Wine Media, I’m Ashamed of You

It’s been in the news for the past week. Researchers from the Australian Wine Research Institute, led by Dr. Chris Curtin, have sequenced the genome of Brettanomyces bruxellensis (aka Dekkera bruxellensis) and are promising that a magic bullet solution to winery “brett” problems will be forthcoming. Decanter and Wine Spectator have both headlined the story.

If either of those publications has a microbiologist – or any biologist or biochemist, really – on staff, he or she should be ashamed of letting this one slip.

Sequencing any organism is unquestionably an accomplishment, though it requires more capital investment than scientific ingenuity in the present rapid-sequencing era. And I have great respect for the Australian Wine Research Institute (AWRI), a government-supported organization that has turned out insightful and important research especially over the past twenty years. But I have to wonder when I see the AWRI managing director Sakkie Pretorius quoted as saying “Sequencing the brett genome, which reveals its genetic blueprint, means the Australian wine industry can future-proof its strategy against brett and the risk of spoilage.”

The Brettanomyces genome gives researchers all sorts of useful information, but it by no means guarantees a solution to winery brett problems. For some context, consider that Pseudomonas aeruginosa, an opportunistic pathogen that causes the death of many people with cystic fibrosis, was sequenced in 2000. We still don’t know how to eradicate it, and it still kills people. Haemophilus influenzae was the first free-living organism to be sequenced (by Craig Venter’s group in 1995) – multiple sequences for different strains have been sequenced since then – and a lot of effort has been put into developing a vaccine for it, but we still haven’t figured out a way to prevent it from making itself comfortable in kids’ ears and causing ear infections. I’m not cherry-picking my examples. Knocking out a problematic infection based on sequencing the causative organism is by far and away the exception, not the rule.

So, as much as it’s thoroughly awesome that the AWRI folks have sequenced brett, it’s a very, very long stretch to conclude that “the Australian wine industry can future-proof its strategy against brett and the risk of spoilage.” With all due respect to everyone at the AWRI, I don’t think so.

One more nagging problem with all of this hubbub is that the research has been published in a magazine called the Wine and Viticulture Journal. Published by the company WineBiz, the Wine and Viticulture Journal is a non peer-reviewed trade publication. Under the ordinary protocols of science, a new genome sequence is introduced to the world via a scholarly article in a peer-reviewed scientific journal in addition. I obviously don’t know the motivations of Dr. Curtin and company – and those motivations might be perfectly reasonable – but publishing something like a genome in a trade magazine is highly unconventional and can’t help but raise the question: why wouldn’t you want the respectability of a peer-reviewed publication? According to the conventions of the scientific community, this research won’t truly be taken seriously until it has been vetted by the peer-review process. In short, the press is jumping the gun a bit.

And by the way, Wine Spectator, misspelling the organism’s name (Dekkara?) and improperly capitalizing the species name doesn’t win you any bonus points, either.

The Problem of Gluten in Wine

Over the past few years, I’ve had several people ask me about gluten in wine. Grapes don’t contain gluten, of course, but gluten-intolerant people do have reasonable reasons to be asking the question.

Wheat paste – made from wheat flour, which does contain gluten – is used to seal the inside of some oak barrels. Some wine is aged in oak barrels, where it could potentially come into  contact with the wheat paste. Therefore, some barrel-aged wine could, potentially, contain gluten. Does this mean that people with severe gluten intolerance or gluten allergies need to avoid (barrel-aged) wine?

While I’ve seen plenty of gluten intolerance-related websites and forums pose and attempt answers to this question, I’ve had trouble taking them seriously. Logic and reason are useful tools, but sometimes scientific evidence turns even the best reasoning on its head. We can all speculate on what sounds reasonable, but some data would be nice.

An experimental report with something to say on the topic was published this year in the Journal of Agricultural and Food Chemistry. (It should be noted that a previous article commented on gluten sensitivity and wine, but the article was published in 2003 in a much less well-known publication – the International Journal of Tissue Reactions – to which I don’t have institutional access.)

