Jamie Goode

The History and Science of Malolactic Fermentation

Jamie Goode — Wed, 01 Jun 2016 00:56:00 GMT

The History

Brad Webb with Ambassador Zellerbach (Photo courtesy of Hanzell Vineyards)

Brad Webb had landed the dream job. It was 1956, and he’d just been appointed winemaker at a new winery, Hanzell, founded by James D. Zellerbach, the wealthy US ambassador to Italy. Zellerbach had spared no expense in pursuing his dream of making classically styled Chardonnay and Pinot Noir to rival the wines of Burgundy. Webb had at his disposal an array of shiny new tanks, casks, and barrels. But there was a problem. The early Pinot Noir that he made in the new facility looked promising, but it wouldn’t do malolactic fermentation (MLF), the conversion of malic acid to the softer lactic acid that usually takes place after alcoholic fermentation.

At the time, a growing awareness of the importance of MLF for high-quality red wines was developing, although a lot of mystery still surrounded this second fermentation. Sometimes it started of its own accord; sometimes it didn’t. Sometimes it finished quickly and easily; other times it dragged on, only to start again at a later stage. One of the inside jokes among winemakers goes something like this: How do you start MLF? Bottle the wine.

The first mention of MLF is thought to be in a 1837 book by Freiherr von Babo. He described a second fermentation occurring in some wines during springtime, when temperatures began to rise, resulting in the release of CO2 and renewed turbidity in the wine. Then, in 1866, the celebrated scientist Louis Pasteur isolated bacteria from wine for the first time. However, he considered all bacteria in wine to be spoilage organisms. A breakthrough came in 1891 when Hermann Müller-Thurgau (yes, that Müller-Thurgau—the grape is named after him) postulated that acid reduction could be because of bacterial activity. This was a bold suggestion; at the time, this change in acid level was generally attributed to tartaric acid precipitation. Müller-Thurgau went on to do valuable work over the next couple of decades in collaboration with a fellow Swiss researcher called Osterwalder.

In 1939, the famous French wine scientist Émile Peynaud wrote an important paper on the role of malic acid in the musts and wines of Bordeaux, pointing out that the absence of MLF was a quality-limiting factor in these wines. “Not only is the acid makeup of the wine completely changed,” he stated, “but [MLF] has an impact on the perfume of these wines and even diminishes the intensity of the color and changes its shade. It is not exaggerating to say that without malolactic fermentation, there would hardly be any great reds of Bordeaux.”

While yeast cultures were starting to become available, no one had successfully cultured the lactic acid bacteria (LAB) responsible for the conversion of malic to lactic acid, with its corresponding softening of acidity. Webb had to resort to experimentation to try to get this fermentation to start. Could the reluctance of his ferments to do MLF be because of the lack of suitable bacterial inoculum in the winery? He tried introducing wine that was undergoing MLF into his tanks, but it didn’t work. Baffled, he turned to John Ingraham, a scientist at UC Davis who had an interest in bacteria. Webb offered his winery as an experimental setting where Ingraham could do trials—after all, in almost all other wineries, MLF took place. So it should be of interest to study a winery where such a fermentation had never occurred. Ingraham was intrigued and willing to use Hanzell as a negative control.

Ingraham had been busy at work, and the timing was good. Research on LAB, which are also important in the production of other foodstuffs, had been flourishing ever since the serendipitous discovery by a dental researcher in Chicago that adding tomato juice to MRS agar, the bacterial growth medium commonly used for making microbial cultures, made these previously tricky bacteria easier to grow in the laboratory. It turns out that tomato juice contains pantothenic acid, a key growth factor for these bugs. Ingraham had isolated 50 strains of LAB from samples of dry wines and lees from California wineries. Of these, with the help of Webb, he selected one that showed the most promising characteristics. They named it ML34. Although this was kept secret at the time, it came from a large redwood tank of Barbera in the Louis Martini winery in Napa.

In 1959, the two took ML34 to Hanzell to study it in winery conditions. After some trials on a small scale, to their great excitement, they got ML34 to carry out a malolactic fermentation in the winery. To their knowledge, they were the first to do this. But, as so often happens in science, other groups had been working on the same problem elsewhere. In France, Peynaud had achieved this with his colleague Domercq, and a short time before, a Portuguese group had also been successful. But this doesn’t take anything away from the achievements of Webb and Ingraham, who published the results of their work in 1960.

Original ML34 Test Tube at Hanzell (Photo courtesy of Hanzell Vineyards)

How Malolactic Fermentation Works

Malic acid is an organic compound with the formula C₄H₆O_5.It is found in all fruit but is most associated with green apples.

During MLF, the LAB convert malic acid to lactic acid, which is softer tasting and less powerful. There is a pH shift and a loss of acidity, the degree of which will depend on how much malic acid was present in the grapes in the first place. MLF happens in almost all red wines, and the choice of whether or not to let it occur in whites is usually a style decision on the part of the winemaker.

But there is a lot more to MLF than this acidic conversion. Just as yeasts have a significant sensory impact above and beyond simply converting sugar to alcohol, LAB change the flavor of wine in ways that are only now becoming appreciated as scientists take a closer look at the activity of these microbes. The sensory impact of LAB can be both positive and negative, and it depends largely on the strain of bacteria that is doing the fermentation, the presence of certain substrates in the wine, and the conditions under which the fermentation takes place.

Grape juice and wine are hostile environments for microbes. On the one hand, grape juice has what microbes really want: sugar. It’s just that it has rather too much of it, around 200 grams per liter. This creates a strong osmotic pressure that threatens to suck all of the water out of the bugs. Grape juice also has too much acidity. Bacteria like to grow in a higher pH medium, with an optimum level around 6 or 7. Grape juice and wine have a pH of about 3 to 3.8, which is quite acidic. Initially, there will be numerous species of yeasts present in grape juice, as well as many strains of bacteria. As fermentation kicks in, however, the alcohol level begins to rise, and the diversity of microbes is much reduced. After four days or so, only one species remains in any significant numbers: Saccharomyces cerevisiae. This comes in many strains, and even wild or native ferments will be carried out largely by this species. There are just a few species of LAB that can cope with the unfavorable environment, and by the time they get to carry out their second fermentation, which normally begins as alcoholic fermentation is finishing, they must contend with extra problems. These include high alcohol, possibly the presence of some sulfur dioxide, and a medium that has been stripped of many vital nutrients by the yeasts.

Bacteria are so tiny that they can be pretty hard to classify, and it’s only with the advent of modern DNA techniques that things have become clearer. The first distinction is between the shape of the bacteria: those that look like rods are called rods, and those that look like little spheres are called cocci. These are further classified depending on the way they carry out metabolism. In this way, LAB can be classified as heterofermentative or homofermentative, depending on how they ferment sugars. The distinction here is that the former ferment sugar to produce lactic acid, acetic acid, and ethanol, while the latter ferment sugar to produce only lactic acid.

Getting Technical

The four genera (a genus is the level of classification just above species) of LAB found in wine are Oenococcus, Leuconostoc, Lactobacillus, and Pediococcus. Of these, there’s one species, Oenococcus oeni, that is of particular interest. Winemakers want this species to carry out the second fermentation, as it produces the best results, and it usually does. Oenococcus oeni is pretty resistant to the hostile conditions found in wines just after alcoholic fermentation has finished—much more so than the other three genera, which tend to grow only when the pH is a little higher.

The LAB feed off any sugars that remain in the wine after the yeasts have finished (yeasts leave a bit of hexose and pentose sugars) and grow in number. The bacteria need less than 1 gram per liter of sugars to create a biomass sufficient enough to carry out MLF. Alongside this activity, they are also able to convert malic acid to lactic, but it’s worth emphasising that this is just one of their metabolic activities.

During the course of their growth, LAB are able to secrete a range of flavor compounds into the wine. This is where bacteria have the potential for enhancing quality or impacting it negatively. Indeed, it is really instructive to taste experimental wines inoculated with different strains of cultured malolactic bacteria. This sort of comparison shows the degree to which MLF can change the taste of wine in addition to modifying its acidity. But there’s very little written on the subject, and most winemakers allow MLF to happen spontaneously, trusting that they are going to get a decent strain of malolactic bacteria.

