It may sound boring, and devoid of art and beauty, but it’s the framework that everything else is built on.
It effects perception of weight, flavour profile and residual sugar. It’s the single biggest factor in good wine matching and probably the element that changes the most (and changes the dish the most). It protects juice and wine, keeps them healthy and lets you leave fruit on the vine longer or forces you to pick it earlier, it determines risk in the winery in exposure to the elements. It effects ageability of the final wine.
There are three major types of acid that are relevant to this discussion. Tartaric acid, malic acid and lactic acid.
Tartaric and malic make up the overwhelming majority of the acids present in the grape (about 90%). Basically up until veraison (the point where are grape changes colour), acid is building up, and that’s the majority of what the grape is at that point, a ball of acid. After veraison, acid begins dropping and sugar increases, along with flavour compounds. This is one of the biggest determining factors of a good vineyard; the ability to develop the fruit, whilst losing acid as slowly as possible. Probably the biggest influence on this is temperature, and that’s why a big diurnal shift in temperature is extremely handy. Lighter soils further exaggerate this effect, whereas heavier soils will retain their temperature much longer.
When we’re deciding when to pick, there are a lot of things to consider but in the background is always the acidity. You want everything to be ripe, and tasting good, and you need the beaume to land somewhere reasonable for the wine you’re going to make, but you’re always tasting to keep an eye on the acidity, if it’s dropping you need to be really careful because the fruit is about to fall away on you, and you really need to pick (even if it’s not ideal), and the flip side is that if the beaume is where it needs to be, but you’ve still got really good acidity, you may leave it for another couple of days because it’s pretty safe and you’re only going to develop the flavour profile further (admittedly, this is over simplified).
Once you’ve picked, you’re going to test your PH, and this is the first time you get a really accurate reading of it, as everything varies drastically throughout the vineyard, and this is the first time you’ve combined it all. What you’re really hoping for is a PH of 3.2 or lower. Basically this means the juice/wine is pretty safe, and you can let it do a really slow wild yeast ferment if you want and chances are everything is going to be healthy and happy the whole way through, and you’re going to develop some really cool flavours. However, if the PH is higher you really need to think. Either you picked too late, or it’s just a fact of life for your vineyard. You need to be looking at adding acid. I know we’re all up in arms these days about natural winemaking, and I’m all for that, and I’ve never added acid to one of my wines, but I absolutely would if the PH crept much over 3.2, because you’re simply going to make a worse wine without it. It’s more vulnerable, so you need to be a lot safer in your decisions, inoculate with a really safe strain of yeast, plunge a lot and really hammer it through the ferment as quick as you can to get it to a more stable point in it’s life. If you do or don’t add acid, you need to be making that call before the ferment; as a rule of thumb, anything you’re going to do, you need to do early or it’s going to stand out.
Then, with regards to the malolactic fermentation, it always seems like it’s going to be a big decision, but in my experience it’s usually a forced decision. It’s not often something you can go either way on; there’s always a clear cut answer.
Basically in this second ferment, bacteria is converting malic acid into lactic acid, which is much softer on the palate. This is why I’ve always found it to be a forced choice, either you need the sharpness of acidity for the wine to make sense, or you don’t. The big advantage of letting something go through MLF is that it stabilises the wine, and the more naturally you’re making the wine, the harder it is to make a stable product, so MLF is of increasing importance.
Skipping ahead a bit, to the maturation both in the winery and in bottle, acid is one of your biggest preservatives. You’ve got tannin, acid, alcohol and sugar as your particularly relevant preservatives, so every decision regarding leaving it in barrel or holding onto a bottle is pretty closely linked to it’s acid backbone.
And to quickly touch on wine matching: the reason acid is so important is because it’s most likely to make or break a wine match, you’re literally increasing the acid of the dish and decreasing the acid of the wine. You use it to cut through heavier elements, and avoid putting it with anything that’s already got abundant acidity (vinaigrette, citrus, etc). You’ll bring out the natural flavour profile of a dish, but you’re also changing the dish pretty drastically, so that decision needs to be weighed. It’s also handy as a palate cleanser, one of my favourite things to do is to take people back from a big Cabernet with the searing acidity of a sparkling blanc de blancs, and start the progression over again. This is a really useful tool, because it takes wine matching from this simple, predictable progression and allows you to take people on a real journey by looping it all back. Rather than always going heavier, go backwards, move sideways, just do something cool.