Unfortunately, the article, “Immunological and Mass Spectrometry Detection of Residual Proteins in Gluten-Fined Wines,” doesn’t speak to the question of wines aged in wheat paste-sealed oak barrels, but to of residual gluten in wines that use gluten for fining – that is, clarified of yeast cells and other things that make wine cloudy – rather than. Fining agents, which also include such unpleasant-sounding if harmless substances as bentonite (a type of clay), egg white proteins, and isinglass (from fish bladders; yes, fish bladders), are added while the wine is still cloudy and help yeast cells and proteins and other things that make wine cloudy settle to the bottom of the container. The now-clearer wine can be “racked” off the top, leaving the cloudy bits in the bottom along with the fining agent, of which there should be none left in the wine. People with food allergies can sometimes be really sensitive to really tiny, trace amounts of allergens, though, so it makes sense to test whether any trace bits of gluten could be hanging around in wine waiting to make a super-sensitive someone sick.

Something really important to note here. This study looked only at wine clarified with wheat gluten, not wine aged in oak barrels sealed with wheat paste. Definitely different things. Still, even looking for gluten reactivity in wine is a start.

The study used anti-gliadin and anti-prolamin antibodies as well as pooled sera from people with wheat allergies to probe wine, then looked directly for gluten proteins in the wine using mass spectrometry. Anti-gliadin and anti-prolamin antibodies (gliadin is a wheat prolamin, and prolamins are proteins found in grains) are formed by people with celiac disease and gluten allergies and have a lot to do with the symptoms of these diseases. Sera (plural of serum, which is blood minus the blood cells) from people with wheat allergies should include these antibodies, too, but might include types that the researchers hadn’t thought to include among the purified antibody selections.

I’m inclined to take issue with the methods this study uses. After mixing together the wine and gluten, the experimenters centrifuged the wine to remove the gluten. This is obviously not what usually happens in a winery (though a few wineries do use gigantic centrifuges for this purpose.) The strength of this study, though, is its use of both antibodies – a relatively direct measure of immune system reactivity – and mass spectrometry, which simply measures whether gluten proteins are present or absent.

The bottom line? These researchers DID find gluten proteins in wine that had been fined with wheat gluten. There is at least the possibility that people with celiac disease or severe gluten allergy will react to wine that has been exposed to gluten during processing. What we still don’t know, it seems, is how likely that possibility is.

*For the record, previous studies over the past ten-ish years have looked for gluten proteins in wine fined with gluten, but this was the first published report using both antibodies and the more sensitive mass spectrometry method.

**Using gluten to clarify wine is an interesting proposition in itself. Presently, both egg proteins and isinglass – a substance derived from fish bladders (yes, I said fish bladders, and I meant it) – are used for the same purpose. Both pose an issue for vegan wine drinkers, who won’t consume anything containing or made with animal-derived substances. The latter is a problem for vegetarians, too, and either might be problematic for people with severe allergies either to eggs or fish. Gluten, then, seems like a great alternative…save that it might cause problems for the gluten-intolerant even as it solves problems for vegans.

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Attempting to drink Norton in Virginia

Norton is not a hybrid. Maybe you knew that, but it’s easy to forget/not realize/assume that it is. Very understandable: Norton obviously isn’t among the top European vinifera varietals – and its name makes it an unlikely candidate for one of those little-known and newly-discovered vinifera esotericals – so that means it must be a hybrid, right? Well, wrong. Norton is a Vitis aestivalis or “summer grape” (aestivalis refers to summertime) and a totally different species from V. vinifera and V. labrusca. In the United States, we usually refer to European varietals as “viniferas, V. labrusca grapes like Concord and Catawba as “natives” for being indigenous to this continent, and intentional “man-made” crosses between European vinifera and American native varietals as “hybrids.” Vitis aestivalis, then, is none of the above.

Or at least that’s the best consensus at this point. Some folks seem to think that Norton might be a very old hybrid between a labrusca called (of all things) Bland and the vinifera Pinot Meunier. I’ve not read genetic data on the subject, but every paper in American Society of Enology and Viticulture as well as the several Norton-related papers indexed on PubMed agreed in identifying Norton as V. aestivalis. Like native labruscas, Vitis aestivalis is also native to eastern North America. state of Missouri markets Norton as “America’s True Grape.”