LAB produce acetic acid from metabolizing sugars, increasing volatile acidity. The amount that volatile acidity increases depends on how much sugar they metabolize, so this is a potential concern when MLF starts before the yeasts have finished their job. Again, this VA increase is quite strain dependent. As well as degrading malic acid, certain strains of LAB can degrade tartaric acid, too. Pasteur called this the tourne disease, and it’s a serious issue. Fortunately, very few strains can do this.

One of the most well-known sensory impacts of LAB is the production of diacetyl (2,3-butanedione). It’s formed by LAB from citric acid and has an odor detection threshold of 0.2 milligram per liter in white wines and 2.8 milligram per liter in reds (this higher threshold is because there are so many other strong flavors in red wine that it’s harder to pick up the dactyl). Diacetyl has a distinctive buttery, creamy character that, at low levels, can be attractive. But higher levels of diacetyl aren’t pleasant and can be considered a fault, and in some circumstances, any detectable diacetyl is undesirable. The factors favoring diacetyl production are the presence of oxygen, high concentrations of citric acid and sugar, temperatures below 18 °C, and the removal of yeast cells before MLF. The levels can be reduced by the presence of viable yeast cells and the addition of sulfur dioxide. Diacetyl can react with cysteine, an amino acid-containing sulfur, to produce thiazole, which smells of toast, popcorn, and hazelnut. The actual level of diacetyl isn’t closely correlated with the extent of MLF, however. Its production is strain dependent, and dependent on the precursors present as well. It’s entirely possible that a part-MLF Chardonnay could show more buttery flavors than one with full MLF.

The production of volatile sulfur compounds (VSCs) is another way LAB impacts wine flavor. These are produced by the metabolism of the sulfur-containing amino acids cysteine and methionine, and the VSCs that result can be good or bad, depending on the context. These are the compounds implicated in reduction problems in wines: sulfites, disulfides, thioesters, and mercaptans (thiols).

The bitter-tasting compound Acrolein is produced by some LAB strains by the degradation of glycerol. It’s undesirable at any level, but thankfully, only a few strains produce it.

An often-discussed byproduct of MLF is the formation of biogenic amines. All fermented products contain them, but malolactic bacteria are capable of producing reasonably high levels. They are formed by the decarboxylation of amino acids, and the major ones found in wine are histamine, tyramine, putrescine, and phenylethylamine. They can have a range of effects on people sensitive to them, including headaches, breathing difficulties, hypertension or hypotension, allergic reactions, and palpitations. People differ in their sensitivity to them, but their presence in wine is undesirable. Not all strains of LAB are able to decarboxylate amino acids. The higher the wine’s pH, the more complex the range of bacterial species that will grow in it. As a result, there will usually be higher levels of biogenic amines. White wines, then, which usually have a lower pH, tend to have lower levels of biogenic amines. Although sulfites are often blamed for allergic reactions to wine, it’s much more likely to be the biogenic amines that are responsible, although this hasn’t been proven conclusively. Using selected strains of LAB to inoculate for MLF is one way to reduce the risk of biogenic amines in wines. Currently, there are no regulations for biogenic amine levels in wine, but this could change.

But biogenic amines aren’t the worst thing produced by certain strains of bacteria. Ethyl carbamate is a carcinogen found in many foods and drinks, and it’s formed through reactions between alcohol and a precursor such as citrulline, urea, or carbamoyl phosphate. The main contributor to ethyl carbamate levels in wine is urea formed by yeasts from the degradation of arginine, but even after alcoholic fermentation, some arginine (0.1 to 2.3 grams per liter) remains in the wine, and LAB can produce citrulline as an intermediate in the degradation process of arginine. The USA has regulations for maximum levels of ethyl carbamate in wine, set at 15 micrograms per liter. Canada allows up to 30 micrograms per liter, and the EU has no uniform maximum level. Typically, wine contains around 10 micrograms per liter, while fortified wines contain around 60, but these levels can vary.

Some strains of LAB are thought to have glycosidase activity, which has a positive effect. Many of the flavor molecules in grape juice are in a chemical state where they need to be converted during fermentation to be active. A glycosidase is an enzyme that removes sugar groups, and in this case it can hydrolyse sugar-bound monoterpenes to release them as volatile aromatic monoterpenes. There is also evidence that LAB are able to synthesize esters, which are fruity-smelling compounds, but this needs to be further verified.

LAB are able to remove green flavors from wine. The reduction of vegetative or grassy aromas that can occur during MLF is thought to happen through the metabolism of aldehydes such as hexenal, which contribute to these green flavors (along with methoxypyrazines). LAB are also thought to be able to improve the body of a wine, through, for example, the production of polyols and polysaccharides.

Though MLF affects flavor in a variety of ways, the most significant occurs through its impact on acidity. It usually increases pH (makes the wine less acidic) by 0.1 to 0.3 units and reduces TA by 1 to 3 grams per liter.

Conclusion

Most MLFs are spontaneous, but it is becoming increasingly common to inoculate with bacterial cultures. These come in different forms. There are freeze-dried cultures that need to be reawakened, making a starter culture with which to inoculate the wine, and there are now active dried cultures as well. Cultured bacteria are a bit fussier than cultured yeasts and need to be handled carefully. Previously, it was thought that the only safe time to inoculate was after alcoholic fermentation had completed. This was because of the risk of volatile acidity increasing (if the bacteria are eating lots of sugar), and the risk of an incompatibility between the bacteria and yeasts, causing fermentation to stick. Now, however, co-inoculation with compatible strains of bacteria and yeasts is commonplace. The advantage is that as long as the two microbe strains work well together, the result is fruitier wines (especially reds), in part because any diacetyl produced by the bacteria is used by the yeasts. As such, co-inoculation is particularly advised for less expensive, fruit-forward red wines, with the added bonus of no risk period between the end of alcoholic fermentation and the beginning of MLF.

Clearly, there’s a lot more to lactic acid bacteria than simply the conversion of malic to lactic acids. These microbes have an important role to play in winemaking, one that we are only just beginning to understand.

How Does a Better Understanding of Wine Science Fit with Understanding Wine?

Jamie Goode — Tue, 27 Oct 2015 18:32:00 GMT

I came to wine from a background as a scientist. I spent six years at university—three each for my undergraduate degree and doctorate—and so I became pretty good at thinking scientifically. Many of you will have come to wine from careers or studies that are similarly quantitative. You can measure things; you can formulate hypotheses; if you study enough you can nail down the answers. Cause and effect.

And then you hit wine. It is frustratingly imprecise. There are so many inconsistencies, it seems, in the world of wine. Wine education itself is often built on a frail scientific basis, with many of the "truths" simply passed down from generation to generation, without firm backing from research.

In particular, there are three areas that struck me as scientifically imprecise and dubious when I entered the world of wine, coming from my own particular scientific perspective.

These were biodynamics, terroir and taste. I’ll consider each in turn. Over the years, my attitudes have changed. I now think that science is an incredibly useful tool for understanding wine, but it’s not the only way we can understand wine. And a broader approach—not a sort of scientific fundamentalism—is needed if we are to make sense of this most complex and wonderful of liquids.

Biodynamics
When I first heard about biodynamics, I couldn’t quite believe it. Here are otherwise smart, capable people being taken in by a system of agriculture with a completely ludicrous theoretical background. Just what were these energetic life forces that were being invoked? And how could the alignments of the planets affect things here on earth? And the homeopathic preparations, fermented in strange bits of animals?

Biodynamics does seem to clash with a scientific understanding of soils and plant physiology. But it would be a mistake to dismiss it without looking a little closer. There are many aspects of biodynamic practice that could be having an interesting effect on grape vines, aside from the rather unusual mechanistic claims of some of its practitioners. For example, scientists are beginning to understand the importance of soil microbes in vine growth. There seems to be a cross-talk between bacteria and fungi in the soil and the vine roots, to the extent that soil microbes can alter the physiology of the vine, causing changes in the grape composition and then in turn the taste of the wine. Much of biodynamic practice is focused on soil health. Who is to say that the various preparations and composting don’t, for example, help the vine find a better balance, producing physiological ripeness in the grapes at lower sugar levels in warm climates, via changes in soil microbial life? There are real limits in our understanding of all the complexes processes that take place in the vineyard. Sometimes people working the vines might have more to contribute to our understanding than the limited scientific studies that have taken place in the vineyards. It’s just that we need to adjust our language in order to have a conversation, and as scientists we can’t always insist that our language is the only legitimate one.

