The Taste of Wine: Acid, Sweetness, and Tannin

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.