So, Norton is not a hybrid and, therefore, I was interested in tasting a few over my weekend at the 2011 North American Wine Bloggers Conference in Charlottesville, Virginia. Hybrids and I don’t get along well for one really quite simple reason: anthranilates. Methyl and ethyl anthranilates are the chemical compounds responsible for the distinctive “foxy” aroma that characterize wines made from hybrid grapes (or pure-bred V. labrusca.) V. aestivalis, however, isn’t known for having a high level of these compounds or the associated “foxy” flavors.

I learned today that Norton is associated with Missouri – Norton is Missouri’s state grape – but I had heard more about Virginia’s iterations of the varietal. Norton is popular in these areas in large part because of its strong mildew resistance, a real boon in often-humid climates. With 100°-ish temperatures and humidity over 50% all weekend, even I was beginning to mildew by the end of my three-day stay in Virginia.  

I somehow managed to miss the several Nortons at the Virginia-only tasting over and around the Friday-evening dinner at Monticello, but there were plenty Missouri versions at the post-prandial “The Other 46 Tasting” (referring to the states other than CA, WA, OR, and NY.) Scientific evidence aside, I’m now more willing to accept the son-of-Bland hypothesis. I wouldn’t exactly call these wines bland, but flavorful they were not. Keeping in-mind that this wasn’t an event designed for in-depth tasting, here are my very brief notes on three Missouri Nortons from that evening:

“Lots of burnt-out fruit up front, nothing to back it up, a bit sour. YUCK.”

“Skunky, smoky, and sweet. Double YUCK.”

“Richer, jammier, a little sweetness, but no tannins, short finish, flat mouthfeel, just not much going on.”

I don’t want to dismiss an entire varietal/region/style based on a handful of examples, so I’ll make an effort to try more Norton wines in the future. HOWEVER, reading a little more about the basic characteristics of the Norton grape makes it sound unlikely as a great winemaking grape. From a 2011 paper in BMC plant biology by a group of viticulturists in Missouri:

-          Norton retains high malic acid at time of ripening → high acidity for a red and, after malolactic fermentation, potentially lots of buttery flavors. I’m just not sure if butter complements the basic Norton flavor.

-          Norton retains high phenols at time of ripening → phenols are such a tremendously large and varied group of compounds that it’s hard to say more about the impact of “high phenols” on the finished wine without more information on the specific phenols involved.

-          The skin of Norton grapes has a higher anthocyanin content than that of Cabernet Sauvignon → deep pigmentation. Usually a good thing, but a bit misleading in this case because it doesn’t match up with intensity of flavor.

 

Interestingly, several of the articles I found that were theoretically in support of Norton angled heavily towards negative comments about Norton’s flavor profile (this profile at Appellation America is a good example.) Ergo, un-foxyness may be the best thing that can be said about Norton. Still, I’ll do my best to keep an open mind. If anyone has anything to contribute about growing V. aestivalis and/or making or drinking wine derived thereof, I’d welcome the education.

Lactose, Lactic acid, Lactase, or the Lac(k) Thereof (and why wine is still okay)

 Some time ago, an inquisitive mind inquired of me as to whether being lactose intolerant could affect the sufferer’s tolerance of wine that has undergone malolactic fermentation. Fair question. “Lactose” and “lactic” are obviously related, and thinking about an intolerance to the “lactic” in wine is a sensible leap with everyone and their brother speculating over what causes wine headaches and the like (derivatory of the overarching food intolerance fad, I expect.)

 The good and the bad news is that lactose intolerance has no bearing whatsoever on the ability to digest malolactically-fermented wine. Good news, as the lactose-intolerant among us can drink wine without reservation. Bad news, as the lactose-intolerant among us are equally as enlightened as everyone else as far as identifying a cause of the wily wine headache, i.e. still in the dark.

 Short answer: lactose intolerance is unrelated to the ability to tolerate wine that has undergone malolactic fermentation.

