, every yeast cell used in inoculation needs to be loaded with magnetite particles to ensure that all of its many, many offspring has at least one magnetite particle.** Don’t even think about generating your own yeast innoculum. And that’s before we get to the magnetic set-up to actually pull down the yeast. I don’t know. Storing wine (and paying that poor guy) is expensive. Maybe this is a cost-effective solution. But if high-end producers aren’t going to be seduced by speed, and if lower-end producers are disinclined to spend more money on production technology, and if the wine industry in general tends to be stuck in the mud, I suspect we needn’t worry too much about drinking magnetite anytime soon.

** Maybe effective clarification doesn’t require that every yeast cell be magnetic, if the yeast tend to stick together (flocculate) and magnetic cells will help pull down their non-magnetic neighbors. Without reading the paper I don’t know, and since I can find neither the paper (maybe it’s not yet been published, or maybe it wasn’t published in English) nor the specific names of the researchers nor any other mention of the research on the University of Ljubljana’s website I have to speculate. It’s disturbing that I can’t find another source backing up the Wine-Searcher article (and I don’t personally know it’s author and can’t locate him via the usual tricks) but, then again, I don’t read Slovenian.

The complicated business of recreating a wine aroma

How wine aroma happens is both very simple and very, very complicated. The simple version: molecules capable of leaving from the surface of the liquid (or carried up to the liquid in the tiny gas bubbles that make sparkling wine sparkle) are carried through the air up to our nostrils where those molecules meet sensory receptors that, when bound to the right kind of molecule, trigger a “smells like” response in our brain. Hence why we swirl and sniff: swirling encourages aromatic molecules to leave the wine; sniffing encourages those molecules to travel into our noses. As you’d expect, the complicated version elaborates on what kinds of molecules are capable of leaving the wine — of moving from the aqueous phase to the gaseous phase, we’d say — and what has to happen between an aroma molecule and a smell receptor for a message to be sent, and how activating a receptor turns into a perception of “smells like” in our conscious minds. But it’s actually even more complicated than that, because molecules that aren’t aromatic and that don’t ever leave the wine for the air influence what we smell, too.

A new study of the aromas of two different Australian shiraz (shirazs just doesn’t look right to me) is a good example of what it takes to make up a wine’s nose.

Wine — talking all wines collectively, not any one in particular — involves at least 800 or so different aroma compounds. The strategies used to figure out that sort of thing, and to analyze the aroma composition of specific wines, are all fundamentally based on separating out all of the many different molecules a wine contains. A common way of doing this is with gas chromatography which, to put it simply, separates molecules by the differences in how they interact with specific solvents. How long it takes a particular molecule to let go of the solvent — its “retention time” — is unique, so seeing a molecule’s retention time is as good as knowing the molecule’s name…at least when someone else has already done the meticulous work of correlating the two. Of those 800-some-odd wine aroma molecules, we can actually only name something like 10-20%. But we know the rest exist, because we can see their retention-time fingerprint pop up on the chromatography results. Even better than gas chromatography, for wine aroma purposes, is gas chromatography-olfactometry, which takes the apparatus for chromatography and adds a smelling port so that scientists can sniff the separated-out molecules as they come up in turn. In the case of the Australian shiraz, the gas chromatography came up with about 100 “odorants,” but the consensus among sniffers was that only about half of those actually smelled like anything. Of those, for only 27 or 28 were their concentrations in the wines high enough to theoretically be detectable (they exceeded their odor threshold). It took 44, added to a wine-alcohol-acid-sugar base, to make something that credibly mimicked the original wine.

Then the researchers asked their trained sensory panel to do something interesting: sniff  the aroma + base wine-like synthetic and the same mix minus one of several key aroma compounds with the goal of identifying which molecules contributed to which perceived smells. The details get long-winded, but the final message stands out. Removing non-aromatic constituents changed aroma perceptions — sometimes more intense, sometimes less, depending on the aroma and the other molecules involved — even when the key aroma compounds themselves were left untouched. And some very obviously smelly compounds present in the wines in quantities far above their odor thresholds had a much smaller impact on wine aroma then their high concentrations would make you think.

