Where does wine acid come from?
Acid occurs naturally in wine and comes from the grapes. Like any other sweet fruit we eat, it must go through a ripening process before we can eat it. We’ve all tried an apple, orange or banana that’s just a bit too green. They don’t have much flavour, they’re not sweet, and they taste tart. Grapes are the same. As they go through the ripening process, their sugar increases. as does their flavour. In the case of red grapes, colour and tannin increases too. Acid, however, begins at its highest level and decreases through the ripening process. The amount of acid left in the grapes is what did not degrade before harvesting.
There are many types of acid present in grapes. The highest proportion of acid is tartaric acid, followed by malic acid. Then there are minute amounts of citric, succinic and other acids. Depending on the climate where the grapes are grown, the rate at which tartaric and malic acid decreases can vary. In cooler climates the decrease in acid is slower. The opposite is true for warm and hot climates. The decision of when to harvest grapes is partly dependant on the remaining amount of acid in the grapes, along with the amount of sugar, flavour, colour and tannin. All these elements are taken into consideration when deciding when to harvest, to match the style of wine we have in mind.
What affects the amount of acid in wine?
McLaren Vale has a Mediterranean climate with a maritime effect. This means it is generally warm with a stabilising and cooling effect from the ocean. Most classic red grape varieties grown in McLaren Vale, such as Shiraz, Cabernet, Grenache, Merlot etc., have a modest amount of acid left at harvest. However, particularly in hotter vintages, the addition of some tartaric acid may be required during the winemaking process to re-balance the wine, which is the case for most wines from warm to hot regions. Different grape varieties also have varying metabolic pathways and respond to the climate differently which, in turn, equates to lower or higher acidity in the resulting wine.
When grapes are first crushed, the internal structure of the berry is partly broken down, which is further disrupted with fermentation on skins in the case of red wine. This releases a large amount of acid into the juice and wine solution, which is too much for the solution to hold and some acid molecules will precipitate out. Have you ever grown a crystal at school? The process is the same. Young red wine and white juice is a saturated solution, so any change in the temperature and solubility will cause a part of the acid molecule to precipitate out of solution and form crystals. The slower the precipitation, the larger the crystals will grow. That’s why we sometimes see ‘wine diamonds’ forming on corks and in the bottle of wines stored in the cellar for many years. This acid precipitation reduces the amount of acid in the wine.
Another way to reduce acid is with a ‘malo-lactic fermentation’. In cool climate regions, the malo-lactic fermentation (or MLF) may be performed on both white and red wines to reduce and soften acid. In warmer climates, the MLF is generally only performed on reds. The MLF is also known as the secondary fermentation and is the conversion of malic acid to lactic acid by bacteria. Lactic is a softer, less ‘tart’ acid than malic. Malic acid is a relatively high energy molecule and, as such, is a food source for bacteria. This fermentation is performed as much for acid reduction, as it is for stability of the wine. In the case of white wines, it is a modern winemaking choice whether to perform MLF, as whites are almost always sterile-filtered into bottles so bacteria cannot feed on the malic acid. In the case of red wines, however, the wines are not filtered to such a degree (sometimes not at all) so there is a greater risk of MLF occurring in the bottle – which is not great. In the winery we ‘seed’ MLF, which means we add a selected strain of bacteria that can quickly and cleanly complete the fermentation before we mature the wine in-barrel.
Why is wine acid important?
Acid is a very important part of the wine ‘matrix’. Wine is not a simple liquid with a bunch of chemicals blended together, but a complex solution with many parts acting in chemical equilibrium. Acid has protective properties to help prevent spoilage. It affects the taste and texture of the wine, the expression of fruit flavours and aromas, and changes the pH. PH is a logarithmic scale based on the concentration of an ion in a solution. The scale goes from 1-14, with 1 being extremely acidic (think concentrated hydrochloric acid) and 14 being extremely ‘basic’ (concentrated sodium hydroxide or lye, used as a drain cleaner among other things).
There are two commonly used measurements of acid in wine. The first is pH scale, mentioned above, and the second is ‘titratable acidity’. The titratable acidity gives us a measure of the total amount of acid in the wine, as compared to tartaric acid. The titratable acidity (TA) generally relates to taste, more than the pH measurement. Even though they are both measurements of acidity in the wine, you can have high pH and high TA, or low pH and low TA, or any combination in between. Other compounds in the wine matrix can act like ‘buffers’ that resist changes to pH, even as we increase the TA. The pH can influence a whole set of characteristics important to the longevity of the wine and the preservative sulphur dioxide. A low pH increases the anti-oxidant and anti-microbial properties of sulphur dioxide, meaning you get a better effect with a lower addition of sulphur.
What does it do to the taste?
As winemakers, we want our wines to not only taste great, but have a good shelf life as well. The best conditions for extending the shelf life of wine involve higher acidity, but the complexing factor is how it affects the taste of the wine. Most of us have seen some kind of ‘tongue map’ which illustrates the different areas on the tongue that perceive different tastes. The general view is there are five main tastes; sweet, sour, bitter, salt and umami. According to the tongue map, the sour taste (acidity) is towards the sides, to the back of the tongue. While there are recent studies that say the tongue map is far too simplistic, and there may be many more tastes then the five listed, we will keep it simple for the purposes of this article.
If you think about sweet drinks, sweet desserts and sweet foods, often there is some acidity involved to ‘balance’ the sweetness. Coca Cola is a great (but terrible!) example. It is both extremely sweet and extremely acidic. Lemon tarts are also a good example. The acidity in the lemon juice balances the sugar sweetness in the lemon curd filling. Sweet and sour pork also demonstrates the sweetness and acid balance in food. Wine is very much the same. Sauternes, late picked whites and botrytis sweet whites all use acidity to balance the flavour on the palate. If we go back to our tongue map, sweetness is perceived near the front of the palate. If we can get the sweetness and acidity balanced properly, it can increase the length of the flavour experience in the mouth.
Through personal experience and anecdotal evidence, acidity also seems to influence the types of flavours perceived. Bench trials are used to test the flavour effects of various additions before we add anything to a tank of wine. Often, when looking at increasing acidity in the same wine, different fruit flavours and aromas become evident. The expression of these flavours and aromas often guide our choice when making these decisions.
What does this all mean for consumers?
Winemakers almost always have this information available on their tasting notes or their website. How it is used is up to you. Depending on whether you are buying the wine to keep, or to drink now, the acidity can provide information that may help your decision. Low pH wines may be more stable and will last longer. A moderate to high TA wine may taste better with certain foods. Apart from sweetness, the acidity in wine can also help balance the food you eat. Rich, fatty protein meals can have a strong savoury/sweet taste that is balanced by the acidity in the wine you drink with the meal. This is also one aspect of the food and wine matching process.
Either way, the enjoyment of the wine is up to you. If you better understand why you enjoy one wine over another, then you may be able to easily find other wines to enjoy.