Longer answer: Most people who react poorly to lactose suffer from an intolerance, not an allergy. Allergies are inappropriate immune responses to specific epitopes, which can be thought of as molecular shapes. An intolerance, on the other hand, isn’t necessarily an immune response. Lactose intolerance is caused by a deficiency in the enzyme responsible for breaking down lactose in the small intestine. Since we can only absorb lactose after it has been broken down into its component parts – glucose and galactose – a lactase deficiency means that undigested lactose builds up in the intestines to cause bloating, diarrhea, gas, and other discomforts. Unlike lactose, lactic acid can be absorbed without first being acted upon by the lactase enzyme.

Incidentally, even if lactic acid absorption was somehow related to lactose absorption, quantity would be a pertinent consideration. Milk contains 2-8% lactose, i.e. relatively a whole lot, while wine contains much less than 1% lactic acid.

In conclusion, then, the lactic acid in wine should be of no concern to most people who need to avoid lactose. A glass of wine makes a far friendlier companion to a good dinner than a glass of milk, don’t you think?

Word of the Day: Delestage (and 2008 Folie a Deux Napa Valley Merlot)

Délestage – (‘dehl-luh-STAJ’) aka “rack and return” (though the French sounds much more refined and romantic, as usual.) Refers to the practice of repeatedly draining fermenting red wine off of its skins through a screen that traps some portion of the seeds, then returning the drained-off juice to continue fermenting on the skins, but minus the seeds entrapped in the draining process. Fewer seeds = lower seed-to-juice ratio = less extraction of seed tannins into juice = less tannic wine.

You know that it can’t really be that simple. There are two reasons why just describing the mechanics of the operation is inadequate. First, the “rack and return” process does more than just remove seeds. Like other methods of cap management*, the process also douses the floating grape skins. Unlike some other methods of cap management, délestage generally incorporates a lot of air into the must when the juice is pumped back over the skins.

Besides stimulating their growth, oxygen discourages fermentation yeasts from producing unsavory cooked cabbage and onion-like sulfides. Oxygen also has far-reaching and often poorly-understood effects on myriad elements of wine chemistry. Tannin polymerization, for example, is influenced by oxygen in complex ways that seem, in general, to lead to softer and rounder wines In fact, the role of oxygen in winemaking is so very complex that I’m going to refrain from saying any more about it here for fear of perjuring myself. In any case, the influence of délestage on a wine can’t just be attributed to removing seeds; oxygen must play a part, too.

The second reason why délestage is more complex than its mechanical description comes from our understanding – or, rather, our lack of understanding – of tannins themselves. We once separated tannins into the two broad categories of seed tannins and skin tannins. Seed tannins were bad: harsh, bitter, and green. Skin tannins were better: softer and malleable. In this context, délestage makes a lot of sense. Decreased exposure to bitter seeds during fermentation should reduce harsh, bitter flavors.

For better or for worse, tannin chemists, led by Dr. Jim Harbertson at WSU, have shattered this simplistic understanding. Tannins are polymers of flavon-3-ols. According to Harbertson’s work, longer tannins are usually perceived as more astringent, yet seed tannins are about a third of the length of skin tannins, averaging ten instead of thirty units. On the other hand, seed tannins take longer to extract than skin tannins; even though seed tannins outweigh skin tannins in magnitude, they release more slowly. To add yet another layer of complexity, the make-up of each tannin polymer influences its sensory characteristics in addition to its sheer length. And even then tannin experts haven’t yet deciphered what happens to tannins over time to make well-aged wine seem softer and less harsh than its youthful counterpart. For more on this topic without delving into the scientific literature, try this palatable Wines and Vines article.

The upshot of how to use délestage in the face of all of this complex chemistry? Taste, taste, taste. I’m no winemaker, but isn’t this self-evident? Superb winemakers have been making superb wine for centuries before anyone ever named or knew of a flavon-3-ol. Intuitively, it makes sense that removing seeds will reduce seed-y flavors. If that makes your wine taste better, go for it. As for oxygen, even if it remains the great unknown variable, scientific uncertainty doesn’t invalidate your taste buds.