Terroir
I was trained as a plant scientist. I learned about root uptake, and how all that plants took from the soil were water and soluble mineral ions. So when I heard people talking about how you could taste the vineyard origin in the wine, I was immediately sceptical. How could a place imprint itself on the grapes so much that subtle changes in grape composition would yield wines—after a complex fermentation process—that would taste completely different, depending on small differences in aspect, climate and subsoil?

Once again, with terroir we meet the gap between what we know to be the case from the experience of those working in the vineyard, and what scientists have been able to show by proper studies. It’s hard to control all the variables in the vineyard, and the sort of work needed to produce publishable results is time-consuming and expensive. And it often doesn’t address the really interesting questions. But when we taste wines from adjoining plots and spot the difference, which in this case we assume must be due to changes in soil type, we want some answers. What is causing this difference? What’s the mechanism? No one has really studied terroir in a truly satisfying way. The result is that terms like "minerality" and "chalky" and "slatey" and "gravelly" get used without any consensus as to what they really mean, and with a vague sort of mechanistic claim implicit in their use.

But science can inform us about terroir. The more we understand vine biology, ranging from fruit uptake to grape composition, the more we can link this with factors such as soil water relations, soil chemistry, soil microbiology, the effect of ultraviolet light exposure, canopy architecture and the role of thermal amplitude, to name but a few of the physical factors that constitute a "terroir." It’s about piecing together a limited set of scientific data points with a set of real world experiences, and then doing some intelligent interpolating to fill in the gaps. Hopefully, then, a clearer picture emerges.

Taste
As a science editor, I’d dipped into the physiology, psychology and neurobiology of flavor perception. I knew that flavor was constructed by the brain from sensory information coming from the senses of smell, taste, touch and vision. I knew that individual differences existed; for example, to the extent that people are either tasters, non-tasters, or supertasters, they have different abilities to taste the bitter compound propylthiouracil (PROP). And I’d seen studies that seemed to indicate that our experience and knowledge of what we were drinking could also change our perceptions. So to suddenly encounter a wine education system where the taste of a wine was regarded to be a property of the wine, and where a careful student with a normally operating sense of taste and smell could nail down the taste of the wine with an almost scientific accuracy, was quite a shock.

I still think that the wine world operates with a rather simplistic understanding of taste and smell. The perception of wine is wonderfully complex, and there’s currently a renewed interest in flavor perception more generally among scientists. The modern view is that flavor is a multimodal sense, wherein smell, taste, touch and vision—and even hearing—are merged together in complex ways at a preconscious level to create this unified sensation of flavor. We don’t operate like measuring devices; instead, we have a sense of flavor that’s really flexible and different factors change our perception without us being aware of it. This flexibility and adaptability helps us navigate the real world, but in wine education, where to pass exams we have to behave like machines, it causes no end of problems.

What we "get" in a wine depends on lots of things. It depends on the vocabulary we have for wine (words act like pegs we can hang sensations on), it depends on what we pay attention to in the wine, and it depends on other factors such as our previous experience of wine, our mood, how hungry we are and the time of day. It also depends on our own unique biology: for example, people differ in their ability to taste bitter substances, in their salivary flow (which affects mouthfeel of red wines) and in their olfactory receptor repertoire.

It’s good to try to taste as scientifically as possible. But it is an illusion to think that if we all worked harder, trained more and became more expert in our use of flavor descriptors, that we’d all reach the same conclusion about a wine. I think the wine world needs to adopt a more sophisticated, nuanced view of what happens when we taste wine, and the notion that we bring quite a bit to the wine tasting experience needs to be factored in to wine exams that have a blind tasting component. Yes, learning does help to iron out some of these creases – because we share our impressions, adopt an aesthetic system of wine, and grow more alike in our assessment of what constitutes a good wine. But there’s no room for control freaks in wine tasting, because so much of what we do in perception is beyond our conscious control.

Closing Thoughts
Wine education is incredibly valuable, but the whole process can be quite frustrating for ambitious, driven wine professionals who want to get to the top. This is because wine is so inexact. This is because we can’t really address its complexity in any rigorous scientific way, and also because the read-out at the end of the process is a sensory one: us tasting wine. Research on interesting questions in the vineyard is so complex and expensive that very little of it is done (who would fund it?), and our sensory systems are very complicated. The perception of flavor is an area of intense current interest because it is fully multisensory, and cross-disciplinary approaches involving neuroscience, physiology, psychology and philosophy are now producing some very interesting results. We need to re-examine the way that tasting is taught in wine education to take these latest results into account. For now, there is a frustrating mismatch between the way we are taught to taste (and are examined in blind tastings) and the real life experience of wine. Science has a lot to offer wine, but it needs to be considered as just one of the useful tools for helping us understand this most remarkable of liquids.

The Taste of Wine: Acid, Sweetness, and Tannin

Jamie Goode — Mon, 11 Mar 2013 11:51:00 GMT

Continuing on from my last article for the Guild, which looked at the visual appearance of wine, this time I’m going to focus on aspects of taste.

Here the term taste is used to refer to the experience of wine in the mouth, but we can’t discount the sense of smell here, because it is pretty much impossible to taste a wine without smelling it at the same time. This is because of retronasal olfaction: volatile molecules get into the smell receptors in the nose through the back of the mouth. And we also need to include the sense of touch here, because as we’ll see later, that’s largely how we sense tannins in the mouth.

Now taste is complicated. The actual sensory experience gained from our taste buds and touch receptors in the mouth is processed sub-consciously by the brain before we are aware of it. So what we smell may alter how we taste. Even factors such as the colour of wine, or our knowledge about it (including factors such as price) can affect our perceptual experience of taste and smell, because of all the processing of this information that goes on in the brain before we are aware of it.

But let’s try to simplify it a bit, and focus on what happens when wine is in our mouths. On our tongues there exist a number of taste buds, each containing a variety of taste receptors. These detect five different modalities, although there is some discussion about whether there might be more. They are sweet, sour, bitter, salty and umami (the savouriness of amino acids). As well as these, there are receptors for heat and touch. So let’s consider the different elements of wine and how they are detected in the mouth.

Acidity

First of all, acidity, which is sensed as sourness. Acid is a vital component of wine, helping to make it taste fresh, but also helping to preserve it. White wines with higher acidity usually age better than those with low. Red wines can get by with a little less acidity because they contain phenolic compounds that help preserve them.

The main organic acids found in grapes are tartaric, malic and citric. Tartaric acid is the key grape acid, and can reach levels of 15 grams a litre in unripe grapes. It’s quite a strong acid and is specific to grapes. In musts it is found in the range of roughly 3–6 grams per litre. Malic acid is abundant in green apples and, unlike tartaric acid, is widely found in nature. Before veraison it can hit levels of 20 grams a litre in grapes. In warm climates, it is found in musts in the range of 1–2 grams per litre, and in cooler climates it occurs at 2–6 grams per litre. Citric acid is also widespread in nature, and is found in grapes at 0.5–1 gram per litre. Other organic acids present in grapes include D-gluconic acid, mucic acid, coumaric acid and coumaryl tartaric acid. Further acids are produced during fermentation, such as succinic, lactic and acetic acids. In addition, ascorbic acid may be added during winemaking as an antioxidant.

This is the bit where it gets quite confusing. There’s no single measurement for acidity in wine. There are two measures, both labeled "TA", but which are different. And there’s pH. And also volatile acidity (VA, largely acetic acid), but we are not going to consider this here, because it is smelt rather than tasted.

Let’s begin with pH. It refers to the concentration of hydrogen ions (known as protons) in a solution. It’s expressed as a negative logarithmic value, which means the lower the number the higher the acidity. And it also means that a solution at pH 3 has 10 times more acidity (defined as protons) than one at pH 4 (corresponding to roughly the range of pH values found in wine, although it can sometimes drop a bit lower than 3). This is where we need to get a bit technical. The ‘acidity’ of an acid depends on something known as its dissociation constant, or pK_a. The lower the pK_a, the more dissociated the acid is, which means it releases more protons into solution. Sulfuric acid has a pK_a of around 1, so it is almost completely dissociated, making it a very strong acid (in terms of protons in solution). Of the organic acids, tartaric has a pK_a of 3.01, which means it is pretty strong. Malic is 3.46, lactic is 3.81 and carbonic acid is 6.52 (which means it has very little dissociation, and is thus a weak acid).

