I watched Jurassic Park again the other day for the first time in a decade or more.

Almost all wines will have a pH of somewhere between three and four

About ten minutes in, you probably remember the bit with the DNA cartoon, where Richard Attenborough shows how his team of scientists extracted workable dinosaur DNA preserved in the blood of a mosquito found in fossilized tree saps. Using adventure-park-friendly words and pictures, the cheerful, non-threatening Mr DNA explains just how these ‘building blocks of life’ translate into a huge six-metre high Tyrannaosaurus Rex that is shortly going to begin ripping the park to shreds.

Now you might think that I should get out more, but what we need, I was thinking as I watched this, is a friendly cartoon to explain the role of pH in wine. It’s something that I often think about – pH, not Jurassic Park cartoon sequences – at this time of year, when you are analyzing great flights of wine from all over the region, trying to figure out the differences between them, and how and why one particular wine might age better or worse than another. And I’m always left thinking that this particular piece of information is much more useful for consumers than we give it credit for. I wish winemakers would routinely put the pH next to the grape variety and food suggestions on back labels, although I may be in the minority here.

In the absence of a cartoon figure zipping across the screen to explain why, I’ll just have to talk you through my reasoning. But let’s start with a definition; pH is a measure of acidity in wine, and it’s by far the easiest one to relate to. Basically winemakers record levels of acidity regularly through the year, first in the grapes and then in the cellars. They will all have their own idea of what makes the ‘right’ level, and it will depend on the vintage, their vineyard site and climate, grape varieties, and the individual style desired. Total acidity is, as the name suggests, a measure of all the different types of acids that are present along the way. It gives you an overall figure, but is not actually hugely useful for us non-oenologists, as all the various acids have a slightly different influence on the taste and structure of the final wine – the most well known here would be the biting malic acid and the softer lactic acid, but there are others. Measuring the pH basically cuts through all of this and gives you a clearer idea of how those acids will be perceived by the final drinkers of the wine. Where total acidity measures quantity, pH measures strength, power, perception.

‘The pH is the wine’s centre of gravity,’ says Pierre Seillan, the winemaker of Chateau Lassegue in Saint Emilion and La Verite in Sonoma. ‘It’s often described as the backbone, but I prefer to think of it as the centre of gravity, because that better conveys the idea of how we are walking a tightrope as we make a wine. If the pH is mishandled it can destroy all our efforts, tipping the whole thing off balance.’

Almost all wines will have a pH of somewhere between three and four – with whites tending to be on the low side of that, and reds on the high. It’s hard to give absolutes, as with anything in wine, but a low pH, say 3.1, which plenty of the whites were in 2014, means high perception of acidity. Low pH is what you want in a white as long as it tastes crisp and fresh, but go too low and you will have an enamel-stripper on your hands. Similarly a high pH, says 3.9 or 4, which you will find many reds were in the 2009 vintage in Bordeaux, or in a normal vintage in many hot climates, means low perception of acidity. These may taste fruity, round and soft, but risk issues such as barnyard aromas from unwanted yeast growth, and general losing of definition as they get older. There is one very simple reason for this – wine needs protection against the potentially disastrous effects of bacteria, yeasts and oxygen, and once wine hits a pH of around four, those protection mechanisms break down. A wine with true ageing potential needs to have a balanced pH, low enough to allow the wine to protect itself over the long years ahead – it’s why there is a clear link between the perception of freshness in Bordeaux wines and their ability to age. And it’s worth adding that it is almost certainly necessary for the wine to have achieved balance naturally, rather than by manipulations in the cellar to de-acidify or otherwise adjust the chemistry.

I started getting interested in this while studying for a tasting diploma at the Bordeaux oenology institute – I have no science background, which may be why the friendly cartoon version of Mr DNA seems so appealing to me – but it is out in vineyards that its true importance has been brought home. Looking at pH levels in a single year might give you an idea about how that individual wine will taste and age, but study them over a vertical from a single property and what you are getting is a snapshot of how healthy the vineyard is.

One of my most instructive pH-related tastings came care of Denis Durantou at L’Eglise Clinet. We tasted through hot, dry vintages such as the 2005 and 2009, against cooler vintages such as 1997, and yet it was striking that the pH remained stubbornly around 3.6, no matter what the alcohol levels were. I had found a similar stability with Cathy Corison in Napa, even if slightly higher, maybe 3.7, as clearly there are no absolutes in terms of the right numbers, and when I commented on it, Durantou ascribed it to old vines with a deep roots structure, and cool clay soils providing a sound terroir.

‘For me the stability of the pH is a shortcut to understanding a vineyard,’ said Durantou. ‘If the roots system is too close to the surface, its pH – and its wines – will be held hostage to the vintage influence. Working on sending roots lower means that they can nourish the vines while being less affected by different levels of sunshine or rain, so the wines can be truer to the nature of the soil’. So, reading it this way, pH is a sign not just of our perception of acidity, but an indicator of how seriously a winemaker takes his or her job, from how much fertiliser he applies to his irrigation choices and even his trellising system – pretty impressive information from a single digit number.

Written by Jane Anson