*Cap management – grape skins are pushed, parachute-like, to the top of the must by CO₂ bubbles created by the fermentation process, creating a “cap” of skins that can literally float above the surface of the must. Free from the protective effects of alcohol and acid and exposed to air, this cap will rapidly submit to spoilage microorganisms if not frequently reincorporated into the must. Hence, in making red wines, the “cap” must be “managed.” 

The fact sheets for these wines state that it they were “fermented using the Délestage method.” Without tasting the délestage and non-délestage samples side-by-side, I can’t help but think part of the benefit of using “rack and return” is being able to incorporate the word “délestage” into promotional materials.

 2008 Folie à Deux Napa Valley Merlot ($18 on the winery website) – Purple-tinged garnet red. Fairly monochrome but very pleasant sweet cherry nose, releasing a bit of cinnamon and clove heat over a few sniffs. Assertive Maraschino cherry hit up front – warm, round, and sweet – made less cloying by overtones of baking spices. A bit alcoholic on the finish with very spare tannins. Pleasant fruit flavors overall, but just a bit too much heat and alcohol for its own shoes.

Folie à Deux Napa Valley Cabernet Sauvignon ($24 on the winery website) – Looks like cranberry juice and smells a bit like cranberry juice, too: bright, astringent, simultaneously fruity and herbaceous. Full, sweet, black raspberry and cherry jam fruit is satisfyingly mouth-filling and sweet before disappearing into an acidic, refreshing finish (again, not unlike cranberry juice.) More tannin in the nose than on the palate with a smooth and fairly light aspect overall. Definitely not a big, chewy, rich, cabernet, but very tasty for a light-weight.

Phenols in train-ing?

I recently received a fascinating email that occasioned my learning a few new and interesting things about phenols.

To excerpt from the intial email:

“…(the taster) commented on the consistent style of the wines as having big but very soft tannins. I told him (as I have told so many others but kind of tongue in cheek) that the softness of the tannins was a result “train settling, ” Because our barrel room is located under a railroad overpass, the barrels are gently vibrated with each pasing train and that this gentle vibration helps create longer phenolic chains. The wines are gently “shaken not stirred” 21,000 times/year (that’s how many trains pass overhead each year. [The taster]was quite intrigued by this explanation and suggested that I should see if, in fact, there might be something to that theory. And so… I am asking you, do you think there could be any merit in thinking that the gentle vibration of our barrels some 21,000 times/year could contribute to the softness of the tannins in our wine?”

And from my response:

“…Though I really don’t know much about tannin chemistry, curiously enough, I work and am friends with two experienced grad students who research that very topic. One of these gentlemen has made wine in Argentina for ten or so years, consistently with an interest in polyphenol chemistry, and can probably rightly be considered an expert on the topic. I consulted them on your question and will do my best to summarize what they had to say. Please keep in mind that this is a very complicated question dealing with a very complicated and not fully understood topic on which active research is underway.
 
First, your assumption that longer phenol chains (i.e. a higher degree of phenol polymerization) are directly associated with ”softer” or less-astringent tannins is not true. You are certainly not the only one to anecdotally think that longer tannins are softer tannins, but several published studies conducted over the last ten years have disproven the theory. Longer chains, appear to be more astringent, if slightly less bitter than shorter ones, with the relationship being non-linear and of relatively small magnitude. Stephane Vidal at the INRA Montpellier (an eminent French agricultural and sensory research institution) has done much of this work, along with Ann Noble’s group at UC Davis.
 
Second, there is no reason to think that vibrations will increase the degree of polymerization of your wine’s phenols. Sorry! To the best of our (my friends’ and my) collective knowledge, there is no data bearing on this question. From simple chemistry and common sense, however, vibration should increase polymerization only if addition of mechanical energy was important in increasing the number of collisions between phenol molecules. Since phenols are already in high concentration in wine, this may not be the case. Moreover, as already mentioned, the “old winemakers’ wisdom” that increased association between phenols and of phenols and acetaldehyde is responsible for the softening of red wines with age has been debunked by combining chemical and sensory analysis.
 