If malolactic fermentation takes place, then the malic acid will be largely converted to lactic acid by the action of lactic acid bacteria. Lactic acid tastes less acid than malic acid, contributing just one proton per molecule whereas malic contributes two. As a result, the pH of the wine shifts upwards through malolactic fermentation by 0.1–0.3 units.

Musts and wines are known as acidobasic buffer solutions. This means you have to work quite hard to change their pH levels. If you add acid to water, you can shift its pH quite quickly, because there is none of this buffering effect. But the presence of other compounds in musts and wines makes it less easy to shift the pH, and it’s a bit easier to shift pH in wine than must. It’s actually tricky to predict the pH of the final wine by looking at the pH of the must, because several things occur during the winemaking process that can change pH. Where acidification is needed, it is usually done with tartaric acid, and as a rule of thumb, 0.5–1 g per litre of tartaric acid is needed to shift pH by 0.1 units. Legally, you could change pH with malic or citric acid, but because these are weaker acids, it would require quite a bit more. And adding citric acid isn’t a great idea where malolactic fermentation is going to take place, because the bacteria turn citric acid into diacetyl, which has a buttery taste and can be quite off-putting. However, I know of some winemakers who use malic acid to make small changes in pH because it doesn’t fall out of solution in the same way that tartaric acid tends to, especially when there is potassium in the must or wine. Some winemakers in warmer climates have illegally used sulfuric acid to change pH, because it is very effective at doing this.

Typical pH levels for a white wine would be 3—3.3, while for reds they would be 3.3–3.6. However, I recently had a New Zealand Riesling with a pH of 2.65, and a while back a South African red that was delicious (and had aged well) despite a pH of 4.0. High pH isn’t necessarily a bad thing: it can confer on a wine a deliciously smooth mouthfeel (think of some Provencale rosés or northern Rhône whites, for example). Generally, though, winemaking at lower pH levels is safer because of the reduced risk of oxidation and microbial spoilage. pH affects the amount of sulfur dioxide (SO₂) that is present in the active molecular form. At pH 3.0, 6% of SO₂ is in the molecular form, whereas at pH 3.5 only 2% is. If the wine gets up to pH 4, then 0.6% of SO₂ is in the molecular form, and so lots would have to be added for it to have any significant effect in protecting the wine. One famous New Zealand boutique winery is known for its rather interventionist red winemaking, acidifying to low pH and then before bottling deacidifying to get the desired pH. This reduces Brettanomyces risk considerably, and helps in other ways, such as fixing colour.

So what about TA? This stands for both total and titratable acidity. Total acidity is the total amount of organic acids in the wine. Titratable acidity looks at the ability of the acid in the wine to neutralize a base (an alkaline substance), which is usually sodium hydroxide. The endpoint is typically pH 8.2, and is indicated by the change of colour of a reagent such as bromophenol blue or phenolphtalein. Total acidity is the best measure to use, but it is hard to measure in practice, so titratable acidity is used as an approximation of this, but it is by definition always going to be a lower figure than the total acidity. So when you see the ‘TA’ of a wine given, you can assume it is the titratable acidity. The units it is expressed in is grams per litre, but here’s another potential source of confusion. Most countries use ‘tartaric acid equivalent’, but in some European countries it is given in ‘sulfuric acid equivalent’, which will be 2/3 of the value of tartaric acid equivalent.

When it comes to the taste of acidity, what is more important, pH or TA? Most of the literature on this suggests that it is the TA that gives the taste of acidity, and so the figure that’s important to look out for is not pH but TA. The confounding factor here is that pH and TA are usually correlated so they are hard to separate, in that low pH wines usually have high TA. But you can get higher pH wines with high TA, and here the acid would taste quite sour. The different organic acids do seem to have different flavours: tartaric is hard, malic is green, and lactic is softer with some sourness. I find that often where warm climate wines have their pH adjusted by tartaric acid, the levels of tartaric acid needed can mean that the acid sticks out as very hard and angular, even where the pH isn’t especially low. Another issue is that added tartaric acid reduces potassium concentrations in the wine (they bind to form potassium bitartarate), and potassium is thought to play an important part in contributing to the weight or body of the wine.

Sweetness

Sweetness in wine is a combination of three factors. First of all, there is sugar itself. This is sensed by sweet taste receptors on the tongue. Second, there is a sweetness that comes from fruitiness. While ‘sweet’ is tasted, some wines can also smell sweet, even though sweetness is a taste modality. Most commercial red wines are dry, in terms of sugar content, but many have sweet aromas from their fruitiness. Very ripe fruity flavours taste and smell sweet even in the absence of sugar. The third source of sweetness is alcohol itself, which tastes sweet. It’s really instructive to try the same red wine at different alcohol levels, where the alcohol has been removed by reverse osmosis or the spinning cone. As the alcohol level drops, with all other components remaining the same, the wine tastes drier and less rounded and full. Where alcohol has been reduced substantially, such as in the new breed of 5.5% alcohol lighter wines, it’s necessary to add back some sweetness, usually in the form of residual sugar. It helps if the starting was had a very sweet fruit profile to begin with, too. For lower alcohol whites, blending in some Muscat or Gewürztraminer, which have sweet aromas, helps quite a bit.

There are a number of ways of making a wine with some residual sugar levels. For some white wines, fermentation stops naturally, or slows to a point where it is very easily stopped by simply chilling and/or adding a little sulfur dioxide. It can of course be deliberately stopped in this way at any stage, but if fermentation is still ticking along nicely then more of both (chilling and sulfur dioxide addition) will be needed. A sweet wine can also be made by blending in must or grape juice concentrate to a dry wine. For commercial wine styles where just a few grams per litre are needed to round the wine off, this is most easily done on the blending bench than by attempting to stop the fermentation at an exact point.

In sweeter white wines and also Champagnes, sugar and acid balance are vital. The two play against each other. Sweetness is countered by acidity, such that a sweet wine with low acid seems much sweeter (and often flabbier) than the same wine with high acidity. In Champagne, a typical dosage for a Brut (dry) Champagne is 8–10 grams per litre, which helps offset the acidity but doesn’t make the Champagne taste sweet. Botrytised sweet wines are prized because as well as concentrating sweetness and flavour, the shrivelling process of noble rot concentrates the acid levels, and the great sweet wines of the world have very high sugar levels as well as high acidity.

Bitterness & Astringency

Tannins are interesting, because they are primarily ‘tasted’ not by the sense of taste, but by touch. But like so many aspects of wine, this is not a simple story. Some tannins are also sensed as ‘bitter’ by bitter receptors in the tongue. Nonetheless, the primary sense of tannins is by touch receptors. Tannins exhibit a mouth-drying, puckering sensation that is an important part of the mouthfeel of red wines, and which can be quite unpleasant in an overly tannic, young wine.

Tannins are very sticky molecules and are particularly good at binding to, and precipitating (removing from solution) proteins. Look at a spittoon next time you have been tasting and spitting red wine. You’ll see unsightly, slimy coloured trails of tannin-protein complexes. Those proteins have been removed from your mouth, and specifically your saliva. Among the salivary proteins that tannins bind are mucins, which act as lubricants in the mouth. They’re really important for keeping the mouth and tongue nice and slippery, and once they are removed, the inside of the mouth feels dry and abraded. It is the tannin-protein complexes, and the loss of this lubrication, that contributes the drying, puckering, astringent sensation of tannins, sensed by touch rather than taste.

It is thought that the larger tannin chains are sensed more as astringent, and the shorter tannin chains are sensed more as bitter. But no one has really done a definitive study on tannin structure and mouthfeel/taste. Tannins form complexes with other components of wine, such as polysaccharides and anthocyanins, which can alter their mouthfeel. It’s not exactly clear how, but it is thought that they become less astringent when this happens.

Both tannins and acids are able to counter the taste of sweetness, providing balance to a wine. Sweetness masks tannins, which is one reason why it has become increasingly common for inexpensive reds to be blended with small quantities to grape juice concentrate. Many commercial reds, especially from California and Australia, have 5-10 g/litre of residual sugar. In a cheap red this can substantially soften the mouthfeel, as well as enhance the fruity sensation of the wine.

Saliva therefore plays an important part in wine tasting. We produce about a litre and a half of it a day, and it’s a complex mixture of proteins, carbohydrates and other molecules. If we taste a lot of red wines in succession, we are likely to be removing the lubricating layer of mucins from the surface of our mouths, making it difficult to assess the mouthfeel of wines accurately. This needs to be borne in mind in the wine trade, where frequently tasters are exposed to 100 or more samples in a day. There is a carry-over effect of astringency, with the gradual lowering of soluble salivary proteins on the repeated ingestion of astringent food, eventually resulting in the rupture of the lubricating film in the mouth and the involvement of deeper layer proteins in the mouth.