In other words, wine aroma isn’t as simple as just pairing up odiferous molecules and their corresponding smells. And we can’t yet predict how or why wine will smell the way it does from first principles. Synthetic wine, then — or at least synthetic wine that replicates real wine — is going to take some time, and a lot more sniffing.

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.

Does cross-flow filtration affect wine flavor? No easy answers.

Cloudy wine (mostly) doesn’t sell. Neither does (most) wine spoiled by spoilage microbes that produce off-aromas. And so, while it is entirely possible to remove floating particles that make wine cloudy and (most) microorganisms via careful winemaking without sterile filtration, most winemakers appreciate the extra insurance it gives. Filters are essentially membranes punctuated by lots of little pores: all good stuff should flow through the pores; bad stuff you want to keep out of the bottle should be too big to pass. To reliably remove yeast and bacteria, filters need to have really tiny pores, and the question always is: are those filters excluding stuff other than the microbes, good-tasting stuff that I want the wine to keep? We’re talking .45 μm pores here, an order of magnitude smaller than the diameter of a red blood cell; anything bigger risks allowing bitsy bacteria through.

Pro-filtration folks say that all molecules important to wine quality are much to small to get caught in even these super-stringent filters. Filtration-shy folks say that wine tastes different post-filtration; no matter what molecular measurements say should happen, something is happening. For an excellent discussion of the arguments on both sides, see Tim Patterson’s excellent review in Wines and Vines (updated here, but this one is still behind a pay wall). Bottom line: scientific evidence suggests that while some important molecules could get stuck to the surface of the filter, all the important stuff can pass through safely; dissenters can still taste a difference, and maybe that has something to do with big conglomerates of molecules.

So it stands for conventional filtration. But what about cross-flow filtration, they shiny new-ish solution to some of the conventional process’s major hassles? Conventional filtration points a stream of flowing wine directly at the filter membrane and waits for it to percolate. Enough push needs to be behind that wine stream to keep things moving, but too much push and the force of the flowing wine will rip right through the filter. And the wine must be almost entirely free of particles in the first place, else biggish stuff will cover the surface of the filter, block the holes, and slow down flow. Either way, the filter membrane will need to be replaced, and they’re expensive. Cross-flow filtration instead points the wine stream across the surface of the membrane. Liquid still flows through but with less direct pressure on the membrane, and the constant stream sweeps pores clean of junk, too.

So cross-flow is better than traditional filtration for a few technical reasons. Is it also better for wine quality?

A study addressing that question has just been published (as a provisional draft; it’s not yet appeared in the print journal) with work done by a team from UC Davis. A few published articles have shown chemical analyses of cross flow-filtered wines, but this study is unique and helpful in two ways: 1. The emphasis was on whether a trained tasting panel could detect flavor differences in filtered wine; and 2. Wines were tested not just immediately after being filtered, but from bottle samples taken at intervals out to eight months post-filtration. Kitchen-sink white and red blends were included.

Though the group’s experiments aimed at looking for sensory differences following filtration, their results uncovered something else. The flavor of the unfiltered red wine changed more over time — more earth, less fruit beginning at two months and continuing to the eight-month end point — while the flavor of the filtered red remained more or less constant; in other words, the filtered wine was more stable. The obvious explanation is that the unfiltered wine suffered from some kind of microbial growth after bottling, even though the idea of a UC Davis-crafted experimental wine having microbial spoilage issues does seem strange.

More to the original point: even though chemical analyses showed that filtration decreased phenolics in the wine — filtered reds had lower pigmentation and up to 26% lower tannin levels — the tasting panel didn’t pick up corresponding differences in astringency. That’s surprising. The only explanation offered in the paper is that the magnitude of the change mustn’t have been big enough to be detectable.

In the end, then, this study probably does more to fuel the filtration debate than to help resolve it. Pro-filtration folk can point to filtration’s apparent lack of sensory impact, and to the likely spoilage of the unfiltered wine. Filtration-caution folk can point to the color and tannin changes and say that, even if those changes didn’t affect flavor in this wine, similar changes might indeed be important in other wines. So instead, we have one more example of what may indeed be Rule No. 1 in winemaking: there are no easy answers.