I’ll admit that the first reaction of my friends to your train-vibration hypothesis was “typical winemaker B-S, looking for a way to distinguish his wines.” Depending on your point of view, I tend to be either a bit more gullible or a bit more creative. Regardless, a bit of musing led us to think of one potential way of relating astringency to your locamotive proximity. As I’m sure you know, yeast lees can have a softening effect on wine. Yeast autolysis, a particular kind of cell death, causes release of polysaccharides and, in particular, mannoproteins into the surrounding wine. Stirring up the lees encourages cell-wall breakdown and the release of these compounds. Yeast-derived polysaccharides associate with phenols (the particulars depend very specifically on the types of polysaccharides involved, but this is a fair generalization) and stabilize the folded molecules, hindering denaturation and the exposure of hydrophobic regions that aggregate to form large and insoluble polymers. Thus, yeast autolysis reduces phenol precipitation. Since the perception of astringency occurs when tannins precipitate, yeast autolysis generally reduces astringency. Returning to your train vibrations, it is possible that the vibrations gently but frequently agitate the lees in your barrels, stirring up dead yeast components, and keeping more of your phenols in solution. This is purely hypothetical, but it’s a thought.
 
Finally, and perhaps needless to say, all of this depends a great deal on the varieties with which you are concerned, your particular vinification practices, and exactly what Paul (and you, and other tasters) mean by “big but soft tannins.” I can see from the Barrister website that you deal with a lot of varieties and your email implies that Paul saw this softness of tannins throughout the reds which, naturally, suggests association with your winemaking techniques and style. With respect to the yeast-stirring hypothesis, how often you rack and the strain of yeast you use and such will be relevant. ”
 

Now, if I could only induce this winemaking gent — or another with access to a busy bit of railway — to store identical batches of wine under “trained” and “untrained” conditions and perform liquid chromatography analyses for phenols and sensory comparisons between the two conditions. Not precisely a widely-applicable study, but unquestionably interesting. And who knows? If the good-vibrations wines performed significantly better than their restive counterparts, there might be a market niche for gently vibrating wine aging platforms!

Microorganism of the day: Schizosaccharomyces pombe

Schizosaccharomyces pombe

 WHAT: a yeast that divides by fission (division in half, rather than budding like most yeast, hence “Schizo”), ferments sugars (hence “saccharomyces” or “sugar-loving”), and was first identified in African millet beer (hence “pombe” meaning “beer” in Swahili.)

Relevance to wine: S. pombe has traditionally been grouped among the spoilage organisms by the wine industry. Unlike its friendly, helpful cousin Saccharomyces cerivisiae (the major player in wine fermentation and bread making), S. pombe tends to throw off a lot of icky-tasting or -smelling byproducts as it turns sugar into alcohol. Sulfur is not a desirable aroma in wine!

S. pombe has one truly nifty feature, however, that is earning it a useful place in winemaking. It can ferment malic acid into alcohol. Malic acid is one of the three major acids in grape juice that carries over into wine (along with tartaric acid and citric acid.) Its fresh, fruity acidity is a boon in fresh, fruity wines, but too much and you’ll find yourself puckering.

The usual savior of malic acid overload is malolactic fermentation – conversion of malic acid into lactic acid by lactic acid bacteria after alcoholic fermentation yeasts have worked their magic. Great for rich, buttery wines — lots of unctuous flavors come along with the malic-to-lactic conversion — but not so great if you were going for a fresh and fruity style in the first place.

Could S. pombe help? What about the sulfur aromas and other issues?

A fair bit of research has investigated ways of using S. pombe in wine: to permit the inclusion of rotten grapes in Sherry and the potential of using genetic engineering to create a Schizoid-Schizosaccharomyces that keeps the good and does away with the bad, for example.

 Lallemande, a major yeast comapny, has recently released ProMalic® “for naturally lowering juice acidity,” based on S. pombe. The yeast is submerged in the wine in something like a big yeast tea-bag, allowed to steep until your pH is up (and your malic acid down) to where you want it, and then pulled out before the yeast gets carried away with making other less-desireable stuff.