The temperature of a wine is important, because it influences the perceptions of astringency, bitterness and sourness, but not sweetness. For example, warm acid is more sour than cold acid, and caffeine is more bitter when it is warm. The perceived astringency of cranberry juice decreases with decreasing temperature, which is surprising. It is likely that serving temperature alters different elements of wine in different ways, making this a complex effect.

In healthy subjects there are large variations in salivary flow rates: high, medium and low responding groups. This could be important in terms of sensitivity to tannins. People with high and medium flow rates perceive astringency sooner. But some studies have found that people with higher flow rates experience more intense astringency. Those with high flow rates are thought to release massive amounts of saliva, and it takes a while to recharge these reserves. This could mean that the sense of astringency is for them a prolonged one. For those with low and medium flow rates, their more rapid mouth re-lubrication makes the duration of astringency shorter, reducing its intensity.

So, here we have sweetness, acidity and tannins all interacting in contributing to the taste of wine in complex ways. Facing up to this complexity can seem a little daunting, but unless we recognize it, then our understanding of the taste of wine can seem a little simplistic and misleading.

The Visual Assessment of Wine

Jamie Goode — Thu, 12 Jul 2012 09:41:00 GMT

It has often been said that we taste with our eyes. How a wine appears in the glass matters a great deal, because ‘taste’ itself is a multimodal perceptive event involving a number of senses, including vision alongside touch, taste and smell. Even the information we have about a wine influences the actual perception of the wine: brain-scanning studies have shown that experienced sommeliers process the taste of wine in different ways to non-experienced wine tasters.

So the appearance of wine matters. It’s also an important cue in blind tasting, and the Master Sommeliers have a visual assessment grid that they use to help teach blind tasting. The visual points in this grid are as follows:

Clarity
Brightness
Color
Concentration of Color
Rim Variation in Red Wines
Presence of Gas or Sediment
Tearing (and staining in the tears for red wines)
Viscosity

I thought it would be good to explore a little more what wine science can tell us about the visual cues that wine gives us. While the grid above is useful, is there anything we can glean from wine scientists that would help us refine it or add to it? There are also some aspects about wine’s appearance that are taught widely, but which may be without scientific basis. It’s always good to question what we are taught.

Let’s begin with color, and white wines. Some people think that the green tinge to the colour in some white wines is attributable to the presence of chlorophyll in slightly under-ripe grapes. I asked some wine scientists whether this might be possible. ‘I don’t think there is any chlorophyll in wine,’ says Liz Waters, R&D Program Manager at the Australian GWRDC. ‘This is reported by Zoecklein (1999), where he states that Amerine (1972) says “White musts can contain traces of chlorophyll, carotene, and xathophyll”,’ says Geoff Cowey, researcher at the Australian Wine Research Institute. ‘He doesn’t state if they are derived from the grapes, but I suspect it is more likely from leaf and stem contact during cold soak/pressing, which contain chlorophylls. The stability of chlorophylls at wine pH is also questionable.’

So what is it that creates colour in white wines? This is widely used in visual assessment. ‘White wine colour is due to polyphenolic-derived pigments,’ says Liz Waters. ‘Not all of them are known or identified, but xanthyllium pigments certainly play a role.’ Geoff Cowey expands: ‘as whites don’t contain coloured anthocyanins in their skins, and white generally have little to no skin contact, I suspect the green/yellow color of white wines are due mostly to hydroxycinnamic acids and phenolics in pulp, which can sometimes have a yellow color, which tend toward brown colours as the wine develops and these compounds become oxidised. There are some proanthocyanins in white grapes that could also contribute towards color.’ It seems as if this is a subject ripe for more research. ‘There doesn’t appear to be much work on it in recent times, but historically the majority of researchers mention flavonoid content, as affected by limited oxidation,’ says Cowey. ‘It is an interesting question: it leads me to think of white grape skin colour albeit with minimal skin contact during processing. The color of ripe chardonnay grape skin colours (yellow/gold) produces yellow/gold coloured must and wine, versus Sauvignon Blanc (bright green skin and must). But then Riesling grapes can come in very yellow but produce very green coloured juice and wine.’

For red wines, color indicates a number of things. First off, it tells us a bit about wine pH. More acidic wines with lower pH tend more to the bright cherry red end of the red spectrum, while those with higher pH are more purple/blue. You can see this effect when you add a bit of tap water (pH around 7) to a bit of red wine in the bottom of a glass (pH 3.2-3.8): the colour goes from red to blue. Second, the colour will vary with the level of initial extraction, the presence or absence of oak during ageing, and the level of sulfur dioxide additions (this can have a bleaching effect, resulting in loss of colour). Third, and perhaps most importantly, the color tells you about the oxygen history of the wine: how much oxygen it has encountered during the winemaking process and ageing. Young wines are typically vividly, brightly coloured with fresh red and purple colours dominating. As they see more oxygen the colour shifts more in the orange/brick red spectrum, tending towards brown with excessive oxygen exposure. Compare a vintage Port at 10 years old with a 10 year old Tawny for an extreme example of how oxygen exposure changes the colour of a wine. Bricking of an older wine is first seen at the rim, because this is where the colour is less intense.

It should be pointed out that the grape variety can also influence wine colour. A good example here is Pinot Noir: it lacks acylated anthocyanins, which are a form of anthocyanin that is more stable, and it generally produces lower levels of anthocyanins, resulting in wines that are generally lighter in colour. The use of oak in ageing wines also makes a difference, because barrels can both introduce phenolics to the wine, and also allow for more oxygen exposure during the winemaking process. Thus oak-aged whites are typically deeper in colour than those aged in tank.

Interestingly, cooler vintages tend to produce deeper coloured red wines. In warm vintages, the grapes develop faster and reach ripeness while the anthocyanin production lags slightly behind. Also, stable colour is due to combinations between anthocyanins and polyphenolic compounds that take place during fermentation, and in the warmer vintages the polyphenolic composition will be different (the tannins will be ‘ripe’ and likely less reactive). This is aside from the extraction process involved, which of course can affect wine colour also. It’s worth mentioning here the phenomenon of co-pigmentation, where small amounts of white grapes (typically Viognier, at 1–6%) are added to a red wine fermentation. The colourless phenolic compounds from the white skins bind with anthocyanins and form stable colour complexes, and thus—paradoxically—the red wine with some white grapes added is actually darker in colour than without.

Is brightness or clarity in a wine just attributable to filtration/long settling? ‘Filtration and settling will increase brightness and clarity, and I can’t think of any other reason for wines to be clear, apart from particles being removed or settling out,’ says Liz Waters. ‘With greater filtration and/or settling, you achieve greater clarification as you remove suspended non-soluble material in the wine, and also decrease the turbidity,’ agrees Geoff Cowey. ‘Thus the clarity of the wine will improve, hence the term used in the industry of “cellar bright”. Note that wines can be really low in turbidity, and sterile filtered, and will have great clarity yet still have more of a dull appearance than bright, but this more a factor of the development of the wine.’

So what about tears? How are they created and what can they tell us about wine? Basically, they are formed by the evaporation of alcohol, which leads to what are known technically as Marangoni stresses, leading to climbing, thin films of wine. These films reach a point where they are no longer stable, and so the wine falls back down in a tear-like pattern. It’s hard to explain, but as the alcohol evaporates at a slightly higher rate from the wine in contact with the glass, this draws in the surrounding wine because of a surface tension gradient (the surface tension is less in the higher alcohol area) which then forms the film climbing the side of the glass. Thus the size of the tears is telling you about the concentration of alcohol in the wine, but also the temperature of the glass. If the glass is warmer, the alcohol will evaporate faster in the wine near the edge of the glass, and the tears will be bigger. Whether or not the tears are highly stained simply reflects the density of colour in the wine. ‘Both high extraction and cooler vintages would enhance colour in wine and thus in the tears, but there would be other reasons also,’ says Liz Waters. ‘This question is really about what leads to high colour in wine.’ In conclusion, tears are not that useful in the visual assessment of wine.