Some super-enthusiastic yeast folk from the Forsberg lab at the University of Southern California say that they have tried fermenting beer with their pombe with results that suggest skunk cabbage more than the local brewpub. With a respectful nod to classic eastern African beverages, however, they note that their attempts involved neither millet nor traditional methods. Anyone tasted any African millet beer?

Some home-brewers out there are apparently giving it a try: http://www.homebrewtalk.com/f12/pombe-brew-138520/ 

For the truly curious yeast fiends out there, see the Forsberg lab Pombe pages at http://www-bcf.usc.edu/~forsburg/main.html#what for a truly excellent discussion of pombe in all its glory.

What are phenols?

Phenols are aromatic alcohols, which means that they are molecules that consist of a six-carbon aromatic ring with a hydroxyl (OH) group attached to one of the ring carbons. To an organic chemist, “aromatic” means that the ring includes an unpaired electron that is shared among all of the carbon atoms of the ring and. Electrons are like people: they like to be paired, are a little unstable when single, and tend to react with other molecules in search of a partner.

The most basic phenol is represented as

 More complex members of the phenol family are distinguished by having other things attached to one or more of the other ring carbons.

Because phenols have a free-floating (“delocalized”) electron, they form very strong hydrogen bonds with each other and other compounds with an unpaired electron. Non-covalent bonds – the bonds that form in-between molecules, including hydrogen bonds – help govern melting and boiling points: the stronger and more numerous the non-covalent bonds, the more energy input it takes to break them. When non-covalent bonds break, molecules move around more and move further from each other, resulting first in the melting of a solid and then, when the input of even more energy causes even more movement, in the boiling of a liquid.

ERGO: phenols have very high melting and boiling points. ERGO: phenols are solids at room temperature. ERGO: phenols, including the colorful ones that make red wine red and the flavorful ones that add depth and character to wine exist in wine as suspended particles. ERGO: they can be removed by filtering. ERGO: enough filtration can cause a wine to lose color, flavor, and mouth-feel (in part) because phenols are lost in the filtration process.

There are three major categories of wine-related phenols: p-hydroxybenzoic acids, cinnamic acids, and flavanoids. What do they do for wine? Phenols are responsible for a major part of the color and flavor of wine. They are, In fact, so important in so many ways that I’ll save more details for another post devoted solely to the topic.

For now, back to Fenema’s Food Chemistry!

The “hows” of sulfites, part I: measurement

What is the aerator-oxidator apparatus, and why am I spending so much time using it?

The short explanation: I’m measuring wine sulfites.

The long explanation: I’m using the aerator-oxidator method to measure free SO_2.

A good day measuring free SO2: no breakage!

The molecular formula for the sulfite ion is SO₃^2-, formed when sulfur dioxide, SO₂, reacts with water to form the bisulfate ion, HSO₃^-, which can then dissociate to SO₃^2- + H⁺. All of these forms are found in wine – wine is an aqueous solution – but SO₂ is what winemakers actually add, usually in the form of potassium metabisulfite or K₂S₂O₅.

For as casually as we toss around the phrase “wine sulfites,” their behavior is remarkably complex. Potassium metabisulfite, sold by the kilo in winemaking supply catalogues, is a salt that releases potassium and molecular sulfur dioxide when dissolved in aqueous solution. We care about molecular SO₂; this is the “species” of sulfur to which we can ascribe antimicrobial properties. Molecular SO₂, however, quickly dissociates into ions which either remain “free” in the wine or become “bound” to other solutes like acetaldehyde, pigments, and sugars.

SO what? Beyond plain old cleanliness and sanitation, SO₂ is probably the most widespread antimicrobial agent used to protect wine, During yeast-driven alcoholic and bacterial-driven malolactic fermentation, (the right kind of) yeast and bacteria in your fresh grape must is a good thing. Thereafter, the presence of most yeast and bacteria in fermented wine means contamination, nasty flavors, or even unsafe wine.

SO₂ diffuses into yeast and bacterial cells and does all sorts of destructive things: damaging DNA, glomming up the structure of important enzymes, and reacting with nutrients so that the microorganism can’t use them for growth (like a game of chemical keep-away.) The end result isn’t always to kill the bug, but the durn thing isn’t likely to keep multiplying. Like humans and most other living things, yeast and bacteria don’t reproduce well when they’re starved and stressed. Microbial susceptibility to SO₂ varies: 20mg/L will suppress some, while others withstand upwards of 40mg/L molecular SO₂.

[By the way, American wineries are allowed to use as much as 350mg of SO₂ per liter, but any wine containing more than 10mg/L must be labeled “contains sulfites.” Wine-dwelling microorganisms can naturally produce as much as 100mg/L in the process of fermentation. Most wineries aim for sulfur dioxide levels around 50-100mg/L, but that’s a big generalization.]

SO back to the original question: what is this great big fragile glass apparatus, and what does it have to do with SO₂? The only name for it seems to be “the aerator-oxidator apparatus” and – go figure – it is used to measure SO₂ levels by the “aeration-oxidation” method. Air bubbles  through a wine sample – “aeration” – carrying volatile “free” bisulfite up and through a condenser and into a vial of hydrogen peroxide, where it reacts with hydrogen peroxide and water to form sulfuric acid. In other letters, SO₂ + H₂O₂ → H₂SO₄, an “oxidation” of SO₂.

Adding a color indicator that flips from royal blue to teal when pH flips from acidic to basic means that sodium hydroxide (a base) can be added to the hydrogen peroxide-plus-sulfuric acid solution to negate the acid. This is just a good old acid-base titration: when the amount of slowly-added sodium hydroxide exceeds the amount of sulfuric acid, pH and color both rapidly change. The final step is calculating sulfuric acid from sodium hydroxide and sulfur dioxide from SO₂. Remember all of those different forms that sulfur dioxide takes in wine? The aerator-oxidator method can be adjusted to measure either free or total SO₂. But wait! I thought that the antimicrobial form was molecular SO₂, and you say that we’re not measuring that? Conveniently, molecular SO₂ can easily be calculated from free SO₂ and the pH of your wine.

To measure free or molecular SO₂, the wine sample is acidified and chilled for the duration of this whole bubbling business. Acidifying the wine – increasing the concentration of H⁺ — forces the equilibrium between SO₂ +H₂O ↔ HSO₃⁼ + H⁺ entirely to the SO₂ side. All of those aforementioned types of SO2are volatile and can make the trip from the wine-containing flask up through the condenser and into the H₂O₂ –containing flask EXCEPT the bisulfate ion that is bound to other compounds in the wine. (Incidentally, chilling the wine sample ensures that heat energy doesn’t break any of those bonds apart.) On the other hand, boiling the wine sample provides enough energy to break (virtually) all of the non-covalent bonds, freeing the bound ion, and allowing the “total” amount of SO₂ in the wine to volatilize. Measuring total SO₂, then, is exactly the same procedure as measuring free/molecular SO₂ save that the wine-containing flask is boiled rather than immersed in an ice bath.

The best part of the whole process? It takes longer to explain and understand than to perform. Yes, I did take general chemistry for a reason!  

Pros of using this method: it’s accurate, easy to read (unless, like my lab-mate, you’re color-blind), inexpensive once you’ve made the set-up, and only takes 10 minutes per sample,

Cons: it takes an entire 10 minutes per sample, all of the SO₂ may not bubble out of the wine, any gaps in the glass-to-glass connections allow SO₂ to escape before reacting with the H₂O₂, and the whole contraption is disturbingly prone to breakage by tired/rushed/frustrated grad students.

By the by, why do some people say that they are allergic to sulfites? A .01%-sized handful of the American population lacks the enzyme sulfite oxidase and literally cannot metabolize sulfites; these folk are also asthmatic and usually have trouble breathing and break out in a rash when they consume sulfites (from wine, dried fruit, lunch meat, ect.) In all great likelihood, you aren’t one of those people.