It needs to be pointed out here that there is a huge difference between viscosity and the sort of surface tension effects that create tears. Tears don’t have anything to do with the viscosity of the wine (and are sometimes wrongly attributed to the presence of glycerol in the wine), unless, of course, viscosity is in some way correlated with alcoholic strength, in which case this is correlation and not causation.

Finally, one question that has always interested me concerned the size of bubbles and their behaviour in Champagne. I’d always been taught that the finer the bubbles and the more persistent, the better the wine. But are the bubbles a property of the sparkling wine or Champagne, and do they really tell us about quality? I asked sparkling wine expert Dr Tony Jordan, previously head of Domaine Chandon, and now working for his own consultancy Oenotech.

‘Bubble size (which increases as a bubble rises in a glass of Champagne) and shape is affected by the composition of the wine, the nucleation sites present and the concentration of CO₂ in the wine.
Carbonated water has totally different bubble behaviour which shows that the wine’s composition (alcohol, surfactants, etc.) has an obvious influence, but it is a complex story.’

‘Some people like to say that the finer the bubble the better the wine or the longer yeast age the wine has had. There may be some truth in the longer ( 10 years +?) cork aged Champagnes/ sparklings (see below) but it is not clear in 2 to 7 years tirage aged wines with moderate cork age and it has little to do with quality (however that is judged). If it was the case that longer yeast aged Prestige Cuvée Champagnes and sparklings had finer bubbles then, for instance, Taittinger would always pour a glass of Comtes de Champagne and show it had a finer bubble than shorter yeast age (NV or Vintage) products. This is usually not observed. It may be finer than some but then not finer than a commercial bubbly made with short yeast age!’

‘Bubble formation requires the presence of nucleation sites with minute air pockets trapped inside them. The pockets of gas arise because of incomplete wetting of the nucleation sites when the wine is poured into the glass. These are now believed to be hollow cellulose fibres from paper and cloth rather than faults in the glass itself. CO₂ diffuses into the pockets of gas until they grow big enough to lift off and interestingly when they do a small pocket of gas is left so another bubble forms from the same site and so on, hence the familiar stream of bubbles from one site. (The nucleation sites can be stuck to the glass or floating in the wine.)’

‘The bubbles at 'lift off' vary in size (10 to 20 micrometres) depending on the nucleation site. As the bubbles rise they continue to grow because CO₂ continues to diffuse into them as they rise. Have a look at a rising stream of bubbles and you can easily see they are bigger as they approach the surface, they grow to up to 1mm in diameter if they have risen through about 10cm ( If the nucleation site is high in the glass then they haven't the time to grow as much).’

‘In the sense that an old cork aged (say 10+ years under cork) Champagne or sparkling may have lost pressure due to diffusion of CO₂ through and past the cork, the bubbles can be expected to not grow as much as they rise because of the lower concentration of CO₂ dissolved in the wine (bubble growth is directly proportional to dissolved CO₂ concentration). That is, the bubbles at a given nucleation site is the same size no matter what the wine but by the time the bubbles have travelled to the surface there could be a difference , bigger for the full pressure bottles, lesser for lower pressure bottles.’

‘However there is also the fact that bubbles at lift off are different in size (i.e. nucleation site dependent) and also Champagnes/sparklings have varying pressures depending on tirage sugar (i.e. a short yeast age sparkling may have a lower pressure hence lower dissolved CO₂) and these contribute to making conclusions about bubble size in a flute, even for old cork aged bottles, unreliable. Then there is the story of how the bubbles are perceived in the mouth. That is another story with less science.’

Jamie Goode is a London-based wine writer with a scientific
background. After completing a PhD in plant biology he worked as a
science editor. Smitten by the wine bug, he began consumer-focused
website www.wineanorak.com in 2000. The success of this site led to
more work, and in 2005 he landed a national newspaper column and a
book deal. Now he devotes all his energies to wine, and his second
book, Authentic Wine, was published by University of California Press
in September 2011, authored in conjunction with consultant winemaker
Sam Harrop.

Questions on Chemistry and the Flavor of Wine

Jamie Goode — Tue, 20 Dec 2011 12:02:00 GMT

I was really pleased with the response to my first article here on wine flavour chemistry. Some of the comments raised interesting questions, and to do these justice I thought I’d use them as the basis for this second piece.

‘As amazing as this article is we must not for forget the statement that the interaction between the wine and the taster has a huge influence on the final outcome and ultimately the perception of the wine. At the end of the day you have to ask yourself... Do you like the wine or not?’(Christopher Hoel)

This is a really important point. Straying into philosophical territory, I think we need to revise the way we approach the practice of wine tasting. Traditionally, the wine trade has regarded the taste of a wine as a property of the wine. That is, the practice of wine tasting is the assessment of the properties of the wine, with the ideal taster behaving like a measuring device. But it’s becoming clear that as tasters we bring something to the wine tasting event: our physiology and our experience of wine will affect the conclusions we come to. What we are actually assessing is our interaction with the wine.

For this reason, wine tasting is quite personal, and as we share a wine together we will be having different experiences. But this doesn’t mean that wine tasting lacks any objectivity. We train our palates and share our tasting notes, and as we do this we do find that there is a strong common experience in wine. While there is always disagreement among a room full of tasters on some wines, there are those bottles that we all seem able to agree on. This whole issue of the subjective and objective issues of wine tasting is a fascinating one—and deserves an article in itself.

‘I know that methoxypyrazine is inherent to certain Bordeaux grapes and that it decreases during ripening through grape exposure to sunlight. I heard from a winery owner recently about perception of rotundone as pepper being associated with cool climate syrah. What is the mechanism for its decrease in warm climates - is it actually temperature or is it also sunlight exposure?’ (Nichole Dishman)

This is a very good comment. Rotundone is a recently discovered compound that gives peppercorns their pepperiness, and has been found in low but significant quantities in some wines, notably Syrah and Grüner Veltliner. A large survey has been carried out of 137 Australian wines: no rotundone was found in 81% of these, and of those that contained it, 62% were Shiraz wines. Those with most rotundone were from cool-climate regions, but no one knows why, yet. It doesn’t seem to diminish through the ripening process in the same way that methoxypyrazines do; the available data suggest that it actually increases. There’s currently very little known about this compound because its discovery in wine is relatively recent (2008). But work is taking place right now that should shed more light on it.

‘Interesting observations regarding the ability (or lack of) smelling all these different compounds. Rotundone? Imagine the consequences when an individual cannot smell this compound and possibly another, which together would help him/her identify the wine as Syrah versus another varietal on a blind tasting? As such, we should consider how we are teaching and/or practicing blind tasting, as it is an essential skill that must be mastered to be successful in the certification process of our chosen profession.’ (Greg Rivera)

‘The last part about everyone having different sensory thresholds for different aromatic chemicals is at the same time troubling and freeing in regards to blind tasting. Everyone, or almost everyone, on this site is concerned about their ability to blind taste, and it is only natural to worry about your own thresholds in this respect. However, I think it is important to remember that there are plenty of other pieces to the blind evaluation equation, and your mind, your ability to deduce, is just as important as your palate. Also I think the latter part of this article puts the focus, rightfully, on the individual interaction with wine and creating a sensory context over time. I think it is very interesting and often helpful to read through lists of varietal markers, but it is probably far too easy to fall into a trap where studying these lists becomes more important than actual time spent with wine. My own, limited, experience with blind tasting has been frequently uncomfortable and produced very uneven results. Even more frustrating is the conversation afterwards where I seemingly "missed" important markers that other people in the group picked up on. I think these conversations can be important, and certainly everyone feels lost at different points in blind tastings--we shouldn't be fixed on results alone. What is liberating is the knowledge that I need to be working to an extent on "what Syrah tastes like to me," rather than what Syrah tastes like.’ (Patrick Miner)

This is a really important point, and it has implications—as you suggest—for the way that we teach wine, and the way we learn to blind taste. To answer it, I first need to separate out two sources of differences in taste perception. First of all, there is our biological make-up; second, our experience.

Biologically there is good evidence that we each live in our own taste worlds. This is only true to a certain extent, of course – it doesn’t make sense to suggest that there can be no shared experience of wine at all. An example of this biological variation is that about a third of people just aren’t very good at detecting ‘bitter’ taste. In sensory science work, those putting together testing panels usually have a screening process where they weed out people who don’t really get bitter. Related to this, but not quite the same (it all gets quite complicated when you delve in more deeply) you may have heard of PROP taster status. This is to do with the ability of individuals to taste PROP (propylthiouracil), an extremely bitter substance to some, but much less bitter or tasteless to others. Those who find it extremely bitter are known as ‘hypertasters’ or ‘supertasters’. And with regard to aromas, it is well known that people differ widely in their detection thresholds. Rotundone is an interesting example, because it seems that some people are ‘smell blind’ to it. But for most aroma compounds, sensitivity will be spread across a continuum. For example, some people can spot TCA (cork taint) at very low levels indeed, and some don’t notice it until the level is a bit higher; most people fall somewhere in the middle.

And then there’s experience. Peoples’ tastes change with time. Most people don’t innately enjoy the flavour of beer, strong cheese or coffee, but grow to like these tastes over a period. Sometimes these acquired tastes are more enduring and compelling than the flavours that everyone seems to like innately. In addition, knowledge of what we are eating or drinking changes the way we approach it. Studies comparing sommeliers with novice wine drinkers have shown that the two groups process the flavour of wine in their brains in quite different ways. Experienced or trained wine drinkers taste wines in a different way. The more we learn, the more we seem to appreciate the flavours of what we are eating and drinking. Basically, experience seems to shape perception.

Interestingly, the learned component of wine appreciation can be shared. If we study the same syllabus, we can have shared knowledge, and this could offset some of the biological differences in perception, bringing us closer to a common experience as we taste wine together. Other interesting questions concern the language we have for wine. Does having an extended vocabulary for wine flavours and aromas alter the way we perceive the wine? Perhaps.

Your point is brilliantly made, though: we should be working on what Syrah tastes like to us, not what Syrah tastes like. This is a really important message, and could help students a great deal in developing their ability to taste wine blind. This enhanced and more realistic take on how we perceive wine should certainly influence wine education more than it currently does.

Our discussions here do suggest that wine education is a little too focused on objective experience. The current expectation is that as tasters develop their expertise, they are able to read a wine correctly, and there is one correct interpretation of the wine. The implication is that if the student disagrees with the examiner’s take on the wine, that the student is wrong.

While I appreciate that this is probably the only way to run tasting exams in the real world, I wonder whether current methods for teaching could be improved on. Would it be possible to acknowledge the subjective elements of wine tasting, and that we bring a good deal to the tasting experience, within the educational syllabus? It would certainly make for a more complicated picture, but the current method of teaching results in a lot of confusion on the part of students who simply don’t get the wine in the same way as their examiners, and it doesn’t correlate with the reality of our interaction with wine, which by its nature is complex.

With regard to biological differences, a common misunderstanding is that it is beneficial to be a PROP hypertaster or supertaster if you want to be a great wine taster. This may not be the case. Hypertasters may well find tannins in red wine objectionable, to the point that they avoid tannic red wines or are unable to evaluate them fairly. [I should add here that there remains some controversy about the importance of PROP taster status.] On the other end of the spectrum, those who are unable to spot bitterness might be similarly compromised in their tasting. Of course, if you are a consumer, it is not a problem: you just avoid the wines you don’t like. But as a professional, you want to be able to assess all wines without some biological hindrance to your tasting. Of course, taste is adaptable: you can learn to appreciate certain flavours, and to some degree biological preferences can be compensated for. But given a choice, as a professional it helps a lot to be somewhere in the middle, without any blind spots (I certainly wouldn’t follow Syrah recommendations from a taster who can’t smell rotundone).

Finally, let’s consider how we can bring together wine flavour chemistry with our actual perception of a wine. Is it possible to link, with a degree of certainty, descriptors in a tasting note with chemical entities in the wine? It’s made tricky by the fact that sensations described by single descriptors might be a result of the combination of several chemicals. But let’s try with a couple of tasting notes taken from my recent blog postings. [Perhaps I might have success with more analytic tasting notes; these are written in a journalistic style. It might be interesting to explore this idea further.]

Kirnbauer Blaufrankisch Classic 2009 Mittburgenland, Austria

13% alcohol. Fresh with lovely raspberry and blackberry fruit to the fore. Vivid with hints of meat and some green notes. Lovely definition to the ripe black fruits with a hint of pepperiness. The green notes here are really attractive.

The raspberry and blackberry fruit are likely to be from fruit esters. The hints of meat? Could this be a little bit of brettanomyces? I don’t think so in this case, but it’s hard to rule out. The pepperiness may be our good friend rotundone. Green notes? Most likely methoxypyrazines, or maybe hexenal. In small doses, greenness can be attractive in a red wine.

Hidalgo La Gitana Manzanilla La Gitana En Rama NV

15% alcohol. Very bracing and aromatic with some salty, nutty savoury characters leading to a tangy palate with fresh, intense, nutty, savoury notes. There are complex flavous of hazelnut, citrus pith and a salty tanginess here. A really impressive, bracing expression of sherry that’s highly food compatible.

So this is a Manzanilla, and the nutty, savoury notes come from ethanal (acetaldehyde), which is formed by the contact between the flor and oxygen (it’s an oxidation product of alcohol). Ethanal has a nutty, appley taste, and it also affects the mouthfeel of the wine. The salty tanginess? Hard to say: perhaps this is a character that’s a result of the combination of the acidity (tartaric and malic acid). Or could it be the traditional explanation, that the proximity of the sea is giving Manzanilla its saltiness?

Wine Flavour Chemistry

Jamie Goode — Wed, 07 Sep 2011 18:10:00 GMT

Wine. It’s a liquid made of chemicals. And some of these chemicals have smells and tastes. That, in a nutshell, is wine flavour chemistry—a dauntingly complex but utterly engrossing topic. It’s my goal with this article to try to introduce the subject, outline some of the emerging concepts, and make sure that you stay reading to the end.

There are two ways of approaching wine flavour chemistry. One is to begin with the chemical composition of wine itself. The other is to look at how we as humans perceive tastes and smells. Both approaches are vital, because it’s possible to argue that the taste of wine isn’t a property of the wine itself, but of the interaction between the wine and the taster. This might seem to be an overly philosophical approach, but it’s important to hold this concept in mind as we explore the whole area of the taste and smell of wine.

Currently, the academic field of wine flavour chemistry is undergoing a quiet revolution. Until recently, most sensory scientists took a reductionist approach to studying wine. That is, they broke it down into its component parts, and studied each in isolation. This is a powerful method of enquiry, but has its limitations, chief of which is that wine aroma and flavour isn’t simply the result of an additive combination of the different flavour compounds present. Instead, there are complex interactions among the different flavour molecules, and also with other chemicals present in the wine, such that the final result isn’t predictable from simply knowing the composition of the wine.

An example of this would be where one aroma compound, present below the concentration at which we are able to smell it, actually affects the way another aroma compound smells. As a result, scientists are now turning to more sophisticated holistic ways of looking at wine aroma. We’ll come to some of these approaches in a while. For now, suffice to say that these sorts of experiments are a lot more tricky than simply looking at chemicals in wine in isolation.

Where wine flavor comes from

Take grape juice, from freshly pressed wine grapes. It’s sweet and not particularly complex, and nothing at all like wine. This tells us that it’s the action of microbes during the process of fermentation, plus the process the French call élevage—the bringing up of the wine—that is responsible for most of the flavour chemistry that we can sniff and slurp in the finished wine. Much of wine’s flavour comes from the action of yeasts and (where malolactic fermentation takes place) bacteria. As these organisms grow in the wine, they take up a range of nutrients from the grape must, and use these to build other chemical components that they need for growth, with sugar as the basic energy source to achieve this growth. The complicated metabolic pathways end up producing a wide array of chemicals that are then excreted into the must. In addition, yeasts die and release chemicals into the developing wine, as well as creating the lees—dead yeast cells at the bottom of the fermenting vessel—that are able to interact with the wine and alter its composition. During élevage, small amounts of oxygen contribute to the chemical reactions taking place, and if wooden barrels are used, these can also directly contribute flavour compounds themselves. And we mustn’t forget that the way that the wine is finished—for example, whether it is filtered or not, and any fining processes employed, plus the bottling conditions and closure choice—also have an impact on wine flavour chemistry.

Studying the flavour of wine

So we have this complicated chemical soup that is wine. How do scientists begin to unravel this complexity and work out the contribution of specific chemicals? It’s important to know which chemicals are important and why, because this then makes the next step of working out how they got to be in the wine at this particular level possible. And from here it’s then feasible to begin to understand how the winemaking process and vineyard environment influences the presence of this chemical (or, if it is produced by yeasts, its precursor compound, i.e. the one that is made from). This sort of scientific understanding is a tool to enable winemakers and viticulturists to have more control. Some may argue whether or not this is a good thing. I like to think that tools like this can be powerful when they are in the right hands. For example, if you desire to make an authentic, profound, complex wine that expresses a sense of place, then this sort of knowledge is your friend.

The long-standing approach to wine flavour chemistry is to take what amounts to a chemical fishing expedition. Individual chemicals are identified one by one from the wine, separated out, and then examined to see whether they smell of anything. This has been a useful approach, especially for flavour chemicals that are present at higher concentrations. Where it has struggled is with those chemicals that are present at very low concentrations and thus are hard to isolate, but which have a significant aromatic influence.

More recently, a technique called gas chromatography/olfactometry (GC/O) in tandem with mass spectrometry (MS) has been employed. Gas chromatography is a powerful way of analysing the volatile composition of wine. It works by separating out the aromatic portion of wine in much the way that a piece of blotting paper separates out the different pigments present in ink. Mass spectrometry can identify each of the chemicals that have been separated out in time, as they leave the chromatography column one by one, and olfactometry is the process where someone sits and smells each of the chemicals in turn as they are released. Now pet owners will be aware that the human nose isn’t as powerful as a dog’s, but it’s still able to pick out some smelly chemicals at incredibly low concentrations. These can be matched to their respective chemical identities, and a list of the volatile aroma compounds in wine can be built up.

The holistic view

One of the leaders in the field of wine flavour and aroma is a researcher called Vicente Ferriera, who is based in Zaragoza, Spain. His group recently published a really interesting paper, which is likely to have a big impact on the way that wine flavour chemists work. In this experiment he took red and white wines, and stripped them of their aromatic components to produce what is referred to as a non-volatile wine matrix. The aromatic components were held in reserve, and then recombined with the non-volatile matrix, to recreate wine. But the clever thing he did was to switch the two components around, so that the white wine volatiles were mixed with the red wine matrix, and vice-versa. You’d expect that the aromatic component would be the bit that determines whether the wine tastes red or white. But what he found was that in fact it was the non-volatile wine matrix that influenced how the wine was perceived. The red wine volatiles, when mixed with a white wine matrix, produced a wine that smelled like a white wine. And the opposite occurred when it was the red wine matrix that had the white wine volatiles added.

This is an important observation, because it shows that it’s not just the volatiles that are present that matter for wine aroma. The non-smelly bit of wine influences how the smelly bits of wine smell. That’s really unexpected and has important implications for how we study wine flavour and aroma. Any approach needs to be holistic.

So what flavour and aroma chemists are now doing is creating what is called a model wine. The first stage in this process is to use GC/O to find out which aroma compounds are present in the wine under study at concentrations where they are above the human olfactory threshold. These are likely to be the most important ones, although, as we discussed earlier, sometimes volatile compounds present below the level at which they can be smelled may be having an effect.

Stage two is to take the wine in question, and strip it of volatiles, so that you are left with the wine matrix. This should be done as cleanly as possible with the minimum effect on the other components present in the wine. Then, the most important volatile compounds—perhaps 20 or 30—are added back in their original concentrations. The result is tested by sensory analysis to see whether it smells and tastes like the original. If it does, then it’s game on. Using this approach it is then possible to create wines where one or more of the volatile compounds are excluded one by one, or in groups, to see what the impact is on the wine. This holistic approach has incredible power, and can yield quite unexpected results.

What’s in a wine?

So what makes wine taste and smell the way it does? It’s possible to categorize flavour compounds in a number of categories.

First, the base aroma. Ferreira’s group has identified 20 aromatic chemicals that are present in all wines, and which form a global odour that has been dubbed ‘wine odour’. Of these 20 aromas, just one is present in grapes (β-damascenone); the rest are produced by the metabolism on yeasts, in many cases working on precursors present in the grape juice.

These are:

• Higher alcohols (e.g. butyric, isoamylic, hexylic, phenylethylic)

• Acids (acetic, butyric, hexanoic, octanoic, isovaerianic)

• Ethyl esters from fatty acids

• Acetates and compounds such as diacetyl

• Ethanol

The influence of alcohol (ethanol) is quite strong. Ethanol has been shown to modify the solubility of many of the aroma compounds, and makes them more reticent to leave the solution, thus making the wine less aromatic.

Second, we have a group of contributory compounds. There are 16 compounds which are present in most wines, but at relatively low levels. They are usually below the level at which they can be smelled on their own, but they have odour activity that is synergistic, contributing to characteristic scents despite being at lower concentrations that would normally lead to them being smelled.

These include:

• Volatile phenols (guiaicol, eugenol, isoeugenol, 2,6-dimethoxyphenol, allyl-2,6-dimethoxyphenol)

• Ethyl esters

• Fatty acids

• Acetates of higher alcohols

• Ethyl esters of branched fatty acids

• Aliphatic aldehydes with 8, 9 or 10 carbon atoms

• Branched aldehydes such as 2-methylpropanol, 2-methylbutanol, 3-methylbutanol, ketones, aliphatic γ-lactones

• Vanillin and its derivatives

Finally, we have impact compounds: the chemicals that are responsible for giving characteristic aromas to certain wines, even when they are present at extremely low concentrations. These are of great interest because they often contribute to distinctive varietal aromas. Examples of impact aromas include:

• Methoxypyrazine: the most important one is 2-isopropyl-3-methoxypyrazine, which has a detection threshold of 2 ng/litre, and is responsible for green, grassy, green pepper aromas. This is one of the few impact compounds formed in the grapes, and it is highly stable through fermentation and ageing.
• Monoterpenes, such as linalool, which is important in many white wines, such as Muscat. It has floral, citric aromas.
• Rose-cis oxide: characteristic of Gewürztraminer, this has a sweet flowery, rose petal aroma.
• Rotundone: a sesquiterpene that gives pepperiness to Syrah, at incredibly tiny concentrations.
• Polyfunctional thiols. These include 4MMP, which has a box tree aroma (4.2 ng/litre detection threshold), 3MHA, which has a tropical fruit scent (60 ng/litre) and 3MH. These three are important in the aroma of Sauvignon Blanc.

The perception of wine

With all this emphasis on what is actually in a wine, it’s possible to forget that the flavour of a wine isn’t the property of the wine: it is what happens when we encounter a wine. And we bring quite a bit to this wine tasting experience. We are far more than measuring devices. So it’s important that we discuss, at least briefly, the nature of sensory perception.

Our experience of wine is based on our perception of chemicals in the wine, but a lot happens to the signals that come from our tongue, mouth and nose. The inputs from the senses smell, taste, touch and vision all affect each other, and are recombined and processed a fair bit by the brain before we are consciously aware of them. Flavour is a multimodal perception: that is, it’s the result of the combination of more than one sensory modality. Added to this, we also bring our previous knowledge and experience of wine into play, and this can affect how we perceive the wine. And there’s also the issue of biological differences that needs to be considered. Some individuals are much more sensitive to certain tastes than others, and it is well known that about a third of people are quite bad at detecting ‘bitter’ taste. We all differ in our ability to smell, too. The best example of this is the compound rotundone, which is responsible for pepperiness in Syrah: about a fifth of people can’t smell this at all.

This sort of complexity is mightily inconvenient, but we need to bear it in mind when we approach wine tasting and evaluation. Currently, the way wine education is taught assumes that everyone is experiencing the same thing when they taste wine, which we now know to be a false assumption. But the fact that what we know about a wine has the potential to shape our perception of that wine reinforces the fact that the culture, history and context of wine is really important, and reinforces the role of the sommelier and wine service in restaurants as vital in helping people to get the most out of their wine.

Concluding remarks

So there we have it: a brief introduction to a topic where our knowledge is currently growing at quite a rate. In some ways, this sort of science talk grates a bit with the wonderfully artistic side of wine. Most of the world’s great wines were made before we knew very little about wine chemistry at all, by people who observed, experimented and had a language for wine totally at odds with scientific thinking and terminology. But I’d argue that in the right hands – of those who really understand what makes wine special, and who can tell the difference between authentic and industrial wine – then the sort of understanding that science brings to wine can help more people produce interesting, compelling wine at prices that normal people can afford. And that has to be a good thing.